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QuickC 2.5 C for Yourself
The following document is from the Microsoft Programmer’s Library 1.3 CD-ROM.
Microsoft QuickC Compiler - C FOR YOURSELF
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Microsoft(R) QuickC(R) Compiler - C FOR YOURSELF
VERSION 2.5
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MICROSOFT CORPORATION
Information in this document is subject to change without notice and does
not represent a commitment on the part of Microsoft Corporation. The
software described in this document is furnished under a license agreement
or nondisclosure agreement. The software may be used or copied only in
accordance with the terms of the agreement. It is against the law to copy
the software on any medium except as specifically allowed in the license or
nondisclosure agreement. No part of this manual may be reproduced or
transmitted in any form or by any means, electronic or mechanical, including
photocopying and recording, for any purpose without the express written
permission of Microsoft.
(C) Copyright Microsoft Corporation, 1988, 1990. All rights reserved.
Printed and bound in the United States of America.
Microsoft, MS, MS-DOS, CodeView, QuickC, and XENIX are
registered trademarks and Windows is a trademark of Microsoft Corporation.
AT&T is a registered trademark of American Telephone
and Telegraph Company.
IBM and PS/2 are registered trademarks of International
Business Machines Corporation.
Hercules is a registered trademark and InColor is a trademark
of Hercules Computer Technology.
Intel is a registered trademark of Intel Corporation.
Olivetti is a registered trademark of Ing. C. Olivetti.
VAX is a registered trademark of Digital Equipment Corporation.
Document No. SY10423-0290
OEM D703-2Z
10 9 8 7 6 5 4 3 2 1
Table of Contents
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Introduction
About This Book
Using the Example Programs
Programming Style Used in This Manual
Key to Document Conventions
Other Books on C Programming
PART I Learning C
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Chapter 1 Anatomy of a C Program
A Typical C Program
Comments
Statements
Keywords and Names
Preprocessor Directives
Functions
Calling Functions
Declaring and Initializing Variables
External and Local Variables
Function Prototypes
A Few Words about printf
Chapter 2 Functions
Functions and Structured Programming
The main Function
Placement and Visibility of Functions
Function Definitions and Prototypes
Calling a Function
Passing Arguments to a Function
Arguments Versus Parameters
Assigning Parameters
Passing by Value
Returning Values from Functions
Using Return Values
Declaring a Function's Return Type
Function Prototypes
Prototyping Functions without Parameters
Prototyping Functions with Variable Parameters
Old-Style Function Declarations and Definitions
Chapter 3 Flow Control
Loops: while, do, and for
The while Statement
The do Statement
The for Statement
Decision-Making Statements: if, else, switch, break, continue,
and goto
The if Statement
The else Clause
The switch Statement
The break Statement
The continue Statement
The goto Statement
Chapter 4 Data Types
Basic Data Types
Specifying Basic Types
Specifying Variables
Specifying Constants
Aggregate Data Types
Arrays
Structures
Unions
Chapter 5 Advanced Data Types
Visibility
Local Variables
External Variables
Visibility in Multiple Source Files
Visibility of Functions
Lifetime
Extending the Lives of Local Variables
Converting Data Types
Ranking of Data Types
Promotions and Demotions
Automatic Type Conversions
Manual Type Conversions through Casting
Register Variables
Renaming Existing Types with typedef
The Enumeration Type
Chapter 6 Operators
Introducing C's Operators
Arithmetic Operators
Relational Operators
Assignment Operators
C's Unique Operators
Increment and Decrement Operators
Bitwise Operators
Logical Operators
Address Operators
Conditional Operator
The sizeof Operator
Comma Operator
Base Operator
Operator Precedence
Chapter 7 Preprocessor Directives
The #include Directive
Specifying Include Files
The #define and #undef Directives
Simple Text Replacement
Function-Like Macros
The #undef Directive
Conditional Directives
The defined Operator
Pragmas
Chapter 8 Pointers
Using Pointers in C
Pointers to Simple Variables
Declaring a Pointer Variable
How Pointers Are Stored
Initializing a Pointer Variable
Using a Pointer Variable
Summary of Pointer Basics
Pointers to Arrays
Arrays and Pointer Arithmetic
Comparing Pointers
PARRAY.C Revisited
Pointers and Strings
Passing Pointers to Functions
Passing Address Constants Versus Passing Pointer Variables
Arrays of Pointers
A Pause for Reflection
Chapter 9 Advanced Pointers
Pointers to Pointers
Equivalence of Array and Pointer Notation
Getting Command-Line Arguments
Null Pointers
Pointers to Structures
Pointers to Functions
Passing Function Pointers as Arguments
A Parting Word on Pointers
Chapter 10 Programming Pitfalls
Operator Problems
Confusing Assignment and Equality Operators
Confusing Operator Precedence
Confusing Structure-Member Operators
Array Problems
Array Indexing Errors
Omitting an Array Subscript in Multidimensional Arrays
Overrunning Array Boundaries
String Problems
Confusing Character Constants and Character Strings
Forgetting the Null Character That Terminates Strings
Forgetting to Allocate Memory for a String
Pointer Problems
Using the Wrong Address Operator to Initialize a Pointer
Declaring a Pointer with the Wrong Type
Using Dangling Pointers
Library-Function Problems
Failing to Check Return Values from Library Functions
Duplicating Library-Function Names
Forgetting to Include Header Files for Library Functions
Omitting the Address-Of Operator When Calling scanf
Macro Problems
Omitting Parentheses from Macro Arguments
Using Increment and Decrement Operators in Macro Arguments
Miscellaneous Problems
Mismatching if and else Statements
Misplacing Semicolons
Omitting Double Backslashes in DOS Path Specifications
Omitting break Statements from a switch Statement
Mixing Signed and Unsigned Values
PART II Using C
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Chapter 11 Input and Output
Input and Output Streams
Screen and Keyboard I/O
Manipulating and Printing Strings
Printing Numeric Values
Using scanf for Keyboard Input
Standard Disk I/O
Creating and Writing to a Text File
Reading a Text File in Binary Mode
Binary and Text Files
Text Format for Numeric Variables
Using Binary Format
Low-Level Input and Output
Low-Level Reading and Writing
Chapter 12 Dynamic Memory Allocation
Why Allocate?
Memory Allocation Basics
Preparing to Allocate Memory
Specifying the Size of the Allocated Block
A Graphic Illustration
Assigning the Address that malloc Returns
Checking the Return from malloc
Accessing an Allocated Memory Block
Allocating Memory for Different Data Types
Deallocating Memory with the free Function
Specialized Memory-Allocating Functions
The calloc Function
The realloc Function
Keeping Out of Trouble
Chapter 13 Graphics
Graphics Mode
Checking the Current Video Mode
Setting the Video Mode
Writing a Graphics Program
Using Color Graphics Modes
Using the Color Video Text Modes
Text Coordinates
Graphics Coordinates
The Physical Screen
Viewport Coordinates
Real Coordinates in a Window
Chapter 14 Presentation Graphics
Terminology
Presentation Graphics Program Structure
Five Example Chart Programs
Palettes
Color Pool
Style Pool
Pattern Pool
Character Pool
Customizing Presentation Graphics
Chart Environment
titletype
axistype
windowtype
legendtype
chartenv
An Overview of the Presentation Graphics Functions
Chapter 15 Fonts
QuickC Fonts
Using QuickC's Font Library
Register Fonts
Set Current Font
Display Text
An Example Program
A Few Hints
Chapter 16 In-Line Assembly
Advantages of In-Line Assembly
The _asm Keyword
Using Assembly Language in _asm Blocks
Instruction Set
Expressions
Data Directives and Operators
EVEN and ALIGN Directives
Macros
Segment References
Type and Variable Sizes
Using C in _asm Blocks
Using Operators
Using C Symbols
Accessing C Data
Writing Functions
Using and Preserving Registers
Jumping to Labels
Calling C Functions
Defining _asm Blocks as C Macros
Optimizing
References and Books on Assembly Language
Appendix A C Language Guide
General Syntax
User-Defined Names
Keywords
Functions
Flow Control
The break Statement
The continue Statement
The do Statement
The for Statement
The goto Statement
The if Statement
The return Statement
The switch Statement
The while Statement
Data Types
Basic Data Types
Aggregate Data Types
Advanced Data Types
Visibility
Lifetime
Type Conversions
User-Defined Types
Enumerated Types
Operators
Preprocessor Directives
Pointers
Appendix B C Library Guide
Overview of the C Run-Time Library
Buffer-Manipulation Routines
Character Classification and Conversion Routines
Data Conversion Routines
Error Message Routines
Graphics 1: Low-Level Graphics Routines
Graphics 2: Presentation Graphics Routines
Graphics 3: Font Display Routines
Input and Output Routines
Math Routines
Memory-Allocation Routines
Process-Control Routines
Searching and Sorting Routines
String-Manipulation Routines
Time Routines
Glossary
Introduction
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Ever since Microsoft introduced the QuickC(R) Compiler, version 1.0 in 1987,
QuickC users have asked for more information on the C programming language.
C for Yourself answers that need, particularly for those who have some
programming experience but are new to the C language.
About This Book
C for Yourself assumes you have programmed before but are not familiar with
C. Thus, it doesn't explain basic programming ideas such as why program
loops are useful. It assumes that you already know about loops in general
and now want to learn how to write them in the C language. The following
list summarizes the book's contents:
■ Part 1, "Learning C," covers basic C language topics such as functions
and data types. The chapters in this section are designed to be read
in order, from beginning to end.
■ Part 2, "Using C," covers practical programming topics such as
input/output and graphics. This section is organized topically, so you
don't have to read the chapters in any particular order.
■ Appendix A, "C Language Guide," summarizes the QuickC implementation
of the C language. You can use this appendix as a quick reference once
you have read Part 1 and have some familiarity with C.
■ Appendix B, "C Library Guide," summarizes the most commonly used
functions in the QuickC run-time library. This appendix is designed
mainly for browsing when you're not using QuickC. When you are in the
QuickC environment, use the Microsoft(R) QuickC Advisor (online help)
to get information about C language features and run-time library
functions.
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NOTE
The pages that follow use the term "OS/2" to refer to the OS/2 systems─
Microsoft (R) Operating System/2 (MS (R) OS/2) and IBM (R) OS/2. Similarly,
the term "DOS" refers to both the MS-DOS (R) and IBM (R) Personal Computer
DOS operating systems. The name of a specific operating system is used when
it is necessary to note features that are unique to that system.
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Using the Example Programs
The example programs in this book are available through online help. This
feature allows you to load, run, and experiment with example programs as you
read.
You can use online help to load and run example programs.
You must be using the QuickC environment to load an example. To load the
program, select Contents from the Help menu, then select the title of this
book. Find the desired program in Help, then copy it into the editor using
QuickC's Copy and Paste functions.
After you copy a sample program into the QuickC Editor, you can treat it as
you would any C source program. You can compile or edit the program, save it
on disk, and so on.
QuickC online help includes all the significant examples in this book (it
doesn't include code fragments and some very short programs). Every program
that is in online help begins with a line in this general form:
/* POINTER.C: Demonstrate pointer basics. */
The line contains the program's name and a brief description of what it
does. The program containing the above line is listed as POINTER.C in online
help.
All the examples available in online help compile without errors at Warning
Level 3, in which QuickC does the most stringent error-checking. At this
Warning Level, some examples will generate the following harmless warnings:
C4103: 'main' : function definition used as prototype
C4035: 'main' : no return value
C4051: data conversion
You can eliminate these warnings by compiling at a lower Warning Level.
Programming Style Used in This Manual
The C language allows considerable flexibility in formatting source code.
The style used in this book is recommended for program readability, but you
do not have to use it when writing your programs. Below is a list of style
guidelines used in this book for example programs:
■ Each example program begins with a comment that names the program and
states what it does.
■ Each statement or function is listed on its own line.
■ Variable and function names are in lowercase. The names of symbolic
constants, such as TRUE or FALSE, are in uppercase.
■ If a function doesn't take any arguments, an opening and a closing
parenthesis follow the function name with no extra space:
getch();
■ If a function takes arguments, a space appears after the opening
parenthesis and before the closing parenthesis:
printf( "Number = %i", num_penguins );
■ Binary operators such as addition and subtraction are preceded and
followed by a space:
3 + 5
■ If parentheses are used to control operator precedence, no extra
spaces are included:
(3 + 5) * 2
■ Opening and closing braces are aligned under the first character of
the controlling keyword. The block underneath is indented 3 spaces:
if( a == b )
{
c = 50;
printf( "%i\n", a );
}
Key to Document Conventions
This book uses the following document conventions:
Example Description
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COPY TEST.OBJ C: Uppercase letters represent DOS commands
and file names.
printf Boldface letters indicate standard
features of the C language: keywords,
operators, and standard library
functions.
expression Words in italics indicate placeholders
for information you must supply, such as
a file name. Italics are also
occasionally used for emphasis in the
text.
main() This typeface is used for example
{ programs, program fragments, and the
} names of user-defined functions and
variables. It also indicates user input
and screen output.
CL options «files...» A horizontal ellipsis following an item
indicates that more items having the
same form may follow.
while( ) A vertical ellipsis tells you that part
{ of the example program has been
. intentionally omitted.
.
.
}
SHIFT Small capital letters denote names of
keys on the keyboard. A plus sign ( + )
indicates a combination of keys. For
example, SHIFT+F5 tells you to hold down
the SHIFT key while pressing the F5 key.
"array pointer" The first time a new term is defined, it
is enclosed in quotation marks. Since
some knowledge of programming is assumed,
common terms such as memory or branch
are not defined.
American National Standards The first time an acronym appears, it is
Institute (ANSI) spelled out.
Other Books on C Programming
This book provides an introduction to the C language and some practical
programming topics. It does not attempt to teach you basic computer
programming or advanced C programming techniques. The following books cover
a variety of topics that you may find useful. They are listed only for your
convenience. With the exception of its own publications, Microsoft does not
endorse these books or recommend them over others on the same subject.
Feibel, Werner. Advanced QuickC,
2d ed. Berkeley, California: Osborne McGraw-Hill, 1989.
An intermediate-level C programming guide using QuickC. It includes data
structures, parsing, simulations, and the DOS interface.
Hancock, Les, and Morris Krieger. The C Primer,
2d ed. New York: McGraw-Hill, 1986.
A beginner's guide to C programming.
Hansen, Augie. Proficient C.
Redmond, Washington: Microsoft Press, 1987.
An intermediate-level guide to C programming.
Harbison, Samuel P., and Guy L. Steele. C: A Reference Manual,
2d ed. Englewood Cliffs, New Jersey: Prentice-Hall, 1987.
A comprehensive guide to the C language and the standard library.
Hergert, Douglas. The ABC's of QuickC.
Alameda, California: SYBEX, Inc., 1989.
A beginner's guide to QuickC programming.
Kernighan, Brian W., and Dennis M. Ritchie. The C Programming Language,
2d ed. Englewood Cliffs, New Jersey: Prentice Hall, 1988.
The first edition of this book is the classic definition of the C
language. The second edition includes new information on the proposed
ANSI C standard.
Lafore, Robert. Microsoft C Programming for the IBM.
Indianapolis, Indiana: Howard W. Sams & Company, 1987.
The first half of the book teaches C. The second half concentrates on
specifics of the PC environment, such as BIOS calls, memory, and video
displays.
Plum, Thomas. Learning to Program in C.
Hasbrouck Heights, New Jersey: Hayden Book Company, 1983.
A widely used introductory college text on computer programming in C.
Schustack, Steve. Variations in C.
Redmond, Washington: Microsoft Press, 1985.
An intermediate-level guide to developing business applications in C.
Waite, Mitchell, Stephen Prate, Bryan Costales, and Harry Henderson (The
Waite Group). Microsoft QuickC Programming.
Redmond, Washington: Microsoft Press, 1988.
Beginning- to intermediate-level C programming, with special emphasis on
the QuickC Compiler.
Ward, Robert. Debugging C.
Indianapolis, Indiana: Que Corporation, 1986.
An advanced guide to the theory and practice of debugging C programs.
Wilton, Richard. Programmer's Guide to PC and PS/2 Video Systems.
Redmond, Washington: Microsoft Press, 1987.
An advanced guide to all the PC and PS/2(R) video modes.
PART I Learning C
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The C language has steadily increased in popularity since it was created in
the early 1970s. C is currently the language of choice for many professional
software developers and is becoming quite popular among nonprofessional
programmers, as well.
C for Yourself is divided into two parts. Part 1 is called "Learning C" and
discusses the C language itself. This part assumes you know the fundamentals
of computer programming but do not know C. Experienced C programmers may
only want to skim these chapters. Part 2, "Using C," discusses practical
programming capabilities, such as graphics, which are provided in the QuickC
run-time library. It should be read after you have finished Part 1 and have
some familiarity with the C language.
Part 1 begins with basic topics such as data types and functions, and it
progresses to more advanced subjects such as pointers. This part of the book
closes with a discussion of common C programming pitfalls.
Chapter 1 Anatomy of a C Program
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As a knowledgeable programmer, you'll probably be tempted to immerse
yourself in C immediately. But before taking that plunge, you should know
the basic model for all C programs. This chapter sketches the anatomy of a C
program without getting bogged down in formal definitions or exceptions to
the rules. (You'll find plenty of rules in the chapters that follow.)
The discussion revolves around a short, reasonably typical C program named
VOLUME.C. To get comfortable with the look of C programs, as well as the
basic ideas that shape them, refer to VOLUME.C frequently as you read.
A Typical C Program
VOLUME.C is a simple program that calculates the volume of a sphere and
prints the following result on the screen:
Volume: 113.040001
Like all of the sample programs in this book, you'll find VOLUME.C in
QuickC's online help. The "Introduction" explains how to load sample
programs. Figure 1.1 illustrates the VOLUME.C program.
(This figure may be found in the printed book.)
Comments
The first line in VOLUME.C is a comment:
/* VOLUME.C: Calculate sphere's volume. */
Comments make a program more readable.
In C, a comment begins with a slash-asterisk (/*) and ends with an
asterisk-slash (*/ ). Because C is a compact language with very few
keywords, comments play an important role in making your programs readable.
You can't nest comments (put one comment inside another). The following line
creates a syntax error:
/* Error! /* You can't */ nest comments in C. */
Statements
C statements always end with semicolons. Here is a statement from the
VOLUME.C program:
result = 4 * PI * result;
Statement blocks are enclosed in braces.
You can also enclose a group of statements in braces, making a "statement
block." Statement blocks contain related statements, such as the statements
in the body of a function.
The C language ignores white space (tabs, blanks, and line breaks) in your
source program, so you can arrange your program in almost any style.
However, a few de facto rules help promote readability. A typical C program
is written with one statement per line. Braces align vertically, and
statements inside braces are indented. The "Introduction" describes the
programming style used in this book.
Keywords and Names
C is a case-sensitive language (it distinguishes between uppercase and
lowercase letters). All of C's keywords are spelled completely in lowercase;
online help contains a complete list of C keywords.
You can declare names in any combination of either case, but many
programmers prefer to use lowercase for variable and function names, saving
uppercase for declaring symbolic constants. (A "symbolic constant" is a
descriptive name that represents a constant value. In VOLUME.C, PI is a
symbolic constant.)
Preprocessor Directives
A preprocessor directive is a command to the QuickC compiler.
Not every line in a C program is an executable statement. Programs can also
contain "preprocessor directives"─ commands for the QuickC compiler. A
directive begins with a number sign (#) and does not end with a semicolon.
The second and third lines of VOLUME.C contain preprocessor directives. The
#include directive in the second line tells QuickC to insert the file
STDIO.H when it compiles VOLUME.C:
#include <stdio.h>
STDIO.H is one of the many "header files" supplied with QuickC. Header files
contain declarations and definitions required by C library functions.
("Library functions" are supplied with QuickC rather than written by you.)
In the VOLUME.C program, the printf library function requires information
from the STDIO.H header file.
The #define directive in the third line defines a symbolic constant named
PI:
#define PI 3.14
Wherever PI appears later in the source program, QuickC substitutes the
text 3.14. The text can be any combination of letters, digits, or other
characters. The effect is much like a search and replace operation in a word
processor.
Functions
A function performs a specific task and can also return a value.
Functions are the building blocks of C. Every C program has at least one
function, and every executable C statement appears inside some function or
another. In plain English, a "function" is a group of statements that
performs a specific task and often returns a value to the statement that
calls it.
C functions serve the same purposes as QuickPascal procedures and functions
or BASIC SUB and FUNCTION procedures. They allow you to write wellorganized
programs that perform different tasks in separate parts.
The C language has no input/output statements.
C also uses functions to perform all input and output (I/O). Unlike other
high-level languages, C has no I/O statements such as PRINT or READ.
Instead, all I/O is done by calling C library functions such as printf.
Every C program has a function named main.
The VOLUME.C program contains two functions, named main and sphere (see
Figure 1.1). The main execution section of every C program is itself a
function named main, which marks where execution starts and ends. When you
run VOLUME.C, execution starts at the beginning of the main function and
stops at the end of main.
Calling Functions
Functions can be called (executed) from anywhere in a program, and they can
receive values as well as return them. A value that you pass (send) to a
function is called an "argument."
Calling a C function is a simple matter. You state the name of the function
and supply in parentheses any arguments you want to pass to it. You must
place a comma between arguments.
The VOLUME.C program contains two function calls, one to the printf library
function and the other to the sphere function, which is defined in the
program. The following statement calls the printf function:
printf( "Volume: %f\n", volume );
The statement passes two arguments to printf. The first, "Volume: %f\n",
supplies text and some formatting information. The second, volume, supplies
a numeric value. See "A Few Words about printf," below, for more
information.
In C, a function does not necessarily have to return a value. It can either
return a value (like a QuickPascal function) or return nothing (like a
QuickPascal procedure).
When a function returns a value, the value is often assigned to a variable.
The following statement from VOLUME.C calls the sphere function and
assigns its return value to the variable volume:
volume = sphere( radius );
A function uses the return keyword to return a value. In VOLUME.C, the last
statement in the sphere function returns the value of the variable result
to the statement that calls sphere:
return result;
Declaring and Initializing Variables
You must "declare" every variable in a C program by stating its name and
type. If you refer to an undeclared variable, QuickC displays an error
message when you compile the program.
The following statement from VOLUME.C declares a float (floating-point) type
variable named volume:
float volume;
After declaring a variable, you should "initialize" it─give it an initial
value─before using it. Uninitialized variables might have any value, so they
are dangerous to use. The VOLUME.C program initializes the variable volume
by as- signing it the return value from a function call:
volume = sphere( radius );
You can also initialize a variable when it is declared, a convenient and
common practice. The following statement from VOLUME.C declares the variable
radius as an int (integer) variable and initializes it with the value 3:
int radius = 3;
External and Local Variables
The place where you declare a variable controls where it is visible. A
variable declared outside any function is "external": you can refer to it
anywhere within the program. (External variables are called "global" in some
other languages.)
A variable declared inside the braces of a function is "local." You can
refer to it inside the function but nowhere else. In VOLUME.C, the result
variable is declared inside the sphere function:
float sphere( int rad )
{
float result;
.
.
.
}
Use external variables only when necessary.
Because it is local to the sphere function, the result variable cannot
be used elsewhere in VOLUME.C. Making variables local whenever possible
minimizes the risk that a variable's value will be changed accidentally in
some other part of the program.
When a function receives arguments, the arguments become local variables
within the function. The sphere function requires one argument, which it
names rad. Within the function, rad is a local variable.
Function Prototypes
Function prototypes allow QuickC to check every function reference for
accuracy.
A function can be declared in much the same way as a variable. Function
declarations, often called "prototypes," allow QuickC to do "type checking."
Given the information in the prototype, QuickC can check every subsequent
use of the function to make sure you pass the right number and type of
arguments and use the correct return type.
A function prototype gives the following information:
■ The function's name
■ The type of value the function returns
■ A list of arguments the function requires
The VOLUME.C program contains one function prototype, for the sphere
function:
float sphere( int rad );
The prototype states that the sphere function returns a float
(floating-point) value and requires one int (integer) argument.
A Few Words about printf
The VOLUME.C program, like most examples in this book, uses the printf
library function to display text. You won't need to know all of the details
of printf to read the rest of this book, but the examples will be easier to
follow if you know a few basic concepts.
The printf function works like the QuickBASIC PRINT USING statement or the
QuickPascal Writeln procedure. It can display string and numeric data in
various formats, and it normally prints to the screen.
You can print a simple message by passing printf a string (characters in
double quotes) as an argument:
printf( "Hi, Mom!" );
The statement prints
Hi, Mom!
The printf function doesn't automatically add a newline character at the end
of a line. The statements
printf( "Le Nozze di Figaro" );
printf( " by W. A. Mozart" );
print the following message on one line:
Le Nozze di Figaro by W. A. Mozart
To start a new line, use the escape sequence \n as follows:
printf( "Hi,\nMom!" );
The statement prints two words on separate lines:
Hi,
Mom!
The f in printf stands for formatting. To print the values of variables and
other items, you supply printf with format codes that tell printf the proper
format for each item. The codes are placed in the first argument, which is
enclosed in double quotes.
The following statement uses the %x code to print the integer 553 in
hexadecimal format. It passes two arguments to printf:
printf( "%x", 553 );
The first argument ("%x") contains the format code and the second argument
(553) contains the item to be formatted. The line displays the following:
229
The printf function accepts several other format codes. For instance, the
VOLUME.C program uses %f to print a floating-point number. Some programs in
later chapters use %d to print integers or %ld to print long integers.
The first argument passed to printf can contain any combination of
characters and format codes. The other arguments contain the items that you
want printf to format. The statement
printf( "%d times %d = %d\n", 2, 128, 2 * 128 );
prints the line:
2 times 128 = 256
The printf function matches the format codes to the items you want to
format, in left-to-right order. In the code above, the first %d formats the
number 2, the second formats the 128, and the third formats the expression 2
* 128 (which evaluates to the number 256).
There's much more to say about printf and other I/O functions, but the rest
can wait until you reach Chapter 11, "Input and Output," which describes I/O
in detail.
Now that you've glimpsed the big picture, we can take a closer look at some
specifics of C programming, beginning with Chapter 2, "Functions."
Chapter 2 Functions
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Chapter 1, "Anatomy of a C Program," introduced functions, the building
blocks of C programs. In this chapter, you'll learn how to use functions in
C programs.
We begin by discussing some function basics, including the role of the main
function. We then show you how to call functions, pass data to them, return
data from them, and declare them. The chapter concludes with a brief look at
old-style function declarations, which you may encounter in some programs.
Functions and Structured Programming
As we mentioned in Chapter 1, a C function is a collection of statements,
enclosed in braces ({ }), which performs a particular task. It can receive
arguments (data) and also return a value.
Functions allow you to program with a "divide and conquer" strategy. Rather
than try to solve a large problem all at once, you break the problem into
several parts and attack each one separately. This approach, known as
"structured programming," allows you to write clear, reliable programs that
perform separate tasks in discrete, logically contained modules. In the C
language, these modules are called functions.
Functions offer several advantages. They can
■ Make programs easier to write and read. All of the statements related
to a task are located in one place.
■ Prevent unexpected side effects by using private (local) variables
that are not visible to other parts of the program.
■ Eliminate unnecessary repetition of code for frequently performed
tasks.
■ Simplify debugging. Once the function works reliably, you can use it
with confidence in many different situations.
If you know QuickPascal or QuickBASIC, you will see many similarities in the
C language. A C function serves the same basic purpose as a QuickPascal
function or procedure or a QuickBASIC FUNCTION or SUB procedure. In later
sections, we'll note some differences between C and these languages.
The main Function
Every C program must have one and only one main function.
Every C program must have a function named main, which tells where program
execution begins and ends. Although main is not a C keyword, it has only one
use: naming the main function. A program must have only one main function,
and you shouldn't use the name anywhere else.
Below is the simplest possible C program:
main()
{
}
The braces ({ }) mark the main function's beginning and end, as they do in
every function. This program doesn't contain any executable statements; it
simply begins and ends.
Most functions have executable statements, of course, and these appear
within the function's braces. The following program contains a statement
which prints Hello, world! on the screen:
main()
{
printf( "Hello, world!\n" );
}
The main function is called by the operating system when it runs your
program. While it's possible to call the main function in a program, you
should never do so, just as you wouldn't write a QuickBASIC program
containing the line
10 GOSUB 10
A program that calls main will start again and again in an endless loop that
eventually triggers a run-time error.
Like all functions, main can accept arguments and return a value. Through
this mechanism, your program can receive command-line arguments from DOS
when it begins execution and return a value to DOS when it ends. Chapter 9,
"Advanced Pointers," describes how to receive command-line arguments via
main.
Placement and Visibility of Functions
A function is normally visible everywhere in the program.
Every C function is normally "visible" to all other functions in the same
program. That is, it can call and be called by any other function. C
functions can even call themselves, a process known as "recursion."
In the program below, the functions whiz and bang are visible to main
and to each other. The main function can call both whiz and bang. In
addition, whiz can call bang, and vice versa.
main()
{
}
whiz()
{
}
bang()
{
}
Functions can appear in any order and at almost any place in your program.
Since main starts and ends the program's execution, this function often
begins the program. But this is a readability convention, not a language
requirement.
C functions can't be nested.
One place where you can't put a function is inside another function. The C
language doesn't allow you to nest functions. Here C differs from
QuickPascal, in which one procedure can contain other "hidden" functions or
procedures. The following program causes a syntax error because the bang
function appears within the whiz function:
main()
{
}
whiz()
{
/* Error! Incorrect function placement */
bang()
{
}
}
Function Definitions and Prototypes
Now that you understand some function basics, we can look at functions in
more detail. A function, or more precisely, "function definition," contains
several parts. Figure 2.1 shows the parts of the sphere function
definition from the VOLUME.C example in Chapter 1, "Anatomy of a C Program."
(This figure may be found in the printed book.)
The function "header" specifies the type of value a function returns and the
function's name. The header also contains an argument list, which specifies
the arguments the function requires. The rest of the function
definition─everything inside the braces─is the function "body."
The ANSI C standard, which QuickC follows, recommends that you supply a
function "prototype" (declaration) for every function definition in your
program. The prototype is identical to the function header except that it
ends with a semicolon. Figure 2.2 shows the sphere function prototype from
VOLUME.C.
(This figure may be found in the printed book.)
The function prototype normally appears near the beginning of the program
and serves a purpose similar to a variable declaration. It provides advance
information about the function, which QuickC can use to check the accuracy
of subsequent calls to the function. We'll examine prototypes in detail in
"Function Prototypes," below.
Calling a Function
You call (execute) a function by stating its name. In the simplest case─when
a function doesn't receive or return any data─the function call consists of
the function's name, followed by an empty pair of parentheses and a
semicolon. The BEEPER.C program, shown below, demonstrates this kind of
function call.
/* BEEPER.C: Demonstrate simple function. */
#include <stdio.h>
void beep( void );
main()
{
printf( "Time to beep\n" );
beep();
printf( "All done\n" );
}
void beep( void )
{
printf( "Beep!\a\n" );
}
When you run BEEPER.C, the program prints:
Time to beep
Beep!
All done
As you may recall from Chapter 1, "Anatomy of a C Program," the \n sequence
represents the newline character. The \a sequence is the "alert" character
(ASCII 7) which makes an audible beep.
In the main function of BEEPER.C, the statement
beep();
calls the beep function. Since beep takes no arguments, the parentheses
of the function call are empty.
The prototype and definition for the beep function use the void keyword
twice, first to indicate that the function returns no value, and second to
indicate that it receives no arguments. We'll return to these points later
in this chapter.
A function call transfers control to that function. The statements within
the function's braces execute in order until the function ends. Then
execution resumes where it left off.
A function can end in one of two ways. The beep function above ends by
"falling off" the closing brace of the function definition. A function can
also end by executing a return statement, which we discuss later in the
section "Returning Values from Functions."
Figure 2.3 illustrates the flow of control in BEEPER.C.
(This figure may be found in the printed book.)
Passing Arguments to a Function
If a function requires arguments, you list them in the parentheses of the
function call. In the BEEPER1.C program below, we revise the beep function
from BEEPER.C to take one argument.
/* BEEPER1.C: Demonstrate passing arguments. */
#include <stdio.h>
void beep( int num_beep );
main()
{
printf( "Time to beep\n" );
beep( 5 );
printf( "All done\n" );
}
void beep( int num_beep )
{
while( num_beep > 0 )
{
printf( "Beep!\a\n" );
num_beep = num_beep - 1;
}
}
The function definition states what kind of arguments the function expects.
In the beep function definition, the header,
void beep( int num_beep )
states that beep expects one int (integer) argument named num_beep
(number of beeps).
The statement that calls beep,
beep( 5 );
gives the value 5 in parentheses, passing that value as an argument. Figure
2.4 shows argument passing in BEEPER1.C.
(This figure may be found in the printed book.)
Function arguments are assigned to local variables inside the function.
When beep receives the value 5, the function automatically assigns the
value to num_beep, which the function can then treat as a local variable.
In this case, the function uses num_beep as a loop counter to repeat the
statement
printf( "Beep!\a\n" );
num_beep times. (The C while loop is very similar to WHILE loops in
QuickBASIC or QuickPascal. You don't need to know the details of loops for
now; they're explained in Chapter 3, "Flow Control.")
If a function expects more than one argument, you separate the arguments
with commas. For instance, the statement
printf( "%d times %d equals %d\n", 2, 16, 2 * 16 );
passes four arguments to the printf function. The first argument is the
string
"%d times %d equals %d\n"
The second and third arguments are constants (2 and 16). The fourth
argument is an expression (2 * 16) that evaluates to a constant.
Arguments Versus Parameters
In the C language, a value passed to a function is called either an
"argument" or a "parameter," depending on viewpoint. From the viewpoint of
the statement that calls the function, the value is an argument. In the view
of the function receiving it, the value is a parameter.
Thus, in BEEPER1.C, the following function call passes an argument to the
beep function:
beep( 5 );
Looking at the same value from the receiving end, the header of the beep
function declares a parameter named num_beep as follows:
void beep( int num_beep );
The argument and parameter refer to the same value─in this case, the value
5. The naming distinction is just a matter of viewpoint, similar to the way
you call a letter outgoing mail if you're sending it, or incoming mail if
you're receiving it.
Assigning Parameters
When you list a parameter in the function header, it becomes a local
variable within the function. This process is easy to follow when it
involves only one argument, as in the BEEPER1.C program above. The function
call passes one value, which the function assigns to one variable. The
variable can be treated like any other variable declared within the
function.
There is a one-to-one correspondence between arguments and parameters.
If a function takes more than one argument, the values are passed in order.
The first argument in the function call is assigned to the first variable,
the second argument is assigned to the second variable, and so on.
The SHOWME.C program below demonstrates this process. Its showme function
takes three arguments. The main function defines three integer variables and
passes their values to showme, which prints the values that it receives.
(You normally wouldn't write a function just to print one line, of course.
We'll add more to SHOWME.C in a later revision.)
/* SHOWME.C: Demonstrate passing by value. */
#include <stdio.h>
void showme( int a, int b, int c );
main()
{
int x = 10, y = 20, z = 30;
showme( z, y, x );
}
void showme( int a, int b, int c )
{
printf( "a=%d b=%d c=%d", a, b, c );
}
Here's the output from SHOWME.C:
a=30 b=20 c=10
The function call in SHOWME.C passes the values of z, y, and x in the
order listed:
showme( z, y, x );
Functions receive parameters in the order they are passed.
These values are assigned, in the same order, to the parameters listed in
the showme function header:
void showme( int a, int b, int c )
The position of the parameters, not their names, controls which arguments
the parameters receive. The first argument ( z ) listed in the function call
is assigned to the first parameter ( a ) in the function header, the second
argument ( y ) to the second parameter ( b ), and so on. Figure 2.5 shows
this process.
(This figure may be found in the printed book.)
Passing by Value
The C language passes copies of function arguments.
In C, all function arguments (except arrays) are passed "by value" rather
than "by reference." That is, a function receives a local copy of each
argument, not the argument itself. These copies are local variables within
the function. They are created and initialized automatically when the
function begins, and they disappear when it ends. Like all local variables,
their values can be changed without affecting variables elsewhere in the
program.
We can clarify this point by adding a few statements to the SHOWME.C
program. The new program, SHOWMORE.C, will change the values of the local
variables in the showme function without changing the values of the
original variables.
/* SHOWMORE.C: Demonstrate passing by value. */
#include <stdio.h>
void showme( int any, int old, int name );
main()
{
int x = 10, y = 20, z = 30;
showme( z, y, x );
printf( " z=%d y=%d x=%d\n", z, y, x );
}
void showme( int any, int old, int name )
{
printf( "any=%d old=%d name=%d\n", any, old, name );
any = 55;
old = 66;
name = 77;
printf( "any=%d old=%d name=%d\n", any, old, name );
}
Here is the output from SHOWMORE.C:
any=30 old=20 name=10
any=55 old=66 name=77
z=30 y=20 x=10
First, note that the showme function in SHOWMORE.C uses new names ( any,
old, and name) when assigning the parameters it receives:
void showme( int any, int old, int name )
Function parameters can have any legal variable names.
Because these variables are local to the function, they can have any legal
names. (The rules for variable names are described in Chapter 4, "Data
Types.") The showme function prints the values of its parameters
immediately after assigning them:
printf( "any=%d old=%d name=%d", any, old, name );
Then the function assigns new values to the variables and prints them again:
any = 55;
old = 66;
name = 77;
printf( "any=%d old=%d name=%d", any, old, name );
Local variables are private to the function containing them.
Changing the local variables in the showme function doesn't affect the
original variables in the main function. Remember, a variable defined inside
a function is only visible inside that function. After control returns to
main, SHOWMORE.C prints the values of the original variables:
printf( " z=%d y=%d x=%d\n", z, y, x );
As the program output shows, the original values are unchanged:
z=30 y=20 x=10
We'll say more about the visibility of variables in Chapter 5, "Advanced
Data Types." For now, just remember that when you pass a value to a
function, the function makes a local copy of that value. The local copy can
be manipulated without changing the original.
────────────────────────────────────────────────────────────────────────────
NOTE
In QuickPascal, you can pass either the value of an argument or the
argument's address. In C, function arguments are only passed by value.
However, that value can be an address. Chapter 8, "Pointers," explains how
to pass addresses to functions.
────────────────────────────────────────────────────────────────────────────
Returning Values from Functions
The return keyword ends a function and can return one value.
Most C functions return a value. This is done with the return statement,
which also ends the function. The VOLUME.C program from Chapter 1, "Anatomy
of a C Program," (see Figure 2.1) contains such a statement. In that
program, the sphere function returns the value of the variable result as
follows:
return result;
The following statement in the main function of VOLUME.C calls the sphere
function and assigns its return value to the variable volume:
volume = sphere( radius );
Figure 2.6 shows the flow of control as the sphere function returns a
value in VOLUME.C.
(This figure may be found in the printed book.)
A return statement can only return a single value. If a function needs to
return multiple values, the normal method is to use pointers─a major topic
that we'll discuss in Chapter 8, "Pointers."
────────────────────────────────────────────────────────────────────────────
NOTE
In QuickPascal, a function returns a value and a procedure does not. The
same distinction applies to QuickBASIC FUNCTION and SUB procedures,
respectively. In the C language, a function can do both. It can return a
value or return nothing.
────────────────────────────────────────────────────────────────────────────
A function can contain more than one return statement, as shown below:
if( error == 0 )
return 0;
else
return 1;
The code returns a different value in different cases. It returns the value
0 when the variable error is 0 and the value 1 when error is nonzero.
(In C, the if and else statements work much like those in other languages.
Chapter 3, "Flow Control," explains these statements.)
A return statement can appear anywhere and need not return a value.
You can place the return keyword anywhere within a function, and the
statement need not necessarily return a value. In the following fragment,
the naked return statement simply ends the function if the value of count
exceeds 500:
if( count > 500 )
return;
else
/* execute more statements... */
A return statement ends the function immediately, no matter where it
appears. In the function shown below, the statements following the return
never execute:
void do_nothing( void )
{
return;
/* The following statements do not execute */
printf( "This function " );
printf( "prints nothing.\n" );
}
If a function doesn't return a value, and you want the function to end by
falling off its closing brace, no return statement is needed. This method is
used to end the beep function in BEEPER.C, discussed earlier in this
chapter:
void beep( void )
{
printf( "Beep!\a\n" );
}
You could add a return to the end of this function, but it's not necessary.
The function ends automatically.
Using Return Values
Function return values are often assigned to variables.
A function's return value can be used in the same way you would use any
value of its type. In the VOLUME.C program from Chapter 1, "Anatomy of a C
Program," the statement that calls sphere assigns the function's return
value to the variable volume:
volume = sphere( radius );
If there's no need to save the return value, you can use it directly. You
may have noticed that the variable volume isn't really needed in the
VOLUME.C program, which simply prints the variable's value and ends. Most
programmers would make the program more compact by replacing the two
statements
volume = sphere( radius );
printf( "Volume: %f\n", volume );
with this one:
printf( "Volume: %f\n", sphere( radius ) );
The second version puts the sphere function call right in the printf
statement, eliminating the superfluous variable. Instead of assigning the
return value to a variable and passing that variable's value to printf, the
statement uses the value directly. (The sphere function is called first.
Then the return value from sphere is passed as an argument to the printf
function.)
While this change streamlines the program, it also makes the code a little
harder to follow. If you don't read carefully, you might overlook the fact
that the printf function call contains another function call.
Unused return values are discarded.
Occasionally, you may have no use for a function's return value. The printf
function, for example, returns the number of characters it displayed, but
few programs need this information. If you don't use a return value, it's
discarded.
You should never ignore the error codes that library functions return to
show whether the function succeeded. See Chapter 10, "Programming Pitfalls,"
for more information about library function return values.
Declaring a Function's Return Type
Thus far, we have explained how a function can return a value─and how the
calling statement can use that value─without paying much attention to what
type of value the function returns. (The C language supports various data
types, such as int for integer values, and float for floating-point values.
Chapter 4 describes data types in detail.)
The return type is important because it controls what the function returns.
If a function returns an integer when you expect a floating-point value,
your program may not work correctly.
A function's prototype and definition control what type of value it
returns.
The function's return type is specified in its prototype and definition.
Below are the prototype and definition of the sphere function from the
VOLUME.C program in Chapter 1, "Anatomy of a C Program." They specify that
the function returns a float value.
float sphere( int rad ); /* function prototype */
float sphere( int rad ) /* function header */
The type name (here, float) in front of the function name shows what type of
value the function returns. If the sphere function returned an int value,
its prototype and header would look like this:
int sphere( int rad ); /* function prototype */
int sphere( int rad ) /* function header */
Use the void type name to show a function returns no value.
You should declare the return type for every function─even for functions
that don't return a value. These functions are declared with the void type
name. In the SHOWME.C program, shown above, the prototype of the showme
function follows this pattern:
void showme( int a, int b, int c );
The void that precedes the function name indicates that showme returns
nothing.
Function Prototypes
Function prototyping is the major innovation of the ANSI standard for C. As
we mentioned earlier, a function prototype gives the same information as the
function's header: the name of the function, the type of value the function
returns, and the number and type of parameters the function requires.
Function prototypes allow QuickC to check function references for
accuracy.
Function prototypes normally appear near the start of the program, before
the first function definition. Given the information in the prototype,
QuickC can perform "type checking." It checks each reference to the
function─its definition, as well as every function call─to make sure that
the reference uses the right number and type of arguments and the correct
return value. Without type checking, it's easy to create bugs by passing the
wrong type of value to a function or assuming the wrong return type.
C programs normally include one prototype for each function they define,
except the main function. Most programmers don't bother to prototype main
unless the program receives command-line arguments or returns a value to DOS
when it ends. (Command-line arguments are discussed in Chapter 9, "Advanced
Pointers.")
Here is the function prototype for the sphere function in VOLUME.C:
float sphere( int rad );
You can see that sphere expects a single int-type parameter and returns a
value of type float. On the other hand, the prototype for showme in
SHOWME.C indicates that showme expects three int-type parameters and
returns nothing:
void showme( int a, int b, int c );
It's common to use the same parameter names in both the function prototype
and the function header. In SHOWME.C, for instance, the showme function
prototype,
void showme( int a, int b, int c );
uses the names a, b, and c, as does the header for that function,
void showme( int a, int b, int c )
Using the same names in both parameter lists makes the program more
readable, but it's not a language requirement. The names in the prototype's
parameter list are merely cosmetic. You can use any names you choose, or
even omit the names completely. The prototype in SHOWME.C works just as well
when written
void showme( int, int, int );
as when you supply the names a, b, and c. Both versions fully specify the
number (three) and type (int) of the parameters the function expects.
Prototyping Functions without Parameters
If a function doesn't expect any parameters, you might be tempted to leave
its parameter list blank. But it's better to put void in its parameter list,
as shown here:
void beep( void );
The void in parentheses specifies that the beep function requires no
parameters. If you leave the parentheses empty, the compiler draws no
conclusion about what parameters the function takes and won't be able to
detect an error if you mistakenly pass an argument to the function.
Prototyping Functions with Variable Parameters
Some functions, such as the printf library function, can handle a variable
number of parameters. This capability can make functions more flexible. As
earlier examples have shown, the printf function can take one parameter or
several, depending on how many values you need to print.
To declare a function with a variable number of parameters, end the
parameter list with a comma and an ellipsis (, . . .). The following
prototype, for example, declares that the sample function expects at least
one int-type parameter and zero or more additional parameters:
void sample( int a,... );
The ellipsis stands for an unspecified number of parameters with types that
are also unspecified. The parameter list in the function header should
follow the same pattern.
Don't declare a variable number of parameters unless it's necessary. Giving
this sort of prototype for a function that takes a fixed number of
parameters defeats the prototype's main purpose. QuickC can't perform type
checking for parameters you leave out of a prototype.
Old-Style Function Declarations and Definitions
This book explains how to declare and define functions under the ANSI
standard for C, which is now the norm. The original C language used slightly
different rules for function declarations and definitions. QuickC can
compile these "oldstyle" programs, but the ANSI standard recommends you use
the full function prototypes we just described.
Still, you may encounter old-style function declarations and definitions in
many existing C programs. So, you should be familiar with the style.
We'll use the VOLUME.C program from Chapter 1, "Anatomy of a C Program," to
demonstrate the old style. First, here's the ANSI-style program presented in
Chapter 1:
/* VOLUME.C: Calculate sphere's volume. */
#include <stdio.h>
#define PI 3.14
float sphere( int rad );
main()
{
float volume;
int radius = 3;
volume = sphere( radius );
printf( "Volume: %f\n", volume );
}
float sphere( int rad )
{
float result;
result = rad * rad * rad;
result = 4 * PI * result;
result = result / 3;
return result;
}
The same program written in the old style would look something like this:
/* OLDSTYLE.C: Old-style function. */
#include <stdio.h>
#define PI 3.14
float sphere();
main()
{
float volume;
int radius = 3;
volume = sphere( radius );
printf( "Volume: %f\n", volume );
}
float sphere( rad )
int rad;
{
float result;
result = rad * rad * rad;
result = 4 * PI * result;
result = result / 3;
return result;
}
You'll notice two distinct differences. First, the old style doesn't allow a
parameter list in the function declaration. In the ANSI version, VOLUME.C,
the declaration of the sphere function specifies that the function takes a
single int parameter:
float sphere( int rad );
The corresponding declaration in OLDSTYLE.C omits the parameter list:
float sphere();
An old-style function declaration cannot provide any information about the
function's parameters.
The other change is in the way the function definition lists parameters. In
VOLUME.C, the function header lists the same information as the function
prototype, giving the type (int) and name ( rad ) of the function's
parameter:
float sphere( int rad )
{
.
.
.
}
In OLDSTYLE.C, the function header gives the parameter's name ( rad ), but
not its type. The parameter's type is declared in a statement directly below
the function header (and before the left brace that begins the function
body):
float sphere( rad )
int rad;
{
.
.
.
}
The rest of OLDSTYLE.C is identical to VOLUME.C.
Now that you understand the basics of functions, we can turn our attention
to the C statements that a function can contain, beginning with flow-control
statements, the subject of the next chapter.
Chapter 3 Flow Control
────────────────────────────────────────────────────────────────────────────
Flow control─diverting execution by looping and branching─is one area where
C closely resembles other languages. If you know how to loop and branch in
QuickBASIC or QuickPascal, learning the C equivalents is mainly a matter of
adjusting to somewhat different syntax. Here, as elsewhere, C never uses two
keywords when one will do. For instance, C has no "then" keyword. Instead,
it uses simple punctuation.
This chapter has two parts. The first part examines the looping statements:
while, do, and for. The second part describes the decision-making
statements: if, else, switch, break, continue, and goto.
Loops: while, do, and for
This section discusses the C statements that create loops: while, do, and
for. These loops repeat while a condition is true or for a set number of
times. We'll begin with the simplest loop, the while statement.
The while Statement
A while loop evaluates its test expression before executing the body of
the loop.
A while loop repeats as long as a given condition remains true. It consists
of the while keyword followed by a test expression in parentheses and a loop
body, as shown in Figure 3.1. The "test expression" can be any C expression
and is evaluated before the loop body is executed. The loop body is a single
statement or a statement block that executes once every time the loop is
iterated. The distinguishing feature of a while loop is that it evaluates
the test expression before it executes the loop body, unlike the do loop,
which we'll examine next.
(This figure may be found in the printed book.)
We've incorporated the simple while loop from Figure 3.1 in the WHILE.C
program, shown below.
/* WHILE.C: Demonstrate while loop. */
#include <stdio.h>
main()
{
int test = 10;
while( test > 0 )
{
printf( "test = %d\n", test );
test = test - 2;
}
}
Here is the output from WHILE.C:
test = 10
test = 8
test = 6
test = 4
test = 2
In WHILE.C, if the variable test is positive when the loop begins, the
test expression evaluates as true and the loop executes. If test has a 0
or negative value when the loop starts, the test expression is false; the
loop body does not execute and the action falls through to the statement
that follows the loop.
(Chapter 6, "Operators," explains true and false values. For now, it's
enough to know that an expression is evaluated as false if it equals 0. Any
nonzero value is true.)
The loop body in WHILE.C happens to be a statement block enclosed in braces.
If the loop body is a single statement, as in the following code, no braces
are needed.
main()
{
int test = 10;
while( test > 0 )
test = test - 2;
}
Occasionally, you'll see a while loop with a test expression such as
while( 1 )
or
#define TRUE 1
.
.
.
while( TRUE )
The test expressions above are always true, creating an indefinite loop that
never ends naturally. You can only terminate this kind of loop with some
overt action, usually by executing a break statement. (See "The break
Statement" later in this chapter.) You can use such a loop to repeat an
action for an indefinite time period─until a certain key is pressed, for
instance.
The do Statement
A do loop is simply a while loop turned on its head. First comes the loop
body, then the test expression. Unlike a while loop, a do loop always
executes its loop body at least once.
A do loop always executes at least once.
The difference is important. A while statement evaluates the test expression
before it executes the loop body. If the test expression in a while
statement is false, the loop body doesn't execute at all. A do statement, on
the other hand, evaluates the test expression after executing the loop body.
Thus, the body of a do statement always executes at least once, even if the
test expression is false when the loop begins.
Figure 3.2 contrasts the while loop from WHILE.C with a comparable do loop
to emphasize this difference. You'll notice that the do keyword comes right
before the loop body, which is followed by the while keyword and a test
expression─the same test expression that WHILE.C uses. Notice the semicolon
that ends the do loop. A do loop always ends with a semicolon; a while loop
never does.
(This figure may be found in the printed book.)
The DO.C program below uses the do loop from Figure 3.2 to perform the same
action that WHILE.C does.
/* DO.C: Demonstrate do loop. */
#include <stdio.h>
main()
{
int test = 10;
do
{
printf( "test = %d\n", test );
test = test - 2;
} while( test > 0 );
}
DO.C gives the same output as WHILE.C:
test = 10
test = 8
test = 6
test = 4
test = 2
The programs do not give the same output if you modify them so that the
value of test is 0 when the loop starts. In that case, the loop body in
DO.C executes once, but the loop body in WHILE.C doesn't execute at all. You
should only use a do loop when you always want the loop body to execute at
least once.
The for Statement
As in QuickBASIC or QuickPascal, the for statement in C is often used to
repeat a statement a set number of times. Let's begin with a simple example.
The FORLOOP.C program, shown below, uses for to repeat a printf statement
five times.
/* FORLOOP.C: Demonstrate for loop. */
#include <stdio.h>
main()
{
int test;
for( test = 10; test > 0; test = test - 2 )
printf( "test = %d\n", test );
}
FORLOOP.C gives the same output as WHILE.C and DO.C:
test = 10
test = 8
test = 6
test = 4
test = 2
Figure 3.3 shows the parts of the for loop in FORLOOP.C.
(This figure may be found in the printed book.)
A for statement is more complex than a while or do statement. The part in
parentheses can contain three expressions separated by semicolons:
■ An "initializing expression" that often initializes a loop counter
■ A "test expression" that states how long the loop continues
■ A "modifying expression" that often modifies a loop counter
Like the test expression in a while statement, the test expression in a for
statement causes the loop to continue as long as the test expression
evaluates as true.
All of the expressions in the parentheses of a for statement are optional.
If you omit the test expression (the second one), the statement repeats
indefinitely. In the following program, for instance, all of the expressions
in the parentheses of the for loop are empty:
main()
{
for( ; ; )
printf( "Hi, Mom!\n" );
}
The loop above repeats indefinitely because it has no test expression that
specifies when to end the loop. It has the same effect as the following
while loop, whose test expression is always true:
main()
{
while( 1 )
printf( "Hi, Mom!\n" );
}
You can use multiple expressions for either the initializing expression or
the modifying expression, as in FORLOOP1.C:
/* FORLOOP1.C: Demonstrate multiple expressions. */
#include <stdio.h>
main()
{
int a, b;
for( a = 256, b = 1; b < 512; a = a / 2, b = b * 2 )
printf( "a = %d b = %d\n", a, b );
}
The output from FORLOOP1.C appears below:
a = 256 b = 1
a = 128 b = 2
a = 64 b = 4
a = 32 b = 8
a = 16 b = 16
a = 8 b = 32
a = 4 b = 64
a = 2 b = 128
a = 1 b = 256
In the FORLOOP1.C program, the initializing expression of the for loop
initializes two variables ( a and b ) instead of one. The modifying
expression changes the values of the same two variables. Use commas to
separate multiple expressions in cases such as this.
Although for and while might seem quite different, they're interchangeable
in most cases. The FORLOOP2.C program demonstrates this principle. Both
loops, while constructed differently, produce the same effect─printing the
numbers from 0 through 9.
/* FORLOOP2.C: Demonstrate similarity of for and while. */
#include <stdio.h>
main()
{
int count;
for( count = 0; count < 10; count = count + 1 )
printf( "count = %d\n", count );
count = 0;
while( count < 10 )
{
printf( "count = %d\n", count );
count = count +1;
}
}
The two loops in FORLOOP2.C function identically. The for loop prints the
numbers from 0 through 9:
for( count = 0; count < 10; count = count + 1; )
printf( "count = %d\n", count );
as does the while loop:
count = 0;
while( count < 10 )
{
printf( "count = %d\n", count );
count = count + 1;
}
Most programmers prefer for over while in a case like this, because for
groups all the loop-control statements in one place. The for statement is
also appropriate when you need to initialize one or more values at the
beginning of the loop. The while and do statements are more appropriate for
cases in which the value used in the test expression has already been
initialized.
Decision-Making Statements: if, else, switch, break, continue, and goto
The C language provides six statements for decision making: if, else,
switch, break, continue, and goto. Like their counterparts in other
languages, these statements transfer control based on the outcome of a
logical test.
The if Statement
The body of an if statement executes when its test expression is true.
An if statement consists of the if keyword followed by a test expression in
parentheses and a second statement. The second statement is executed if the
test expression is true, or skipped if the expression is false.
The IFF.C program contains a simple if test. It prints a prompt and waits
for you to press a key.
/* IFF.C: Demonstrate if statement. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press the b key to hear a bell.\n" );
ch = getch();
if( ch == 'b' )
printf( "Beep!\a\n" );
}
In the IFF.C program, the statement
ch = getch();
calls the getch library function to get a keypress from the keyboard and
then assigns the result to the variable ch. If you press the b key, the
program prints
Beep!
and sounds a beep. (To simplify the code, IFF.C tests only for a lowercase b
character. A program would normally use a library function such as tolower
to test for both upper and lowercase.)
Figure 3.4 illustrates the parts of the if statement in the IFF.C program.
(This figure may be found in the printed book.)
The test expression of the if statement
ch == 'b'
The equality operator (==) tests if values are equal.
is true when the variable ch equals the letter b. In C the equality
operator (==) tests if two values are equal. (Chapter 6 discusses
operators.)
The body of the if statement in IFF.C happens to be a single statement, but
the body can also be a statement block, as in the following fragment:
if( ch == 'b' )
{
printf( "Beep!\a\n" );
printf( "You pressed the 'b' key\n" );
}
You can also nest if statements, as shown below:
if( ch == 'b' )
{
printf( "Beep!\a\n" );
beep_count = beepcount + 1;
if( beep_count > 10 )
{
printf( "More than 10 beeps...\n" );
if( beep_count > 100 )
printf( "Don't wear out the 'b' key!\n" );
}
}
The code nests three if statements. The first if tests whether ch equals
the letter b; the second tests whether the variable beep_count is greater
than 10. The third tests whether beep_count exceeds 100.
The else Clause
An else clause can follow an if statement.
The else keyword is used with if to form an either-or construct that
executes one statement when an expression is true and another when it's
false. The ELSE.C program demonstrates else by adding an else clause to the
code from IFF.C. It sounds a beep and prints Beep! if you type the letter
b, or it prints Bye bye if you type any other letter.
/* ELSE.C: Demonstrate else clause. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press the b key to hear a bell.\n" );
ch = getch();
if( ch == 'b' )
printf( "Beep!\a\n" );
else
printf( "Bye bye\n" );
}
To create an else-if construct, place an if statement after an else.
The C language has no "elseif" keyword, but it's easy to create the same
effect, because the statement that follows else can be any C
statement─including another if statement. The ELSE1.C program uses if and
else to test three conditions. It sounds a beep when you type the letter b,
it prints Enter when you press the ENTER key, or it prints Bye bye when
you press any other key.
/* ELSE1.C: Demonstrate else-if construct. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press the b key to hear a bell.\n" );
ch = getch();
if( ch == 'b' )
printf( "Beep!\a\n" );
else
if( ch == '\r' )
printf( "Enter\n" );
else
printf( "Bye bye\n" );
}
The else keyword is tied to the closest preceding if that's not already
matched by an else. Keep this rule in mind when creating nested if-else
constructs. (See the section "Mismatching if and else Statements" in Chapter
10, "Programming Pitfalls.")
The switch Statement
The switch statement can perform multiple branches.
The switch statement offers an elegant option in situations that require
multiple branches. It tests a single expression that can have several
values, providing a different action for each value.
One disadvantage of if and else is that they only allow one branch per
keyword. The program either executes the statement that follows the if or
else, or it doesn't. To perform more complex tests, you have to pile on more
if and else statements, as in the ELSE1.C program above.
A program that handles keyboard input, for instance, may require several
different responses to various keypresses. The ELSE1.C program above used
combinations of if and else to process keyboard input. We've used a single
switch statement in the SWITCH.C program below to do the same job:
/* SWITCH.C: Demonstrate switch statement. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press the b key to hear a bell.\n" );
ch = getch();
switch( ch )
{
case 'b':
printf( "Beep!\a\n" );
break;
case '\r':
printf( "Enter\n" );
break;
default:
printf( "Bye bye" );
break;
}
}
The SWITCH.C program produces the same output as ELSE1.C. Figure 3.5
illustrates the switch statement in SWITCH.C, comparing it with the if-else
construct in ELSE1.C.
(This figure may be found in the printed book.)
As in other decision-making statements, the parentheses after the keyword
contain the expression to test. This can be any expression that yields a
constant value. The test expression in SWITCH.C evaluates the value of the
variable ch:
switch( ch )
The switch statement branches to one of several labeled alternatives.
The test expression is followed by a statement block enclosed in curly
braces. The block contains alternate sections of code that you want to
execute under various circumstances. The program's action branches to one of
the alternatives, depending on the value of the test expression.
Each alternative in the statement block starts with a "case label," which
consists of the case keyword, a constant or constant expression, and a
colon. (The only other C statement that uses labels is goto, which we'll
discuss later in this chapter.)
Below is the first case label in SWITCH.C:
case 'b':
This case label lists the character constant 'b'. If the variable ch
equals 'b', the program's action branches to this label. If ch equals
'\r', the program branches to the following label:
case '\r':
The basic effect of switch is similar to the SELECT CASE statement in
QuickBASIC. The program can branch to many different alternatives, but only
one at a time.
A switch statement can have as many case alternatives as you need. Each
alternative must be labeled with a constant value. (You can't use a variable
in the label.)
────────────────────────────────────────────────────────────────────────────
NOTE
In previous versions of QuickC, the constant in a case label could only be a
char or int value. In QuickC 2.5, the constant can be any integral type,
including a long or unsigned long as well as a char or int.
hapter 4, "Data Types," describes the char, int, and long types.
────────────────────────────────────────────────────────────────────────────
The default keyword is used only in switch statements.
SWITCH.C also shows how to use the default keyword in a switch statement.
The statements after the default label execute if the value of the test
expression doesn't equal any of the values listed in other labels. In
SWITCH.C, the code following default executes when the variable ch equals
anything other than 'b' or '\r'.
Not every switch statement requires a default label. If no default is
present, and the test expression doesn't match any of the values listed in
the other case labels, no statements are executed.
Use the break keyword to exit a switch statement.
You normally place a break statement at the end of each alternative, as
shown in SWITCH.C. The break statement exits the switch statement block
immediately. If you don't put a break at the end of the alternative, the
action falls through to the next statement.
For instance, say that you remove all the break statements from SWITCH.C, as
shown below:
switch( ch )
{
case 'b':
printf( "Beep!\a\n" );
case '\r':
printf( "Enter\n" );
default:
printf( "Bye bye" );
}
If you run the revised program and type the letter b, the program executes
the first alternative, producing this output:
Beep!
then goes on to execute the statements that follow:
Enter
Bye bye
Occasionally, you may want to fall through from one case alternative to
another. But you should be careful not to omit break statements
accidentally. (See the section "Omitting break Statements from a switch
Statement" in Chapter 10, "Programming Pitfalls.")
If you end each alternative with a break, as in SWITCH.C, the order of the
alternatives isn't critical. The program branches to the label containing
the correct value, no matter where that label appears in the switch
statement block. For instance, you can reverse the order of the alternatives
in SWITCH.C without changing the program's output. For readability's sake,
many programmers put default at the end of a switch statement and arrange
the other alternatives alphabetically or numerically.
Sometimes you'll want to execute the same code for more than one case. This
is done by grouping all the desired labels in front of one alternative. For
instance, if you revise the second alternative in SWITCH.C to read
case '\r':
case '\t':
case ' ':
printf( "What a boring choice!\n" );
break;
the program will print
What a boring choice!
when you press the ENTER key, the TAB key, or the SPACEBAR.
The break Statement
The previous section explained how to use break to exit from a switch
statement. You can also use break to end a loop immediately. The BREAKER.C
program shows how to do this. The program prints a prompt, then displays
characters as they are typed until the TAB key is pressed.
/* BREAKER.C: Demonstrate break statement. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press any key. Press Tab to quit.\n" );
while( 1 )
{
ch = getche();
if( ch == '\t' )
{
printf( "\a\nYou pressed Tab\n" );
break;
}
}
}
The while statement in BREAKER.C creates an indefinite loop that calls the
getche function again and again, assigning the function's return value to
the variable ch. The if statement in the loop body compares ch to the tab
character. When TAB is pressed, BREAKER.C prints You pressed Tab and
executes the break statement, which terminates the while loop and ends the
program.
A break statement exits only one loop.
It's important to remember that the break statement only ends the loop in
which it appears. If two loops are nested, executing a break in the inner
loop exits that loop but not the outer loop. BREAKER1.C shows how break
works within nested loops. The program's inner loop checks for the TAB key
and the outer loop checks for the ENTER key.
/* BREAKER1.C: Break only exits one loop. */
#include <stdio.h>
#include <conio.h>
main()
{
char ch;
printf( "Press any key. Press Enter to quit.\n" );
do
{
while( ( ch = getche() ) != '\r' )
{
if( ch == '\t' )
{
printf( "\a\nYou pressed Tab\n" );
break;
}
}
} while( ch != '\r' );
printf( "\nBye bye." );
}
The BREAKER1.C program includes a while loop nested within a do loop. Both
loops test the same condition─whether the variable ch equals the ENTER key
(\r). The while loop also calls the getche function, assigning the
function's return value to ch.
When TAB is pressed, the program prints You pressed Tab and executes a
break statement, which terminates the inner loop. The break does not end the
outer loop, however. The program continues until ENTER is pressed, providing
the condition that ends both loops.
Note that break can only be used to exit a loop or switch statement. While
you might be tempted to use break to jump out of complex if or else
statements, the break statement cannot be used for this purpose. It has no
effect on if and else.
The continue Statement
The continue statement skips remaining statements in the loop body where it
appears.
The continue statement, like break, interrupts the normal flow of execution
in a loop body. But instead of ending the loop, continue skips all following
statements in the loop body and triggers the next iteration of the loop.
This effect can be useful within complex loops, in which you might wish to
skip to the next loop iteration from various locations.
The CONT.C program shows how continue works. It increments the count
variable, counting from 0 through 9, but stops printing the value of count
when that value exceeds 3.
/* CONT.C: Demonstrate continue statement. */
#include <stdio.h>
main()
{
int count;
for( count = 0; count < 10; count = count + 1 )
{
if( count > 3 )
continue;
printf( "count = %d\n", count );
}
printf( "Done!\n" );
}
Here's the output from CONT.C:
count = 0
count = 1
count = 2
count = 3
Done!
The continue statement occurs within the body of the for loop. When the
value of count exceeds 3, the continue skips the rest of the loop body─a
statement that calls printf─and causes the next iteration of the loop.
The goto Statement
The goto statement jumps from one part of the function to another.
Similar to the GOTO statement in BASIC, goto in C performs an unconditional
jump from one part of a function to any other part. The target of the goto
statement is a label which you supply. The label must end with a colon, as
do case labels, which we discussed earlier.
Most C programmers avoid using the goto statement. It's a bit inconsistent
with the overall philosophy of C, which encourages structured, modular
programming. And, regardless of philosophy, it can be very difficult to read
and debug a program that is littered with haphazard unconditional jumps.
Nevertheless, goto has at least one sensible use. If a serious error occurs
deep within a nested series of loops or conditional statements, goto offers
the simplest escape. The following code has several levels of nesting, with
a goto statement at the innermost level. If the value of error_count
exceeds 15, the goto statement executes, transferring control to the label
bail_out.
if( a == 1 )
{
while( b == 2 )
{
for( c = 0; c < 3; c = c + 1 )
{
if( d == 4 )
{
while( e == 6 )
{
if( error_count > 15 )
goto bail_out;
}
}
}
}
}
bail_out: /* The goto statement transfers control here. */
To achieve the same effect without goto, you'd have to add extra conditional
tests to this code, making the code more complex and perhaps less efficient.
Names in goto labels are governed by the rules for variable names, which
we'll discuss in the next two chapters. For now, just remember that a goto
label is visible only in the function in which it appears. You can't execute
a goto statement to jump from one function to another function.
The next two chapters explain how to create and manipulate data─variables
and constants─in C programs. Chapter 4, "Data Types," describes the basics,
such as how to declare and initialize variables of different types. Chapter
5, "Advanced Data Types," describes more advanced topics, such as the
visibility of variables.
Chapter 4 Data Types
────────────────────────────────────────────────────────────────────────────
This chapter explains the C data types and shows how to declare and use C
variables. The chapter begins by describing the basic data types from which
all other data types are derived. We then discuss more complex data types,
including arrays and structures. In Chapter 5, "Advanced Data Types," we'll
explore more advanced data-handling topics, such as variable visibility and
automatic type conversions.
Basic Data Types
All data in C programs is either a constant or variable, and each has an
associated data type. The concept of types is common to all high-level
languages. For instance, an integer (whole) number has the INTEGER type in
QuickBASIC, the Integer type in QuickPascal, and the int type in C. This
section describes the basic data types in C and explains how to specify
variables and constants using these types.
All of the basic data types contain a single value. Types that contain more
than one value─arrays, structures, and unions─are called "aggregate types."
We'll discuss aggregate types later in this chapter.
Specifying Basic Types
The C language has four basic data types, which are specified with the
keywords char, int, float, and double. The char (character) type is used for
text and the int type for integers. The float and double types express real
(floating-point) values.
The TYPES.C program creates variables of the four basic types and prints
their values:
/* TYPES.C: Illustrate basic data types. */
#include <stdio.h>
main()
{
char char_val = 'a';
int int_val = 543;
float float_val = 11.1;
double double_val = 66.123456789;
printf( "char_val = %c\n", char_val );
printf( "int_val = %d\n", int_val );
printf( "float_val = %f\n", float_val );
printf( "double_val = %2.9f\n", double_val );
}
Here is the output from TYPES.C:
char_val = a
int_val = 543
float_val = 11.100000
double_val = 66.123456789
Each basic data type requires a different amount of memory, as illustrated
in Figure 4.1. In QuickC, a char contains one byte, an int has two bytes, a
float has four bytes, and a double type has eight bytes.
(This figure may be found in the printed book.)
────────────────────────────────────────────────────────────────────────────
NOTE
The C language is designed to run on many different computers, with machine
architectures that may be quite different. To accommodate these differences,
some C data types are "implementation dependent," meaning their sizes depend
on which computer you're using. For instance, the int (integer) type
contains two bytes on IBM PC computers and four bytes on VAX(R)
minicomputers. These differences are important only if you're transporting a
program from one operating system to another. Since QuickC runs only under
one operating system (DOS), this book describes C data types in DOS.
────────────────────────────────────────────────────────────────────────────
Special Type Specifiers
The C language has four special type specifiers─signed, unsigned, long, and
short. These act as "adjectives" to modify the range of values expressed by
a basic data type.
The char and int data types are signed by default.
The signed keyword signifies that a value can be either negative or
nonnegative. If you don't specify, a char or int value is signed.
You can preface a char or int with unsigned to extend the range of
nonnegative values. An unsigned int can have a value in the range 0 through
65,535, and an unsigned char can have a value of 0 through 255.
The long keyword is used to increase the size of an int or double type. A
long int value contains four bytes (twice as many as an int) and expresses
an integer in the range -2,147,483,648 through 2,147,483,647. A long double
value contains 10 bytes and can express a floating-point number with 19
digits of precision.
In QuickC, the short int type is identical to the int type. (This is not the
case in some operating systems other than DOS.)
Table 4.1 lists the basic data types and the range of values each can
express.
Table 4.1 Basic Data Types
╓┌────────────────┌────────────────────────┌─────────────────────────────────╖
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
char signed char -128 to 127
unsigned char none 0 to 255
int signed, signed int -32,768 to 32,767
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
int signed, signed int -32,768 to 32,767
unsigned unsigned int 0 to 65,535
unsigned short unsigned short int 0 to 65,535
short short int, signed short -32,768 to 32,767
signed short int
long long int, signed long -2,147,483,648 to
signed long int 2,147,483,647
unsigned long unsigned long int 0 to 4,294,967,295
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
float none Approximately 1.2E-38 to 3.4E+38
(7-digit
precision)
double none Approximately 2.2E-308 to
1.8E+308 (15-digit
precision)
long double none Approximately 3.4E-4932 to
1.2E+4932 (19-digit precision)
────────────────────────────────────────────────────────────────────────────
Most programmers take advantage of type defaults. If a type qualifier
appears alone, the type int is implied. By itself, short is a synonym for
short int. Where long appears alone it is a synonym for long int, and where
unsigned appears alone it is a synonym for unsigned int.
Specifying Variables
As we mentioned in Chapter 1, "Anatomy of a C Program," you must declare
every variable in a C program by stating the variable's name and type.
Variable names are governed by the following rules, which also apply to
other userdefined names such as function names:
■ C is case-sensitive. For example, myvar, MyVar, and MYVAR are
different names.
■ The name can't be a keyword (see online help for a list of keywords).
■ The first character must be a letter or underscore ( _ ). Many of
QuickC's system-defined names, including some library-routine names,
begin with underscores. To avoid conflicts with such names, don't
create names that begin with underscores.
■ Other characters can be letters, digits, or underscores.
■ The first 31 characters of local variable names are significant. The
name can contain more than 31 characters, but QuickC ignores
everything beyond the thirty-first character. Global variable names
are normally significant to 30 characters.
All C keywords are lowercase, and it's common to use lowercase for variable
names. Mixed case is becoming popular in some contexts, however. OS/2 and
Microsoft Windows(tm) use mixed case for most system-defined names.
Specifying Constants
Constants─values that don't change during the life of the program─can be
numbers, characters, or strings. Your program can also define "symbolic
constants," which are names that represent constant values. This section
describes how to specify C constants.
Numeric Constants
A numeric constant can have any basic data type, and can be specified in
decimal, hexadecimal, or octal notation. Table 4.2 shows how to specify
numeric constants.
Table 4.2 Constant Specifications
╓┌──────────────────┌────────────────────────────────────────────────────────╖
Constant Type
────────────────────────────────────────────────────────────────────────────
255 decimal int
0xFF hexadecimal int (255)
0377 octal int (255)
255L long int
255U unsigned int
0xFFul long unsigned hexadecimal int (255)
15.75E2 floating point (1575)
-.123 floating point
.123 floating point
3e0f floating point
Constant Type
────────────────────────────────────────────────────────────────────────────
3e0f floating point
────────────────────────────────────────────────────────────────────────────
A number without a suffix, such as 255, is treated as decimal. The 0x
prefix specifies a hexadecimal number, and the 0 (zero) prefix specifies
octal (base 8).
If a number doesn't have a decimal point, it is an integer. Integers are
signed by default; you can use the suffix U or u to specify an unsigned
constant. To specify a long integer, place the suffix L or l after the
number.
A floating-point constant contains either a decimal point or an exponent
preceded by e or E . It can optionally include the suffix F or f to
denote the float type or the suffix L or l to denote the long double
type.
Character and String Constants
The C language uses different notation for character and string constants. A
single character enclosed in single quotes is a character constant:
'a'
A string constant is 0 or more characters enclosed in double quotes:
"Hello"
A string also ends with a null character (\0), as we'll see in the section
"Strings."
The difference between character and string constants is important when you
perform comparisons. The character constant 'a' contains 1 character, but
the string constant "a" contains 2 characters: the letter a plus a null
character. Because the two values have a different number of characters, any
comparison of them is invalid. (See "String Problems" in Chapter 10,
"Programming Pitfalls.")
You can specify special characters, such as the tab and backspace, with a
multi-character sequence that begins with a backslash ( \ ). These sequences
are sometimes called "escape sequences." Table 4.3 shows the special
character sequences.
Table 4.3 Special Characters
╓┌───────────────────────┌───────────────────────────────────────────────────╖
Sequence Character
────────────────────────────────────────────────────────────────────────────
\a Alert (bell)
\b Backspace
\f Form feed
\n Newline
\r Carriage return
\t Horizontal tab
\v Vertical tab
\' Single quote
\" Double quote
\\ Backslash
\ooo Octal notation
Sequence Character
────────────────────────────────────────────────────────────────────────────
\ooo Octal notation
\xhh Hexadecimal notation
\0 Null
────────────────────────────────────────────────────────────────────────────
Some unusual characters don't have a predefined sequence. You can specify
these with a backslash ( \ ) followed by the hexadecimal or octal number
representing the character's ASCII value. For instance, a telecommunications
program might need to specify ASCII 21, the NAK ("not acknowledged")
character. You can specify this character in either hexadecimal notation,
'\x15', or octal notation, '\25'. Note that the hexadecimal number begins
with \x, while the octal number starts with a backslash alone.
Symbolic Constants
A "symbolic constant" is a user-defined name that represents a constant.
Symbolic constants are usually typed in uppercase. For instance, the
directive
#define PI 3.14
declares a symbolic constant named PI. Wherever PI occurs in the program,
the compiler substitutes 3.14. Chapter 7, "Preprocessor Directives,"
discusses symbolic constants and the #define directive.
Aggregate Data Types
This section describes aggregate data types, which contain organized
collections of data in a definite order. In C, the aggregate types are
arrays, structures, and unions.
An "array" is a collection of data items of the same type. Programs use
arrays in cases where a standard data format is repeated many times. For
example, you might use an array to store numbers representing the population
of Minnesota for all the years from 1950 to 2000. C-language arrays are very
similar to arrays in QuickPascal and QuickBASIC.
A "structure" is a collection of data items of different types. Programs use
structures in cases where a variety of data have a close association. For
example, you might use a structure to store information about a given
employee─name, months of employment, and hourly wage. Structures are similar
to QuickPascal records or QuickBASIC user-defined types.
A "union" allows you to use different data formats to access the same area
of memory. It can hold different kinds of information at different times.
Unions are similar to variant records in QuickPascal.
Arrays
An array is a group of data items of the same type under one name.
The simplest aggregate data type is an array: a group of data items that
share the same type and a common name. You can make an array from any data
type, including basic types such as char and int and more complex types such
as structures. This section shows how to declare, initialize, and access
arrays, including arrays with more than one dimension. We'll begin with a
simple example that creates a one-dimensional array.
Creating a Simple Array
The ARRAY.C program creates the array i_array, which contains three
integers.
/* ARRAY.C: Demonstrate one-dimensional array. */
#include <stdio.h>
main()
{
int j;
int i_array[3];
i_array[0] = 176;
i_array[1] = 4069;
i_array[2] = 303;
printf( "--- Values -------- --- Addresses -------\n\n" );
for( j = 0; j < 3; j = j + 1 )
{
printf( "i_array[%d] = %d", j, i_array[j] );
printf( "\t&i_array[%d] = %u\n", j, &i_array[j] );
}
}
Here is the output from ARRAY.C:
--- Values -------- --- Addresses -------
i_array[0] = 176 &i_array[0] = 3506
i_array[1] = 4069 &i_array[1] = 3508
i_array[2] = 303 &i_array[2] = 3510
As you can see, ARRAY.C prints the values in i_array and the memory
address where each array element is stored. You usually don't have to worry
about actual memory addresses in C, but it's useful to have some idea how
array elements are stored in memory. Depending on factors such as the amount
of memory in your system, you may see different addresses when you run
ARRAY.C.
(The second printf statement uses the "address-of" operator (&) to determine
the address of each array element. Chapter 6, "Operators," explains this
operator. For now, it's sufficient to recognize that the operator allows
ARRAY.C to print addresses.)
Figure 4.2 shows how i_array is stored in the addresses from the ARRAY.C
output.
(This figure may be found in the printed book.)
Declaring the Array
You declare an array variable by stating its type and its name, as you would
a simple variable. You must also declare the size of the array, stating the
number of elements with an integer constant in square brackets. For example,
the line
int i_array[3];
from ARRAY.C declares a three-element integer (int) array named i_array.
Multidimensional arrays are declared the same way, except you must give the
size of each dimension. The following statement, for instance, declares a
twodimensional int array named two_dim:
int two_dim[2][3];
We'll return to multidimensional arrays a little later in this chapter.
Initializing the Array
Arrays, like simple variables, should be initialized before use. ARRAY.C
initializes i_array with these statements:
i_array[0] = 176;
i_array[1] = 4069;
i_array[2] = 303;
An array can be initialized when it is declared.
ARRAY.C declares an array in one statement and then initializes its elements
one by one. You can also initialize an array when you declare it. The
following statement does both jobs at once:
int i_array[3] = { 176, 4069, 303 };
Note the curly braces around the initializing values. The braces are
mandatory in this kind of initialization.
Under the ANSI C standard, which QuickC version 2.5 follows, you can
simultaneously declare and initialize an array within a function. Pre-ANSI
compilers, including QuickC version 1.0, don't allow this unless the static
keyword precedes the array declaration. Chapter 5, "Advanced Data Types,"
discusses static.
When you declare and initialize an array at the same time, the initializing
values are normally constants, as shown above. Occasionally, you may want to
initialize an array as you declare it using variables instead of constants.
QuickC version 2.5 allows this, but only within a function. The sample
array in the following example is initialized legally under QuickC version
2.5 but illegally under QuickC version 1.0:
func()
{
int val = 5;
int sample[3] = { val, val, val };
}
If you initialize a local array in this way, you must include the size of
the array within the square brackets following the array name. If the
example initialized the sample array with the following line:
int sample[ ] = {val, val, val};
QuickC would issue an error because the size of the array (3) is not
specified.
Specifying Array Elements
Array subscripts are enclosed in square brackets ( [ ] ).
You specify an array element by giving its position, using an integer value
called a "subscript." Square brackets ([ ]) enclose each subscript. In the
ARRAY.C program above we specify the first element of i_array as
i_array[0]
Notice that the first element of a C array has the subscript 0, not 1.
Unlike QuickPascal and QuickBASIC, the C language does not give you the
option to start at an index number other than 0.
Since array subscripts begin at 0, the subscript of the last array element
is 1 less than the number used to declare that dimension of the array. In
ARRAY.C, the last element of i_array is i_array[2] , not i_array[3].
C doesn't check array subscripts.
Unlike QuickBASIC and QuickPascal, C doesn't check the validity of array
subscripts. If the ARRAY.C program included the expression
i_array[55];
it would refer to a nonexistent array element. (The expression refers to the
element 55, but i_array contains only three elements.) This would not
trigger a compiler error or run-time error, however. It's your job to
remember the size of the array and avoid references that go outside the
array's boundaries. This rule is also important when you're accessing arrays
with pointers (see Chapter 8, "Pointers").
Strings
A string is an array of characters.
You may have wondered why we didn't mention strings in our earlier
description of basic data types. The reason is that strings aren't a formal
data type. In the C language, a string is simply an array of characters
(char values).
The STRING.C program below creates the string c_array and displays its
contents in the same format as the previous example. The program prints the
value of each array element and its address.
/* STRING.C: Demonstrate string arrays. */
#include <stdio.h>
main()
{
int j;
char c_array[] = "Hello";
printf( "--- Values -------- --- Addresses -------\n\n" );
for( j = 0; j < 6; j = j + 1 )
{
printf( "c_array[%d] = %x %c", j, c_array[j], c_array[j] );
printf( "\t&c_array[%d] = %u\n", j, &c_array[j] );
}
}
Here is the output from STRING.C:
--- Values -------- --- Addresses -------
c_array[0] = 48 H &c_array[0] = 3522
c_array[1] = 65 e &c_array[1] = 3523
c_array[2] = 6c l &c_array[2] = 3524
c_array[3] = 6c l &c_array[3] = 3525
c_array[4] = 6f o &c_array[4] = 3526
c_array[5] = 0 &c_array[5] = 3527
Figure 4.3 shows how c_array is stored in memory. Again, the addresses in
the output may differ depending on factors such as the amount of available
memory.
(This figure may be found in the printed book.)
A string ends with a null character.
The figure illustrates another important feature of strings. Although
c_array has five printing characters ( Hello ), it actually contains six
characters─five letters plus a null character (\0) that marks the end of the
string. As noted earlier, the C language automatically adds a null character
to every string enclosed in double quotes.
STRING.C uses a shortcut when it initializes c_array. You may have noticed
that the array declaration
char c_array[] = "Hello";
doesn't declare the array's size (the square brackets are empty). When an
array is initialized at the same time it's declared, QuickC can figure out
how many elements the array has by counting the number of initializing
values to the right of the equal sign.
You can use this shortcut for any type of array, not just a char array. If
the array has more than one dimension, however, you can only omit the size
of the first dimension.
Multidimensional Arrays
A "multidimensional" array contains two or more array dimensions. The
TWODIM.C program below creates a two-dimensional array named i_array.
/* TWODIM.C: Demonstrate multidimensional arrays. */
#include <stdio.h>
main()
{
int j, k;
int i_array[2][3] = { { 176, 4069, 303 }, { 6, 55, 777 } };
printf( "--- Values -------- --- Addresses -------\n\n" );
for( j = 0; j < 2; j = j + 1 )
{
for( k = 0; k < 3; k = k + 1 )
{
printf( "i_array[%d][%d] = %d", j, k, i_array[j][k] );
printf( "\t&i_array[%d][%d] = %u\n", j, k, &i_array[j][k] );
}
printf( "\n" );
}
}
Here's the output from TWODIM.C:
--- Values -------- --- Addresses -------
i_array[0][0] = 176 &i_array[0][0] = 3498
i_array[0][1] = 4069 &i_array[0][1] = 3500
i_array[0][2] = 303 &i_array[0][2] = 3502
i_array[1][0] = 6 &i_array[1][0] = 3504
i_array[1][1] = 55 &i_array[1][1] = 3506
i_array[1][2] = 777 &i_array[1][2] = 3508
Each subscript of a multidimensional array appears in its own set of square
brackets, as the TWODIM.C output shows. When you declare the array, the
first subscript states the size of the first dimension, the second states
the size of the second dimension, and so on. In TWODIM.C, the declaration of
i_array,
int i_array[2][3]
states that i_array contains two rows of values, each row containing three
integers. The statement that declares i_array also initializes the array,
listing the initializing values in curly braces to the right of the equal
sign:
int i_array[2][3] = { { 176, 4069, 303 }, { 6, 5, 77 } };
The braces clearly show that the array contains two groups of three values.
Two-dimensional arrays are often pictured in rows and columns, as in Figure
4.4. Of course, since computer memory is linear, i_array is actually
stored with its rows end-for-end, as in Figure 4.5.
(This figure may be found in the printed book.)
You refer to a multidimensional array element the same way you would a
onedimensional array element, except that you use one subscript for each
dimension of the array. For instance, the statement
printf( "%d\n", i_array[0][1] );
specifies two subscripts. It prints the value stored in element 0, 1 of
i_array, which is 4069.
Figure 4.5 shows how to specify every element of i_array in TWODIM.C.
(This figure may be found in the printed book.)
Structures
A structure is a group of related data items of different types under one
name.
The second aggregate data type is the structure: a group of related data
items under one name. While array elements are all the same type, the
elements of a structure, known as its "members," can be of different types.
Structures are equivalent to records in QuickPascal or user-defined types in
QuickBASIC. As in those languages, the ability to group different types in
the same construct provides powerful, very flexible data-handling
capabilities.
Creating a Simple Structure
We'll write a simple program to demonstrate the basics of structures.
Suppose you want to write a payroll program that records these facts about
an employee:
■ Name
■ Number of months of service
■ Hourly wage
Each of these data items requires a different data type. The name can be
stored in a string (character array), while an integer will do for the
months of service. The hourly wage may contain a fraction; we'll store it in
a floating-point variable.
Although each of these variables has a different type, we can group all of
them in a single structure. The EMPLOYEE.C program below contains the
structure.
/* EMPLOYEE.C: Demonstrate structures. */
#include <stdio.h>
#include <string.h>
struct employee
{
char name[10];
int months;
float wage;
};
void display( struct employee show );
main()
{
struct employee jones;
strcpy( jones.name, "Jones, J" );
jones.months = 77;
jones.wage = 13.68;
display( jones );
}
void display( struct employee show )
{
printf( "Name: %s\n", show.name );
printf( "Months of service: %d\n", show.months );
printf( "Hourly wage: %6.2f\n", show.wage );
}
Here is the output of the EMPLOYEE.C program:
Name: Jones, J
Months of service: 77
Hourly wage: 13.68
Declaring a Structure Type
Since a structure can (and normally does) contain different data types,
creating it is a little more complicated than making an array or simple
variable. Before you can create a structure variable, you must declare a
structure type that tells the compiler how many members the structure
contains and what types they are.
A structure-type declaration starts with the keyword struct, which is
followed by a list of the structure's members enclosed in braces. Between
the struct and the list of members, you can also specify a "structure tag"─a
name that other parts of the program can use to refer to the type.
The structure declaration from EMPLOYEE.C,
struct employee
{
char name[10];
int months;
float wage;
};
A structure declaration makes a template for variables of the type it
defines.
creates a "template" for an employee structure that structure variables of
this type can use. It's as if you created a brand new data type, tagging it
employee. Figure 4.6 illustrates the employee structure type.
(This figure may be found in the printed book.)
Creating a Structure Variable
Once you have declared a structure, you can create variables of that type
using the structure tag. Each variable can contain values of the types
defined in the structure type. In EMPLOYEE.C, the statement
struct employee jones;
declares a structure variable of the type employee named jones. The
struct states that the variable is a structure. The employee tag specifies
the variable's structure type, and jones is the variable's name.
You can also declare the variable in the same statement that declares the
structure type. The following code declares the employee structure type
and a variable of that type named jones:
struct employee
{
char name[10];
int months;
float wage;
} jones;
The variable name ( jones ) appears at the end of the declaration.
Use the member-of operator (.) to specify structure members.
You specify structure members by name, using the "member-of" operator (.) to
separate the variable name and the member name. These are the names of the
members of the jones structure variable in EMPLOYEE.C:
jones.name
jones.months
jones.wage
Like other variables, structure variables should be initialized before use.
After jones is declared in EMPLOYEE.C, the statements
strcpy( jones.name, "Jones, J" );
jones.months = 77;
jones.wage = 13.68;
initialize the members of the jones variable. The first statement
initializes the jones.name member by calling the strcpy ("string copy")
library function; this function is described in Chapter 11, "Input and
Output."
Figure 4.7 shows how the jones structure is stored in memory. Again, since
computer memory is linear, the members of the structure are laid out
end-to-end.
(This figure may be found in the printed book.)
You can initialize a structure when you declare it. The following code would
perform both operations in EMPLOYEE.C:
struct employee jones =
{
"Jones, J",
77,
13.68
};
This code declares the jones structure variable and lists the initializing
value for each of its members.
Using Structure Variables
A structure member can be treated like any other variable of its type. You
can assign a value to it, change its value, and so on. For instance, the
statement
jones.months = 83;
would change the value of the jones.months member in EMPLOYEE.C.
Assigning one structure to another copies the entire structure.
You can also assign an entire structure to another structure of the same
type. This copies the entire contents of the first structure to the second.
You might do this to save time when creating a new structure whose contents
differ only slightly from those of an existing structure.
To illustrate, let's modify the EMPLOYEE.C program. Say you have a second
employee named Lavik whose wage rate and months of service are the same as
those of Jones and you want to create a second structure. You could begin by
declaring a second employee structure variable named lavik in this
fashion:
struct employee lavik = jones;
Now the members of the lavik structure contain the same data as the
members of the jones structure. The lavik.name member contains the
string Jones, J, the lavik.months member contains the value 77, and the
lavik.wage member contains the value 13.68. You could add the statement
strcpy( lavik.name, "Lavik, B" );
to place a new string in the lavik.name member.
Structure variables can be passed as function arguments.
When you pass a structure name to a function, the function creates a local
structure variable of that type. Like all local variables, the new variable
is private to the function that includes it.
For example, if you add the statements
strcpy( show.name, "King, M" );
printf( "%s\n", show.name );
to the end of the display function in EMPLOYEE.C, then a new string is
copied into the show.name member of the function's structure variable. The
printf statement in the second line prints
King, M
Since this structure is local to the display function, the change doesn't
affect the structure defined in the main function. If you add the statement
printf( "%s\n", jones.name );
to the end of the main function, the program prints
Jones, J
The original structure is unchanged.
While you can pass a structure name to a function as we did above, it's more
common to pass the function a pointer to the structure. This not only
permits the function to access a structure defined elsewhere in the program,
but it conserves memory (since the function doesn't create a local copy of
the structure). Chapter 9, "Advanced Pointers," explains how to access
structures using pointers.
Arrays of Structures
An array of structures is a group of structures of the same type.
Since it's rare for a company to have a single employee, a more practical
version of the EMPLOYEE.C program would have an array of structures─one
structure per employee. The concept may sound intimidating, but this is a
common use of structures.
The following statement declares a 50-element array named payroll, with
each element a structure of the type employee :
struct employee payroll[50];
To specify members in such an array, you combine array notation and
structure notation, giving the array name, a subscript, and a member name.
For instance, the name
payroll[0].months
specifies the months member of the first structure in the payroll array.
The first part of the name ( payroll[0] ) contains the array name and
subscript that identify the structure. The second part ( months ) identifies
the member within that structure.
Figure 4.8 depicts the first three elements of the payroll array.
(This figure may be found in the printed book.)
Once you grasp the basic idea, it's easy to imagine practical uses for an
array of structures. Many programs, from an address book to a library card
catalog, might use a structure to store different types of information about
an individual item, then store many such structures in an array.
Structures of Structures
As noted earlier, a structure can contain members of any data type─including
other structures. So you can create a structure of structures: a structure
whose members are structures.
To illustrate, suppose you write a group of functions that draw various
kinds of graphic windows and message boxes. You could define a small
structure something like the following:
struct title
{
char text[70]; /* Title text */
int color; /* Color of title text */
short justify; /* Left, center, or right */
};
to aid in drawing titles. The title structure's three members specify the
title's text, its color, and how its text is justified.
Once the title structure is defined, you can make it part of other, larger
structures that use titles. If you define a window structure type to draw
windows, for example, that structure could include a title along with
other structure members:
struct window
{
struct title wintitle; /* Window title */
/* Other structure members go here... */
};
In this structure type, the title member is named wintitle.
You specify members of such structures using member-of operators and the
appropriate names. If you create a variable of the window type named
mywindow, the name
mywindow.wintitle.color
specifies the color member of the wintitle member of the mywindow
structure.
If you program using QuickC's Presentation Graphics library, you'll find it
useful to understand the notation we just explained. Our fictitious title
structure is a simplified version of the Presentation Graphics titletype
structure type (see Chapter 14, "Presentation Graphics").
Bit Fields
A "bit field" is a specialized structure that provides a way to manipulate
individual bits or groups of bits. One use for this advanced feature is to
access hardware addresses such as the computer's video memory.
The members of a bit-field structure are groups of bits.
You declare and use a bit-field structure much as you would any other
structure. The difference is that every one of its members must be a bit or
group of bits. You can't include other data types in a bit field.
The following statement declares a bit-field structure type with the tag
SCREEN:
struct SCREEN
{
unsigned character : 8;
unsigned fgcolor : 3;
unsigned intensity : 1;
unsigned bgcolor : 3;
unsigned blink : 1;
} screenbuf[25][80];
The colons in the declaration tell QuickC these are bit fields rather than
normal structure members. The number following each colon tells how many
bits the field contains. In the SCREEN type the character member has 8
bits, intensity has 1 bit, and so on. Figure 4.9 illustrates the SCREEN
type.
(This figure may be found in the printed book.)
Figure 4.10 illustrates memory allocation for the SCREEN type. The members
of the SCREEN type mirror the arrangement of bits in screen memory.
(This figure may be found in the printed book.)
Take another look at the structure declaration. In addition to declaring a
structure type, the statement declares a two-dimensional array variable
screenbuf, of the same structure type. You could use this array as an
alternate video buffer. Many graphics programs use a similar arrangement to
switch between an alternate video buffer and the computer's video memory.
The five members of the SCREEN type happen to take up a full int (two
bytes, on DOS machines). A bit field need not fill up a byte or int; the bit
field can contain as many bits as you need up to the maximum number of bits
for the field's
base type. The base type for each field in the example is unsigned (unsigned
int), so each field can contain a maximum of 16 bits.
The members of a bit-field structure are accessed with the structure-member
operator─like other structure members. For instance, the name
screenbuf[13][53].blink = 1;
specifies the blink member of element 13, 53 of the screenbuf array.
The range of values you can assign to a bit-field member depends on the
member's size. Since the blink member of the SCREEN type contains one
bit, blink is limited to the value 0 or 1. The fgcolor member contains
three bits and can have any value from 0-7.
Unions
A union is a group of variables of different types that share storage
space.
A union is a variable that can hold any one of several data types at
different times, using the same storage space. Unions are a rather advanced
feature. One use of them is to access DOS registers, which you may sometimes
need to access as bytes and at other times as words.
As with a structure, you must start by declaring a union type to tell the
compiler the number and types of the union's members. You include one of
each type that you expect to use.
The following code creates a union that can hold a char, int, or long value.
It declares a union type with the tag u_sample and declares a variable of
that type named example.
union u_sample
{
char c_val;
int i_val;
long l_val;
} example;
When you declare a union, QuickC allocates as much storage as the largest
data type in the union requires. Since the largest type in u_sample is
long, this union contains four bytes.
The elements of a union are called members and use the same notation as
structure members. Thus, the members of the example union are named
example.c_val
example.i_val
example.l_val
The contents of a union depend on how you access it. For instance, the
statement
example.c_val = '\0';
stores a char value in the example union. Since a char value takes one
byte, the statement uses only one byte of the space in example. The
statement
example.i_val = 77;
uses two bytes of the union, because an int value requires two bytes of
storage. Likewise, the statement
example.l_val = 75621;
stores long value in example, taking up all four bytes of its storage
space. Figure 4.11 shows memory allocation for the three members in the
example union.
(This figure may be found in the printed book.)
It's your job to keep track of what is stored in a union. If you store a
long value in example and mistakenly treat that value as a char value
later, the result may be nonsense. It's especially important not to confuse
integer and floating-point types, which are stored in different internal
formats.
Now that you're familiar with the data types that C offers, you are ready to
tackle more advanced data-handling concepts. The next chapter discusses
several of these.
Chapter 5 Advanced Data Types
────────────────────────────────────────────────────────────────────────────
In Chapter 4, "Data Types," we described the basic C data types and showed
how to declare and use different kinds of variables. This chapter examines
more advanced data topics, including the visibility and lifetime of
variables and the conversion of values from one data type to another.
If you know QuickPascal or QuickBASIC, some of these topics, such as
visibility, should be familiar. For example, a variable declared within a
function is visible (accessible) only in that function. One area in which C
differs notably from QuickPascal is type conversion. The C language gives
programmers the freedom to convert a value from one type to another type,
whereas QuickPascal does not.
Visibility
Every variable in a C program has a definite "visibility" that determines
which parts of the program can "see," or access, the variable. Another term
for visibility is "scope."
As we mentioned in Chapter 1, "Anatomy of a C Program," there are two basic
kinds of visibility: local and external. A "local" variable─one declared
within a function─is visible only within that function. An "external"
variable─one declared outside all functions─is visible to all functions that
follow it in the program.
This section begins by describing local and external visibility, then goes
on to discuss visibility in multiple-file programs and the visibility of
functions.
────────────────────────────────────────────────────────────────────────────
NOTE
While the examples in this section use simple int variables, visibility
rules apply equally to aggregate types such as arrays and structures.
────────────────────────────────────────────────────────────────────────────
Use external variables only when necessary.
C programmers normally limit the visibility of each variable to those parts
of the program that need to access the variable. For instance, if a variable
is needed only within one function, it should always be local to that
function. By restricting a variable's visibility, you can prevent other
parts of the program from accidentally changing the variable's value. Such
haphazard side effects were common in older interpreted BASIC programs, in
which every variable had unlimited visibility.
Local Variables
Variables declared within a function are local to that function.
As we noted in Chapter 1, "Anatomy of a C Program," and Chapter 2,
"Functions," the place where you declare a variable controls where the
variable is visible. Declaring a variable within a function makes the
variable local to that function; the variable can be seen only within the
function.
The VISIBLE.C program below demonstrates local visibility. It contains a
function named be_bop that tries to print the value of the local variable
val.
/* VISIBLE.C: Demonstrate local visibility. */
#include <stdio.h>
void be_bop( void );
main()
{
int val = 10;
be_bop();
}
void be_bop( void )
{
printf( "val = %d", val ); /* Error! */
}
Notice where the val variable is declared. The declaration
int val = 10;
occurs within the main function, so val is local to main. When you compile
VISIBLE.C, QuickC stops, providing this error message:
C2065: 'val' : undefined
What happened? The printf statement in the be_bop function
printf( "val = %d", val ); /* Error! */
can't "see" the variable val, which is declared locally within main.
Outside the main function, in which val is declared, the variable doesn't
exist.
You could eliminate the error message by declaring val externally, but
most programmers would avoid that solution. If a variable has external
visibility, any part of the program might change its value accidentally. A
better solution is to pass the value of val to be_bop as a function
argument, as shown in the program VISIBLE1.C below.
/* VISIBLE1.C: Demonstrate local visibility. */
#include <stdio.h>
void be_bop( int param );
main()
{
int val = 10;
be_bop( val );
}
void be_bop( int param )
{
printf( "%d\n", param );
}
The VISIBLE1.C program is identical to VISIBLE.C except for two changes. The
be_bop function now can accept an argument, and the statement that calls
be_bop passes the value of val as an argument. These changes allow the
be_bop function to print the value of val without the drawback of making
val external.
Most local variables are declared at the beginning of the function and are
visible throughout the function. If you declare the variable later in the
function, it is visible only to statements that follow the declaration.
The reason for this rule is simple: QuickC, like all language compilers,
reads your program line by line, from beginning to end. Until the compiler
sees the variable's declaration, it must treat the variable as undefined.
This rule applies to all variables, including external variables, as we'll
see in the next section.
Although the practice isn't common, you can restrict a local variable's
visibility even further by declaring it in a statement block inside a
function. For instance, you might declare a variable within the body of the
loop or conditional statement. In fact, any pair of curly braces limits the
visibility of a variable declared within that pair.
External Variables
If you declare a variable outside all functions, the variable has external
visibility; every function that follows the declaration can see the
variable. External variables are called "global" in some other languages.
Experienced C programmers use external variables only when necessary─for
instance, when two or more functions need the ability to change the same
variable or communicate with each other by changing a variable. Even in
those cases, however, you may be able to avoid the dangers of external
visibility by passing a pointer to the variable as a function argument. See
the section "Passing Pointers to Functions" in Chapter 8 ("Pointers") for
more information.
Most external variables are declared near the beginning of the program,
before any function definitions. In this way, you can make the variable
visible to every function in the program. You could do this in VISIBLE1.C by
placing the declaration of val,
int val = 10;
immediately before the main function.
If you declare the variable val later in the program, it is not visible to
functions that precede the declaration. The VISIBLE2.C program below
demonstrates this principle.
/* VISIBLE2.C: Demonstrate external visibility. */
#include <stdio.h>
void be_bop( int param );
main()
{
be_bop( val ); /* Error! */
}
int val = 10;
void be_bop( int param )
{
printf( "val = %d\n", param );
}
The VISIBLE2.C program is identical to VISIBLE1.C except that val is
declared externally
int val = 10;
following the main function, rather than locally within main.
Because the declaration occurs outside all functions, the variable is
external. However, because the declaration follows the main function, the
variable is not visible within main. When the printf statement in the main
function refers to val, QuickC issues the error message:
C2065: 'val' : undefined
Remember, QuickC reads the program line by line, from start to finish. Since
the compiler knows nothing about val when it reaches the reference in
main, it must treat val as undefined. In this program, only the be_bop
function can refer to val.
Visibility in Multiple Source Files
A "source file" is the file containing your program's text. Source files
normally have the .C file extension, to distinguish them from other files
such as executable (.EXE) files.
Simple programs have only one source file, but large programs are often
split into several source files. If you write a word-processing program, for
instance, you might place all the program's screen-output functions in one
file, all the file-handling functions in a second file, and so forth.
Use the extern keyword to make an external variable visible in more than
one source file.
Normally, an external variable is visible only in the source file in which
it is declared. In a multi-file program, however, a function in one file
might need to access a variable in a second file. To make the variable
visible in more than one source file, you must declare it with the extern
keyword.
Let's look at a short two-file program that shows how to use extern. The
first source file, FILE1.C, declares two external variables, chico and
harpo. The file contains one function (main) that calls a second function
named yonder.
/* FILE1.C: Visibility in multiple source files. */
int chico = 20, harpo = 30;
extern void yonder( void );
main()
{
yonder();
}
The second source file, FILE2.C, contains the yonder function that is
called in FILE1.C. This file also declares the variables chico and harpo,
but it prefaces their declarations with extern to show that the variables
are defined externally in some other file. Once this is done, any function
in FILE2.C can refer to chico and harpo as if they are defined in the
same file.
/* FILE2.C: Visibility in multiple source files. */
#include <stdio.h>
void yonder( void )
{
extern int chico, harpo;
printf( "chico = %d, harpo = %d\n", chico, harpo );
}
You can compile this program in one of two ways. In the QuickC environment,
choose Set Program List from the Make menu and add FILE1.C and FILE2.C to
the list. Then choose Build Program from the Make menu.
You can also enter this command from the DOS command line:
qcl FILE1.C FILE2.C
In either case, the executable file is named FILE1.EXE. The program's
output,
chico = 20, harpo = 30
shows that the yonder function in FILE2.C can access the variables defined
in FILE1.C.
Sometimes you may want an external variable to be visible only in the source
file where it's declared. The variable can be shared by functions in one
file, but it is hidden to all other files, thus minimizing the risk of
naming conflicts.
The static keyword can limit a variable's visibility to one source file.
To limit a variable's visibility to one file, precede the variable's
declaration with the keyword static. For example, if FILE1.C declared the
harpo variable as static in this manner,
static int harpo;
it would prevent FILE2.C from accessing harpo at all, even though FILE2.C
declares (with extern) that harpo is defined somewhere else.
Visibility of Functions
Functions are normally visible in multiple source files.
Unlike variables, functions are external by default. That is, they are
normally visible to every file in a multi-file program. You'll notice that
in FILE1.C we declared the yonder function with the extern keyword. We did
this merely to improve readability; the keyword shows clearly that the
function is defined in some other file. If we removed the extern from the
declaration of yonder in FILE1.C, the program would work just as well as
before.
At times you may want to restrict the visibility of a function in a
multi-file program, making it visible in some files but not in others. By
"hiding" a function from other parts of a program, you can reduce the danger
of naming conflicts. For instance, if you write a library of functions to
sell commercially, you probably would hide all of the library's local
function names, to prevent conflicts with function names your customers
might create.
The static keyword can limit a function's visibility.
As with external variables, you limit a function's visibility using the
static keyword. A function declared as static is visible only in the source
file that declares it. If we add static to the header of the yonder
function, for example,
static void yonder( void )
the function could no longer be called from the FILE1.C file.
Lifetime
In addition to visibility, every variable also has a certain "lifetime"─that
is, the period during the program's execution when the variable exists.
External variables exist for the life of the program. Memory is allocated
for them when the program begins and remains until the program ends.
An automatic variable disappears when the function ends.
Local variables have shorter lifetimes. They come into being when the
function begins and disappear when the function ends. For this reason, a
local variable is said to be "automatic." The variable comes and goes
automatically, each time the function is called.
Automatic variables conserve memory in a couple of ways. First, since they
evaporate when the function ends, automatic variables don't consume memory
when not in use. Second, they are stored in the "stack" memory area, which
the program allocates at run time. So, automatic variables don't enlarge the
executable program.
The C language provides the auto keyword for declaring automatic variables.
However, this keyword is seldom used, since all local variables are
automatic unless you specify otherwise. In the following function, both val
and example are automatic variables:
void sample( void )
{
int val;
auto int example;
}
The auto preceding the declaration of example has no practical effect. The
variable example is automatic even if you remove the auto from its
declaration.
Extending the Lives of Local Variables
Occasionally, you may want a local variable to retain its value between
function calls. The static keyword, introduced earlier as a means of
limiting the visibility of external variables, also performs this task.
A static local variable retains its value through subsequent function
calls.
If you precede a local variable declaration with static, the variable exists
for the life of the program─the same lifetime as an external variable. The
variable still has local visibility, however.
The STATIC.C program below shows how to create and use a static local
variable. In STATIC.C, the value of the methuselah variable persists
through all calls to the add_val function, which adds values to
methuselah and prints the variable's value.
/* STATIC.C: Demonstrate static variables. */
#include <stdio.h>
void add_val( int value );
main()
{
add_val( 1 );
add_val( 5 );
add_val( 20 );
}
void add_val( int value )
{
static int methuselah;
if( value == 1 )
methuselah = 0;
methuselah = methuselah + value;
printf( "methuselah = %d\n", methuselah );
}
The add_val function in STATIC.C accepts one parameter and also declares a
static local variable named methuselah. Each time add_val is called, it
adds the passed value to methuselah.
The main function calls the add_val function three times, passing the
values 1, 5, and 20 to add_val as arguments. The program's output
methuselah = 1
methuselah = 6
methuselah = 26
shows that the value of methuselah persists through all three function
calls.
If we remove the static keyword from the declaration of methuselah, the
variable's value is not preserved between function calls. The value of
methuselah is unpredictable the second and third times that add_val is
called.
Notice that extending a local variable's lifetime with static doesn't affect
its visibility. The methuselah variable keeps its value between function
calls, but you can't refer to the variable outside the add_val function.
Converting Data Types
It's usually best to avoid mixing data items of different types in the same
expression. You wouldn't normally add a character variable to a
floating-point variable, for instance. Some languages, such as QuickPascal,
generally treat type mixing as an error. However, the C language gives you
the freedom to mix data types when necessary.
For example, since the char and int types both can store whole numbers,
there may be times when you have a good reason to add a char value to an int
value. When you mix types, QuickC does not issue an error message. Instead,
the compiler converts both data items to the same type and then performs the
requested operation.
Type conversion can occur in one of two ways. The first way occurs
automatically when you combine different types in an expression. You can
also use special syntax to intentionally "cast" (convert) one type to
another. We'll discuss both methods in the following sections.
Knowing how C converts types will help you to find bugs that result from
unintended type clashes and to minimize errors when you deliberately mix
types.
Ranking of Data Types
For purposes of conversion, the C language ranks data types in the order
shown in Figure 5.1.
(This figure may be found in the printed book.)
The ranking illustrated in Figure 5.1 generally reflects the amount of
storage that each type requires. As you may remember from Chapter 4, "Data
Types," larger data types require more storage than smaller types. Thus, an
int, which requires two bytes of storage, outranks a char, which requires
one byte.
Within this ranking, an unsigned type outranks the corresponding signed
type. An unsigned char value is of higher rank than a signed char, and so
forth.
Promotions and Demotions
A promotion is usually harmless.
A type conversion always involves two data items of different types.
Whenever possible, QuickC converts the lower-ranking (smaller) data item to
the higher-ranking (larger) type. This kind of conversion, called a
"promotion," is normally harmless. For example, since a two-byte int has
more than enough room to store a one-byte char, it's generally safe to
promote a char value to an int.
A demotion usually causes a loss of data.
Sometimes, the compiler is forced to convert a higher-ranking value to a
lower-ranking type. This kind of conversion, called a "demotion," usually
causes loss of data. For example, the int value 32,000 is much too large to
be stored in a char type, which can't hold a number larger than 255. If you
assign the value 32,000 to a char variable, some data must be lost.
A demotion of an integer type truncates the higher-ranking type, throwing
away the data from high-order bytes that can't fit in the smaller-ranking
value. Some demotions of floating-point types round off a value rather than
truncate it.
Automatic Type Conversions
C does an automatic type conversion when you mix different data types.
When a program statement mixes two different data types, QuickC performs an
automatic type conversion. The following code, for instance, adds the char
variable a to the int variable b.
char a = 5;
int b = 32000;
b = a + b;
In the statement
b = a + b;
the addition operation to the right of the equal sign triggers an automatic
type conversion. QuickC promotes the char value to an int and then adds the
two int values.
If you're not sure whether QuickC is doing an automatic type conversion, set
Warning Level 2 or higher in the Compiler Flags dialog box. The compiler
generates the warning message
C4051: data conversion
whenever an automatic conversion occurs. This monitoring helps you readily
identify unwanted conversions.
If you carelessly mix different types, you may create subtle errors. The
CONVERT.C program below has a deliberate error that shows what can happen
when types are mixed. It adds four variables and assigns their sum to a
fifth variable, causing three promotions and one demotion.
/* CONVERT.C: Demonstrate type conversions. */
#include <stdio.h>
main()
{
char c_val = 10;
int i_val = 20;
long l_val = 64000;
float f_val = 3.1;
int result;
result = c_val + i_val + l_val + f_val; /* Error! */
printf( "%d\n", result );
}
The CONVERT.C program adds the numbers 10, 20, 64000, and 3.1. Instead of
the correct result, 64033.10, the program prints
-1503
Something definitely went wrong. The problem lies somewhere in the line
result = c_val + i_val + l_val + f_val;
which triggers four automatic type conversions. We'll examine the
conversions in order.
The first conversion occurs when the char variable c_val is added to the
int variable i_val:
c_val + i_val
Since the variables are different types, QuickC automatically converts the
lower-ranking char value to the higher-ranking int type before adding them.
This promotion doesn't create any problems, since there's more than enough
room to store the one-byte char value in the two-byte int. The sum of this
addition is 30, another int value.
The next operation adds that partial sum to the long value of l_val (to
make the expression easier to read, we'll show the sum from the previous
addition):
30 + l_val
This addition triggers another promotion. The compiler promotes the int
result of the first addition to a long value before adding it to l_val,
which is long. Since the four-byte long type has more than enough room to
store a two-byte int, this promotion is also harmless.
Now the partial sum equals 64030. The last addition from CONVERT.C
64030 + f_val
triggers another harmless conversion: the compiler converts the long result
of the previous addition to a float value before adding it to f_val. Even
though floating-point and integer values are stored in different internal
formats, no data is lost when the long is converted to a float.
The result of these additions and conversions is the float value 64033.10,
which is correct. So where does the mistake occur?
The problem arises when CONVERT.C assigns the final sum to the wrong type of
variable. You'll recall that the line containing these operations begins
with the assignment result =.
Earlier in the program, we declared the variable result as an int. The
twobyte int variable created to store the result of these additions is too
small to contain the four-byte float sum that was finally produced.
The assignment forces QuickC to demote the larger float value to the smaller
int type. It's impossible to store such a large floating-point value in the
two bytes of an int, so the final result is incorrect.
Figure 5.2 shows the progression of automatic type conversions that the
CONVERT.C program produces.
(This figure may be found in the printed book.)
We can fix the conversion error by declaring the variable result as a
float, substituting
float result;
for the earlier declaration. We'll also need to change the format string in
the printf function call to print a float value, as shown below:
printf( "%6.2f\n", result );
Now CONVERT.C prints the expected value of 64033.10.
Manual Type Conversions through Casting
A cast forces a value to a particular type.
The C language also allows you to force a type conversion that would not
otherwise happen, a process known as "casting." Using casts, it is possible
to convert a data item to any C data type.
Sometimes you must use a cast to make the program work properly. When
calling the malloc library function, for instance, you should perform a cast
on the value that the function returns. (Chapter 12, "Dynamic Memory
Allocation," explains malloc and other memory-allocating functions.)
Casts can also make a program more readable. QuickC does most automatic type
conversions silently. So if you write a tricky bit of code that relies on
automatic conversions, you, or some other programmer, may not notice the
conversions later. To make such code more readable─and easier to debug─you
can add explicit type casts in places where silent conversions might go
unnoticed.
To cast a value to a different type, place the desired type name in
parentheses in front of the value. For instance, the statement
f_val = (float)any_val;
casts the value of the variable any_val to type float before assigning it
to f_val. Here the type name in parentheses,
(float)
performs the cast. No matter what type any_val has, the cast converts that
type to float before assigning it to f_val.
When you cast a variable, the cast affects the value the variable yields,
but not the variable itself. Suppose that any_val is an int variable with
the value 333. The above cast converts the value 333 to float format before
assigning it to f_val. But any_val remains an int variable after the
cast.
Remember, you can detect automatic type conversions by setting Warning Level
2 or higher in QuickC and watching for the following warning:
C4051: data conversion
You can then add explicit casts to eliminate the warning where the
conversions are desirable. (See "Automatic Type Conversions" above.)
Register Variables
Register variables are stored in processor registers instead of addressable
memory.
You can use the register keyword in variable declarations to request that a
variable be stored in a processor register. Because processor registers can
be accessed more quickly than addressable memory locations, this storage can
make a program run faster. Programmers use register to speed access to
heavily used variables, such as counter variables in loops.
The register specifier is much less important than it used to be, now that
most C compilers, including QuickC, can perform optimizations (improvements)
during compilation. If you compile with the Optimizations option turned on,
QuickC automatically stores variables in registers when needed. So you
probably won't need to use register except in special cases.
────────────────────────────────────────────────────────────────────────────
IMPORTANT
If you compile with Optimizations on, an explicit register declaration can
override register storage that QuickC would do automatically. Declaring one
variable with register might prevent QuickC from storing some other variable
in a register. In the worst case, this can make a program run slower.
────────────────────────────────────────────────────────────────────────────
You can use register only with short integer types (char, int, and short
int). Other types─including aggregate types such as arrays─are too large to
fit in a register.
Only two registers are available for variable storage at any given time.
(They are DI and SI, for those who have programmed in assembly language.) If
you request more registers than are available, QuickC stores the extra
variables in addressable memory, as it does non-register variables.
The following declaration uses register to ask the compiler to store the int
variable val in a processor register:
register int val;
You can ask the compiler to store more than one variable in a register. For
instance, the statement
register int val, count;
declares val and count as register variables.
────────────────────────────────────────────────────────────────────────────
NOTE
Since registers are not addressable, you can't use the address-of (&)
operator to get the address of a variable declared with register. This rule
applies whether or not QuickC is actually able to store the variable in a
register. Thus, if you need to access a variable through a pointer, don't
declare that variable with register. See the section "Initializing a Pointer
Variable" in Chapter 8, "Pointers."
────────────────────────────────────────────────────────────────────────────
Renaming Existing Types with typedef
The typedef keyword creates a new name for an existing data type. This is a
convenience feature that you can use to make programs more readable. For
instance, the declaration
typedef int integer;
allows you to use integer as a synonym for int.
One more practical use of typedef is to substitute a short, descriptive name
for an aggregate type. For instance, the QuickC Presentation Graphics
library uses typedef to create descriptive names such as windowtype and
titletype for structures used in that library.
────────────────────────────────────────────────────────────────────────────
NOTE
The typedef keyword doesn't create a new data type. It merely allows you to
use a different name for a type that already exists.
────────────────────────────────────────────────────────────────────────────
You can also use typedef to minimize portability problems. By using typedef
declarations for data types that are machine dependent, you need only change
the typedef declaration if you move the program to a different operating
system.
The Enumeration Type
The "enumeration type" specifies a set of named integer constants, similar
to the enumerated type in QuickPascal. In the C language, enumeration types
are declared with the enum keyword.
Use enum to name a set of integer constants.
The enum type is useful mainly for improving a program's readability. With
enum, you can use meaningful names for a set of constants whose purpose
might not otherwise be apparent.
Suppose you're writing a calendar program in which the constant 0 represents
Saturday, 1 represents Sunday, and so on. You might begin by declaring the
enumeration type day in the following manner:
enum day
{
saturday, sunday, monday, tuesday,
wednesday, thursday, friday
};
Notice this declaration's similarity to a structure declaration. As with
structures, the type declaration creates a template that you can use to
declare variables of this type. (See the section "Declaring a Structure
Type" in Chapter 4, "Data Types.")
Unless you specify otherwise, the first value in an enumeration type equals
0 and others are numbered sequentially. In the enum type shown above,
saturday equals 0, sunday equals 1, and so forth.
The values in an enumeration type need not be sequential, however. If you
want some other order, you can declare explicit values for each member of
the type. The following declaration, for example, assigns the names zero,
freeze, and boil to the constants 0, 32, and 220, respectively.
enum temps
{
zero = 0,
freeze = 32,
boil = 220
};
After declaring an enumeration type, you can create a variable of that type
and assign it a value from the type. The statement
enum day today = wednesday;
declares the variable today, assigning it the value wednesday from the
day enumeration type.
After you assign its value, you can use the variable today as you would an
int variable. Although the variable is considered to have the enum type, it
is an ordinary int for all practical purposes.
Enumeration types aren't used very often, partly because you can achieve a
similar effect using the #define directive. (Chapter 7, "Preprocessor
Directives," explains #define in detail.) For example, the code
#define SATURDAY 0
#define SUNDAY 1
#define MONDAY 2
#define TUESDAY 3
#define WEDNESDAY 4
.
.
.
int today = WEDNESDAY;
uses #define to create symbolic constants named SATURDAY, SUNDAY, MONDAY,
TUESDAY, and WEDNESDAY, assigning them the values 0 through 4. The last
line in the example creates the int variable today and assigns it the
value of WEDNESDAY. The result is identical to the statement shown earlier:
enum day today = wednesday;
One advantage of using enum over #define directives is that it groups
related names in one place and can be more compact than a long series of
directives.
This concludes our main discussion of data types. The next chapter,
"Operators," examines the C language's rich set of operators, which allow
you to manipulate data in many different ways.
Chapter 6 Operators
────────────────────────────────────────────────────────────────────────────
Compared with other languages, C is very compact, using fewer than 50
keywords. One reason C can get by with so few reserved words is its
abundance of powerful operators─well over 30.
Most C operators are easy to understand and remember. Even if you have never
seen a C program, you probably understand that the statement
val = val * 5;
multiplies the variable val by 5 and assigns the result to val.
Because the printable ASCII character set has only so many unique symbols, C
uses some ASCII symbols in more than one operator. For instance, the
asterisk (*) performs either a multiplication or pointer operation,
depending on context. Similarly, the ampersand (&) is part of three C
operators. Depending on context, the ampersand can obtain an address or
perform a logical or bitwise AND operation. Be careful not to confuse
operators that look similar but do different jobs.
This chapter describes the C operators, beginning with those that are common
to most languages, and then discussing those unique to C.
Introducing C's Operators
We'll start by discussing C operators that look and behave similarly to
operators in other languages. These include the following groups:
■ Arithmetic operators, which do operations such as addition and
multiplication
■ Relational operators, which compare two values and give a true or
false result
■ Assignment operators, which make one value equal to another
Arithmetic Operators
The C language's arithmetic operators closely resemble those in other
languages. Table 6.1 lists C's arithmetic operators.
Table 6.1 Arithmetic Operators
╓┌─────────────────────────┌─────────────────────────────────────────────────╖
Operator Description
────────────────────────────────────────────────────────────────────────────
* Multiplication
/ Division
% Modulus
+ Addition
- Subtraction
────────────────────────────────────────────────────────────────────────────
The "modulus operator" (%) may be unfamiliar. It divides a value and gives
the remainder. For instance, the statement
remainder = 20 % 3;
assigns the value 2 to the variable remainder (20 divided by 3 equals 6,
with a remainder of 2). If the division doesn't produce a remainder, the
modulus operator yields the value 0.
Relational Operators
"Relational operators" evaluate the relationship between two expressions,
giving a true result (the value 1) or a false result (the value 0). C has
six relational operators, which are listed in Table 6.2.
Table 6.2 Relational Operators
╓┌──────────────────────┌────────────────────────────────────────────────────╖
Operator Description
────────────────────────────────────────────────────────────────────────────
Operator Description
────────────────────────────────────────────────────────────────────────────
< Less than
<= Less than or equal
> Greater than
>= Greater than or equal
== Equal
!= Not equal
────────────────────────────────────────────────────────────────────────────
The "equality operator" (==), shown above, tests whether two expressions are
equal.
Don't confuse the equality operator with the assignment operator (=)
discussed in the next section. The assignment operator sets one value equal
to another, as we'll see shortly. (Chapter 10, "Programming Pitfalls,"
discusses this common programming error.)
The C language gives the value 1 for true and 0 for false but recognizes any
nonzero value as true. The following code fragment demonstrates this
difference:
printf( "C generates %d for true\n", 2 == 2 );
printf( "C generates %d for false\n", 2 == 4 );
if( -33 )
printf( "C recognizes any nonzero value as true\n" );
The output from this code,
C generates 1 for true
C generates 0 for false
C recognizes any nonzero value as true
shows that the true expression ( 2 == 2 ) gives the value 1 and the false
expression ( 2 == 4 ) gives the value 0. The last output line shows that C
recognizes the nonzero value -33 as a true value.
Assignment Operators
The "assignment operator" (=) sets one value equal to another. The following
statement assigns the value of sample to val:
val = sample;
You can combine an assignment with a bitwise or arithmetic operation.
In a convenient shorthand, C allows you to combine the assignment operator
with any arithmetic or bitwise operator (see the "Arithmetic Operators" and
"Bitwise Operators" sections). For example, the statement
val = val + sample;
can more conveniently be written
val += sample;
Both statements add val to sample and then assign the result to val.
Table 6.3 lists C's special assignment operators.
Table 6.3 Special Assignment Operators
╓┌───────────┌─────────────┌─────────────────────────────────────────────────╖
Expression Equivalent Operation
────────────────────────────────────────────────────────────────────────────
x *= y x = x * y Multiplication
x /= y x = x / y Division
x %= y x = x % y Modulus
x += y x = x + y Addition
x -= y x = x - y Subtraction
x <<= y x = x << y Left shift
x >>= y x = x >> y Right shift
x &= y x = x & y AND
x ^= y x = x ^ y Exclusive OR
x |= y x = x | y Inclusive OR
────────────────────────────────────────────────────────────────────────────
Note that the equal sign always follows the other operator. In the following
code,
val ^= sample;
val =^ sample;
the first statement is meaningful, but the second is a syntax error.
C's Unique Operators
The remaining sections in this chapter describe C operators that fall into
two categories: those that are unique to C and those that look or behave
differently in C than in other languages.
Increment and Decrement Operators
The C language's unique "increment" (++) and "decrement" (- -) operators are
very useful. They increase or decrease an expression by a value of 1.
Table 6.4 Increment and Decrement Operators
╓┌─────────────────────┌─────────────────────────────────────────────────────╖
Operator Operation
Operator Operation
────────────────────────────────────────────────────────────────────────────
++ Increment expression by 1
- - Decrement expression by 1
────────────────────────────────────────────────────────────────────────────
Thus, the two statements
val = val + 1;
val++;
are equivalent and so are these statements:
val = val - 1;
val--;
You can use the ++ and - - operators before or after an expression.
The ++ and - - operators can precede or follow an expression. Placed before
an expression, the operator changes the expression before the expression's
value is used. In this case, the operator is said to be a "prefix" operator.
Placed after an expression, the operator (known as a "postfix" operator)
changes the value of the expression after the expression's value is used.
In the DECRMENT.C program, shown below, the decrement operator is used both
as a prefix operator and a postfix operator.
/* DECRMENT.C: Demonstrate prefix and postfix operators. */
#include <stdio.h>
main()
{
int val, sample = 3, proton = 3;
val = sample--;
printf( "val = %d sample = %d\n", val, sample );
val = --proton;
printf( "val = %d proton = %d\n", val, proton );
}
Here is the output from DECRMENT.C:
val = 3 sample = 2
val = 2 proton = 2
In the first use of the decrement operator, the statement
val = sample--;
assigns the value of sample (3) to the variable val and then decrements
sample to the value 2. Contrast this with the statement
val = --proton;
which first decrements proton to the value 2 and then assigns that value
to val.
Bitwise Operators
The "bitwise operators," listed in Table 6.5, manipulate bits in data of the
integer type. These operators are often used in programs that must interact
with hardware.
Table 6.5 Bitwise Operators
╓┌─────────────────────────┌─────────────────────────────────────────────────╖
Operator Description
────────────────────────────────────────────────────────────────────────────
~ Complement
<< Left shift
>> Right shift
& AND
^ Exclusive OR
| Inclusive OR
────────────────────────────────────────────────────────────────────────────
The ~ operator, known as the "one's complement," acts on only one value
(rather than on two, as do most operators). This operator changes every 1
bit in its operand to a 0 bit and vice versa.
The and >> operators, known as the "shift operators," shift the left
operand by the value given in the right operand. These operators offer a
fast, convenient way to multiply or divide integers by a power of 2.
The & operator, known as the "bitwise AND," sets a bit to 1 if either of the
corresponding bits in its operands is 1, or to 0 if both corresponding bits
are 0. It is often used to "mask," or turn off, one or more bits in a value.
The ^ operator, known as the "bitwise exclusive OR," sets a bit to 1 if the
corresponding bits in its operands are different, or to 0 if they are the
same.
The | operator, known as the "bitwise inclusive OR," sets a bit to 1 if
either of the corresponding bits in its operands is 1, or to 0 if both
corresponding bits are 0. It is often used to turn on bits in a value.
Each of the bitwise operators is used in the BITWISE.C program, shown below.
/* BITWISE.C: Demonstrate bitwise operators. */
#include <stdio.h>
main()
{
printf( "255 & 15 = %d\n", 255 & 15 );
printf( "255 | 15 = %d\n", 255 | 15 );
printf( "255 ^ 15 = %d\n", 255 ^ 15 );
printf( "2 << 2 = %d\n", 2 << 2 );
printf( "16 >> 2 = %d\n", 16 >> 2 );
printf( "~2 = %d\n", ~2 );
}
The output from BITWISE.C,
255 & 15 = 15
255 | 15 = 255
255 ^ 15 = 240
2 << 2 = 8
16 >> 2 = 4
~2 = -3
shows the results of the program's various bitwise operations.
The fourth and fifth output lines show you how to use shift operators to
multiply and divide by powers of 2. The program multiplies 2 by 4 by
shifting the value 2 twice to the left:
2 << 2 = 8
Similarly, the program divides 16 by 4 by shifting the value 16 twice to the
right:
16 >> 2 = 4
Logical Operators
C has three logical operators─AND, OR, and NOT─that allow you to test more
than one condition in a single expression. Table 6.6 lists C's logical
operators.
Table 6.6 Logical Operators
╓┌──────────────────────────┌────────────────────────────────────────────────╖
Operator Description
────────────────────────────────────────────────────────────────────────────
! Logical NOT
&& Logical AND
| | Logical OR
────────────────────────────────────────────────────────────────────────────
The logical OR ( | | ) and AND (&&) operators are often used to combine
logical tests within a conditional statement. For example, the if statement
if( val > 10 && sample < 10 )
printf( "Oh joy!\n" );
prints Oh joy! if both conditions in the test expression are true (if val
is greater than 10 and sample is less than 10). Here, the relational
operators (> and ) have higher "precedence" than the logical AND operator
(&&), so the compiler evaluates them first. We discuss operator precedence
later in this chapter.
The logical NOT operator (!) reverses an expression's logical value. For
instance, if the variable val has the value 8, the expression (val == 8)
is true but the expression !(val == 8) is false.
The NOT.C program below shows a common use of this operator.
/* NOT.C: Demonstrate logical NOT operator. */
#include <stdio.h>
main()
{
int val = 0;
if( !val )
printf( "val is zero" );
}
The expression if( !val ) is equivalent to the expression if( val == 0 ).
When used in this way, the logical NOT operator converts a 0 value to 1 and
any nonzero value to 0.
────────────────────────────────────────────────────────────────────────────
NOTE
Don't confuse the logical OR and AND operators with the bitwise OR and AND
operators discussed in the previous section. The bitwise operators use the
same ASCII symbols, but have only one character. For instance, logical AND
is &&, whereas bitwise AND is &.
────────────────────────────────────────────────────────────────────────────
Address Operators
The C language has two operators that work with memory addresses. Table 6.7
lists C's address operators.
Table 6.7 Address Operators
╓┌──────────────┌────────────────────────────────────────────────────────────╖
Operator Operation
────────────────────────────────────────────────────────────────────────────
Operator Operation
────────────────────────────────────────────────────────────────────────────
& Yield address of the operand
* Yield value contained at the operand's address
────────────────────────────────────────────────────────────────────────────
Both address operators are often used with pointers─variables that contain
the addresses of other variables. Chapter 8, "Pointers," and Chapter 9,
"Advanced Pointers," are devoted to explaining pointers, including the use
of these two operators with them. Since you must understand pointers in
order to understand these operators fully, we'll describe them briefly here
and elaborate on their use in Chapter 8.
The "address-of operator" (&) yields a constant equal to the machine address
of its operand. For instance, if the variable val contains the value 10,
and its storage is located at address 1508, the expression val yields the
value 10, while the expression &val yields the constant 1508.
Since the address-of operator yields a constant, you can't assign a value to
an expression that uses it. The statement
&val = 20;
is illegal for the same reason that the statement
1508 = 20;
won't pass muster.
The "indirection operator" (*) yields the value contained in the address
referenced by its operand. If you declare ptr as a pointer variable, the
expression
*ptr
yields the contents of the address to which ptr points.
Conditional Operator
The "conditional operator" (? :) is made up of two symbols and requires
three expressions. It is similar to an if-else construct. If the first
expression evaluates as true, the first operand is assigned the value of the
second operand. If the first expression is false, the first operand is
assigned the value of the third operand.
The following statement gives the absolute value of the variable val. The
variable is assigned its original value if it is nonnegative or is negated
if its original value is negative:
val = (val >= 0 ) ? val : -val;
The statement is equivalent to the following if-else construct:
if( val >= 0 )
val = val;
else
val = -val;
The sizeof Operator
The "sizeof operator" yields the number of bytes contained in its operand,
which can be either a general data type or a specific variable. If you apply
sizeof to a type name in parentheses, as in the expression
sizeof( int )
the operator yields the size of that data type in bytes. This example yields
the value 2, indicating that an int contains two bytes on DOS machines. You
can use this feature to determine the sizes of types that are implementation
dependent when transporting a program from one machine to another.
If you place sizeof in front of a variable name, the operator returns the
number of bytes in the variable. For instance, if you create the string
char my_string[] = "Hello";
the expression
sizeof my_string
yields the value 6, showing that the string contains 5 printing characters
and a null character.
Comma Operator
Preceding chapters have shown various ways to use the comma (,) in C
programming. For instance, commas can separate multiple function arguments
or variable declarations. In such cases the comma is not an operator in the
formal sense but merely punctuation, like the semicolon that ends a
statement.
The comma is used as punctuation and as an operator in C.
In C, the comma can also perform as an operator. The commas that separate
multiple expressions determine the order in which the expressions are
evaluated, and the type and value of the result that is returned. The comma
operator causes expressions to be evaluated from left to right. The value
and type of the result are the value and type of the rightmost operand.
For example, the statement
val = sample, sample = temp;
first assigns the value of sample to val, then assigns the value of temp
to sample.
The comma operator often appears in for statements, where it can separate
multiple initializing expressions or multiple modifying expressions. The
FORLOOP1.C program from Chapter 3, "Flow Control," demonstrates both uses.
Here is the for statement from that program:
for( a = 256, b = 1; b < 512; a = a / 2, b = b * 2 )
printf( "a = %d \tb = %d\n", a, b );
The statement initializes two variables ( a and b ) and contains two
modifying expressions ( a = a / 2 and b = b * 2 ). Chapter 3 explains the
FORLOOP1.C program in detail.
Base Operator
The base operator (:>) associates a base expression with a based pointer.
Based-object support is a highly advanced feature included in QuickC 2.5 for
compatibility with Microsoft C version 6.0; please refer to your C 6.0
documentation for information about based objects.
Operator Precedence
Like all languages, C has precedence rules that control the order for
evaluating the elements in expressions containing more than one operator. If
you're familiar with precedence rules in other languages, you won't find any
surprises in C. Table 6.8 shows the "pecking order" established for C's
operators.
Three general rules control the order of evaluation:
1. When two operators have unequal precedence, the operator with higher
precedence is evaluated first.
2. Operators with equal precedence are evaluated from left to right.
3. You can change the normal order of precedence by enclosing an
expression in parentheses. The enclosed expression is then evaluated
first. (If parentheses are nested, inner parentheses have higher
precedence than outer ones.)
We'll demonstrate operator precedence with a simple example. Since the
multiplication operator (*) has higher precedence than the addition operator
(+), the statement
val = 2 + 3 * 4
assigns to val the value of 14 (or 2 + 12) rather than 20 (or 5 * 4). Since
parentheses have higher precedence than any operator, they can change the
normal precedence order. If you enclose the addition operation in
parentheses, as follows
val = (2 + 3) * 4
the addition is done first. Now the statement assigns to val the value 20
(or 5 * 4).
Table 6.8 lists the C operators and their precedence values. The lines in
the table separate precedence levels. The highest precedence level is at the
top of the table.
Table 6.8 C Operators
╓┌───────────────────────┌────────────────────────────────────┌──────────────╖
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
( ) Function call Left to right
[ ] Array element
. Structure or union
member
-> Pointer to structure
member
────────────────────────────────────────────────────────────────────────────
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
- - Decrement Right to left
++ Increment
────────────────────────────────────────────────────────────────────────────
:> Base operator Left to right
────────────────────────────────────────────────────────────────────────────
! Logical NOT Right to left
~ One's complement
- Unary minus
+ Unary plus
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
+ Unary plus
& Address
* Indirection
sizeof Size in bytes
(type) Type cast [for example, (float) i]
────────────────────────────────────────────────────────────────────────────
* Multiply Left to right
/ Divide
% Modulus (remainder)
────────────────────────────────────────────────────────────────────────────
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
+ Add Left to right
- Subtract
────────────────────────────────────────────────────────────────────────────
<< Left shift Left to right
>> Right shift
────────────────────────────────────────────────────────────────────────────
< Less than Left to right
<= Less than or equal
> Greater than
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
> Greater than
>= Greater than or equal
────────────────────────────────────────────────────────────────────────────
== Equal Left to right
!= Not equal
────────────────────────────────────────────────────────────────────────────
& Bitwise AND Left to right
────────────────────────────────────────────────────────────────────────────
^ Bitwise exclusive OR Left to right
────────────────────────────────────────────────────────────────────────────
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
| Bitwise OR Left to right
────────────────────────────────────────────────────────────────────────────
&& Logical AND Left to right
────────────────────────────────────────────────────────────────────────────
|| Logical OR Left to right
────────────────────────────────────────────────────────────────────────────
? : Conditional Right to left
────────────────────────────────────────────────────────────────────────────
= Assignment Right to left
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
= Assignment Right to left
*=, /=, %=, +=, -=, Compound assignment
<<=, >>=, &=, ^=, |=
────────────────────────────────────────────────────────────────────────────
, Comma Left to right
────────────────────────────────────────────────────────────────────────────
Chapter 7 Preprocessor Directives
────────────────────────────────────────────────────────────────────────────
This chapter describes preprocessor directives─commands that control the
QuickC compiler. It explains how to insert the contents of one source file
into another file, how to do text substitutions throughout a file, and how
to compile different parts of a file in different situations.
A "preprocessor directive" is a command to the QuickC compiler. Although
they appear in the same source file as executable statements, preprocessor
directives aren't statements in the formal sense. Unlike executable
statements, they are not translated into machine code. Instead, they tell
the compiler itself to take some action while it translates your source
program. For instance, an #include directive tells QuickC to insert another
file into the source file.
The term "preprocessor" refers to the time when these commands are carried
out. Like most language compilers, QuickC translates your source program in
several phases, the first of which is called the "preprocessor phase."
QuickC first "preprocesses" all the directives in your source program, then
processes the program's executable statements.
All preprocessor directives begin with a number sign (#), which must be the
first nonblank character in the line on which it appears. Since directives
aren't statements, they don't end with semicolons. You can't put other
statements or directives on the same line with a preprocessor directive,
except for a comment, which must appear to the right of the directive.
Because the compiler reads your source file sequentially, line by line, the
position of directives is important. A preprocessor directive only affects
statements that follow it in the source file.
The #include Directive
The #include directive inserts another file in the source file.
The #include directive inserts the contents of another file into your source
file. The inserted file is called an include file or header file.
When the compiler encounters an #include, it searches for the file named in
the directive. This directive makes QuickC look for the standard include
file STDIO.H:
#include <stdio.h>
If the designated file is found, the compiler inserts its contents at the
spot where the #include directive appears. Figure 7.1 illustrates a program
SAMPLE.C that includes the file STDIO.H.
(This figure may be found in the printed book.)
When QuickC compiles the SAMPLE.C program shown in Figure 7.1, it inserts
the contents of file STDIO.H into SAMPLE.C at the spot marked by the
#include directive.
Most include files contain commonly used declarations and definitions.
Standard include files, supplied with QuickC, contain declarations and
definitions for QuickC library routines. You can also write include files of
your own.
Standard include files end with the .H file extension (STDIO.H is an
example). You can use any extension for include files you create, but most
programmers stick with the .H extension.
────────────────────────────────────────────────────────────────────────────
NOTE
In some languages, it's common to put executable statements, as well as
declarations and definitions, in include files. This practice is legal but
not recommended in QuickC. Microsoft debugging tools such as the Microsoft
CodeView(R) debugger may not recognize executable statements in include
files.
────────────────────────────────────────────────────────────────────────────
The #include directive doesn't support wild cards, so you can't insert a
group of related files with a single directive. Each #include directive
inserts only one file.
Include files can be nested. For instance, the source program SAMPLE.C might
include a file named INOUT.H. The INOUT.H file, in turn, might contain a
second #include directive that includes a file named KEYBOARD.H. Figure 7.2
illustrates this process.
(This figure may be found in the printed book.)
Although it's rarely necessary to nest include files more than two or three
levels, nesting may continue up to 10 levels.
Specifying Include Files
There are two ways to tell QuickC where to search for an include file. You
may have noticed that the #include directive shown earlier encloses the file
name STDIO.H in angle brackets (<>). If you enclose the file name in angle
brackets, as in the directive
#include <stdio.h>
the compiler searches the "standard directories" for the file.
In QuickC, the standard directories are one or more directories that you
define by a DOS environment variable named INCLUDE. An advantage of
specifying the standard directories is that QuickC can automatically search
more than one directory.
Alternatively, you can enclose the file name in double quotes, in the
following manner,
#include "myfile.h"
to cause QuickC to start searching in the directory that contains the
current source file. If the target file isn't in that directory, the
compiler searches the standard directories.
────────────────────────────────────────────────────────────────────────────
NOTE
You can specify additional directories on the DOS command line when you
invoke QuickC with the QCL command. See Chapter 1, "Creating Executable
Programs," in the Microsoft QuickC Tool Kit.
────────────────────────────────────────────────────────────────────────────
The #define and #undef Directives
The #define directive performs a text substitution in the source file. This
directive has two main uses: simple text replacement and creation of
function-like macros. It is also used with the #undef directive to control
conditional compilation, as we'll discuss later.
Simple Text Replacement
The #define directive works like the search and replace function of a
word processor.
At the simplest level, the #define directive works much like the "search and
replace" function of a word processor, replacing one piece of text in the
source file with another piece of text. The #define directive is commonly
used to create a symbolic constant─a meaningful name for a "magic number"
whose meaning might not otherwise be apparent. This improves the program's
readability.
For instance, in the VOLUME.C program in Chapter 1, "Anatomy of a C
Program," the directive
#define PI 3.14
defines a symbolic constant named PI. The directive causes QuickC to
replace every occurrence of the text PI in the VOLUME.C source program
with the text 3.14. For example, when the compiler encounters the program
line
result = 4 * PI * result;
it expands the line to read
result = 4 * 3.14 * result;
Besides making your program more readable, symbolic constants can streamline
its maintenance. For instance, say you later decide to use 3.14159265 rather
than 3.14 in VOLUME.C. All you need to change is one #define directive at
the beginning of the program.
The replacement text can be longer than the 3.14159265 we used above. A
replacement text can't be longer than 512 bytes in QuickC, but you'll
rarely, if ever, have to worry about this limit.
Function-Like Macros
A function-like macro accepts arguments, like a function.
Some languages use the term "macro" when referring to replacement text. In
C, a macro can do more than simply replace text. It can also accept
arguments in much the same way that a function does. In this case the
replacement text is called a "function-like macro."
A well-designed macro can be every bit as useful as a function. In fact,
some C library routines are implemented as macros rather than C functions.
The MACRO.C program below has a macro that works like the abs library
function, returning the absolute value of any integer. The macro uses the
conditional operator (? :), which we explained in Chapter 6.
/* MACRO.C: Demonstrate macros. */
#include <stdio.h>
#define ABS(value) ( (value) >= 0 ? (value) : -(value) )
main()
{
int val = -20;
printf( "result = %d\n", ABS(val) );
}
The ABS macro behaves much like a function. You can "call" it by name,
passing it an argument you want to process. The macro is defined in the
following program line:
#define ABS(value) ( (value) >= 0 ? (value) : -(value) )
The parameter value appears four times in the macro─once in the macro name
ABS and three times in the replacement text that follows the name.
Always enclose macro parameters in parentheses.
To avoid unwanted side effects, you should always enclose macro parameters
in parentheses. If the parameter passed to the ABS macro is an expression
containing operators, the lack of parentheses could cause
operator-precedence errors. See the section "Omitting Parentheses from Macro
Arguments" in Chapter 10, "Programming Pitfalls."
Function-like macros, like other #define directives, are expanded during the
preprocessor phase of compilation, before QuickC translates any executable
statements. When QuickC encounters the line
printf( "result= %d\n", ABS(val) );
it expands it to read:
printf( "result= %d\n", ( (val) >= 0 ? (val) : -(val) ) );
Macros can improve readability.
A macro can improve a program's readability by describing the nature of an
operation while hiding its complex details. Most people find the first of
the two statements above easier to understand than the expanded version.
Macros are faster than functions but can make a program bigger.
Function-like macros are fast, too. Since a macro creates in-line code, it
doesn't have the overhead associated with a normal function call. On the
other hand, each use of a macro inserts the same code in your program,
whereas a function definition occurs only once. So while macros can be
faster than functions, they can also bloat the size of the executable file.
The #undef Directive
The "#undef directive" is related to #define. As the name suggests, #undef
removes ("undefines") a name that was created with #define. For instance, if
you create the symbolic constant PI with the #define directive,
#define PI 3.14
you can then remove the name PI with the following #undef directive:
#undef PI
You can use #define and #undef to create a name that has meaning in only
part of a source program. The next two sections explain why you might want
to do this.
Conditional Directives
Conditional directives are useful for making different versions of a
program.
Conditional directives can make QuickC skip part of a source file. They are
used primarily to create different versions of a program. While developing a
program, for instance, you might want to include debugging code at some
times but not others. Or, if you plan to move a program to some other
machine, you can compile machine-specific sections of code only for a
certain machine.
The C-language conditional directives are listed below.
#if #endif
#else #ifdef
#elif #ifndef
────────────────────────────────────────────────────────────────────────────
NOTE
The #ifdef and #ifndef directives are obsolete under the ANSI C standard;
see "The defined Operator" below.
────────────────────────────────────────────────────────────────────────────
The #if directive works like the if statement.
The #if and #endif directives work like an if statement, allowing you to
compile a block of source code if a given condition is true. The #if
directive is followed by a constant expression, which the compiler tests at
compile time. If the expression is false, the compiler skips every line
between the #if and the next #endif.
The example below calls the display function only if the name DEBUG was
previously defined as 1 (with #define).
#if DEBUG == 1
display( debuginfo );
#endif
Here, the "conditional block" is a single line (the display function
call). A conditional block can contain any number of valid C program lines,
including preprocessor directives as well as executable statements.
The test expression for a conditional directive can be almost any expression
that evaluates to a constant, with a few minor exceptions (the expression
can't use the sizeof operator, type casts, or the float and enum types).
The #else directive works like the else keyword.
The #else and #elif directives work like the else keyword and can perform
more complex conditional tests. For example, you could use code like that in
the following example to build different versions of a program for various
IBM PC computers, including different files for each computer.
#if XT == 1
#include "XT.H"
#elif AT == 1
#include "AT.H"
#else
#include "PS2.H"
#endif
The code includes the file XT.H if the name XT is defined as 1 and it
includes the file AT.H if the name AT is defined as 1. If both XT and
AT are undefined, the third conditional block executes, causing QuickC to
include the file PS2.H.
You can nest conditional directives in the same way as you would conditional
C language statements.
The defined Operator
The defined operator tests whether a name has been defined.
The test expression of an #if or #elif directive can use the defined
operator to test whether a name has been defined. You can use this feature,
along with #define and #undef, to turn various parts of a program on and
off, compiling different parts under different conditions.
The defined operator is true if its argument has been defined and false
otherwise. A name is considered defined if it has been created with #define
(and not later removed with #undef).
The DEFINED.C program below prints Hi because the name DEBUG is defined
when the compiler encounters the #if defined directive.
/* DEFINED.C: Demonstrate defined operator. */
#define DEBUG 12345
main()
{
#if defined( DEBUG )
printf( "Hi\n" );
#endif
}
The defined operator tests only whether a name is defined, not whether it
has a certain value. Thus, the DEFINED.C program will print Hi no matter
what value is assigned DEBUG. You could substitute the directive
#define DEBUG 0
to define DEBUG as 0, or the directive
#define DEBUG
to define DEBUG as having no value at all. Both directives define the name
DEBUG, so the program would print Hi in both cases.
You can use the logical NOT operator (!) to reverse the logic of an #if
defined directive. (Logical operators are explained in Chapter 6.) The code
#if !defined( DEBUG )
printf( "Hi\n");
#endif
prints Hi if DEBUG is not currently defined.
A plain #if directive treats undefined names a little differently than does
an #if defined directive. If a name is not currently defined, the #if
directive treats the name as having the value 0.
In the following code, the #if directive explicitly tests whether DEBUG
equals 0.
#undef DEBUG
#if DEBUG == 0
printf( "Hi\n" );
#endif
The result is the same as that of the previous example.
────────────────────────────────────────────────────────────────────────────
NOTE
The defined operator is new under the ANSI C standard. You may see older
programs that use the older directives #ifdef and #ifndef for the same
purpose. These directives are obsolete, but QuickC version 2.5 supports them
for the sake of compatibility. The #ifdef directive is followed by a name
(not in parentheses) and works the same as #if with defined. If the given
name has been defined, #ifdef is true. The #ifndef directive is the opposite
of #ifdef. It is true if the given name is not currently defined.
────────────────────────────────────────────────────────────────────────────
Pragmas
Pragmas are implementationspecific compiler commands.
Although portability is a hallmark of C, the language's creators recognized
that every C compiler will need to support some features unique to its host
machine. The "#pragma directive" offers a way for each C compiler to offer
machine-specific features while retaining overall compatibility with the C
language. Since pragmas are machine-specific by definition, they can be─and
usually are─different for every C compiler.
Pragmas have the same general syntax as preprocessor directives. The pragma
must begin with a number sign (#) and it can't share a line with other
directives or statements except a comment, which must appear to the right of
the pragma.
QuickC supports four pragmas: check_stack, check_pointer, message, and pack.
Each of these pragmas is described in online help.
Some pragmas take arguments, which come after the #pragma keyword. In the
following code, the message pragma displays different messages during
compilation depending on the outcome of an #if test:
#if XT == 1
#pragma message( "Building XT version" )
#elif AT == 1
#pragma message( "Building AT version" )
#endif
The message displayed by the message pragma is visible only if you compile
from the DOS command line with the QCL command.
Chapter 8 Pointers
────────────────────────────────────────────────────────────────────────────
The next two chapters explain pointers─a large and important topic in C.
This chapter explains fundamental techniques: how to use pointers with
various data types and pass them to functions. In Chapter 9, "Advanced
Pointers," we'll explore more advanced pointer techniques, such as multiple
indirection.
If you have never used pointers before, you may want to read this chapter
now and then turn to Chapter 9 after you have had some practice using
pointers in your own programs.
Don't panic!
There's a lot of new information in these two chapters. Don't be discouraged
if you don't grasp it all on a first reading. The idea behind a pointer is
simple, but some advanced pointer techniques are not so easy to follow at
first.
Using Pointers in C
Almost every real-world C program uses pointers in some way or another. Much
of the usefulness of pointers stems from the fact that in C all function
arguments are passed by value. Because a function only receives local copies
of such arguments, it can't change the original values that the arguments
represent. Pointers make this possible.
Here are some common uses of pointers:
■ Manipulating strings
■ Passing command-line arguments to a program at run time
■ Returning more than one value from a function
■ Accessing variables that wouldn't otherwise be visible to a function
■ Manipulating an array by moving pointers to its elements instead of
using array subscripting
■ Accessing the address of a memory area that your program allocates at
run time
■ Passing the address of one function to another function
Pointers to Simple Variables
A pointer variable contains the address of a data object.
Although pointers have many different uses, it takes only a few words to say
what a pointer is. A "pointer" is a variable that contains the address of
some other data object─usually a variable. Because a pointer contains the
other variable's address, it is said to "point to" that variable.
This section uses the program POINTER.C to demonstrate the basic mechanics
of pointers─how to declare and initialize a pointer and use it to access a
simple variable:
/* POINTER.C: Demonstrate pointer basics. */
#include <stdio.h>
main()
{
int val = 25;
int *ptr;
ptr = &val;
printf( " val = %d\n", val );
printf( "*ptr = %d\n\n", *ptr );
printf( "&val = %u\n", &val );
printf( " ptr = %d\n", ptr );
}
Here is the output from POINTER.C:
val = 25
*ptr = 25
&val = 5308
ptr = 5308
(The third and fourth output lines show addresses. These may differ when you
run POINTER.C depending on factors such as available memory.)
POINTER.C creates a pointer variable named ptr and makes ptr point to an
int variable named val. Then it prints the two values to show that ptr
can access the value stored in val. The program goes on to print the
address where val is stored and the address contained in ptr, to show
they are the same.
Declaring a Pointer Variable
Like any variable, a pointer variable must be declared before it is used,
and its value can change in the course of a program. A pointer variable can
have any legal variable name. Here is the pointer declaration from
POINTER.C:
int *ptr;
This declaration states the program has a pointer variable named ptr that
can point to a data object of the int type.
Notice the similarity to a simple variable declaration. As in other cases,
the declaration gives a type (int) and name ( ptr ) for the variable.
Use the indirection operator ( *) to declare a pointer variable.
The indirection operator (*) in front of the name ptr shows this variable
is a pointer. This operator has two different uses in C. In declarations,
such as the one above, it simply means "this is a pointer." In other
contexts, as we'll elaborate throughout this chapter, it means
indirection─using the data object that a pointer points to.
A pointer declaration shows what type of data object a pointer references.
A pointer doesn't have a type in the same sense as other variables. When you
declare a simple variable, the type specifier shows what type of value the
variable stores. When you declare a pointer variable, the type specifier
shows what type of data object the pointer points to.
Thus, in POINTER.C the declaration of the variable val indicates val
stores a value of the type int,
int val = 25;
while the declaration of the variable ptr indicates it points to a data
object of the type int:
int *ptr;
To declare pointers to other types of variables, you can use whatever type
specifier is appropriate. These statements, for instance, declare pointers
to char and float variables:
char *c_ptr, *ch;
float *f_pointer;
Note that if you declare more than one pointer variable in the same line,
each name must be preceded by the indirection operator. The first line in
the previous example declares two pointer variables: c_ptr and ch. Each
pointer can point to an object of the char type. If you omit the second
indirection operator from the first line,
char *c_ptr, ch;
the line declares a pointer variable named c_ptr and an ordinary char
variable named ch.
A pointer declared with type void can point to any type of data object.
In most cases a pointer points to a particular type of object, such as an
int. You can also declare a pointer with type void, which allows it to point
to any type of object.
One use of void pointers is to write a general-purpose function, such as a
sort, that can operate on data of more than one type. Each time you use a
void pointer, you must perform an explicit type cast to show what type of
object it points to on that occasion.
Figure 8.1 shows the relationship between val and ptr in POINTER.C,
immediately after ptr has been declared. The figure shows that the
variable val is stored at memory location 5308, as in the output shown
above. Again, the actual address may differ when you run POINTER.C.
(This figure may be found in the printed book.)
Figure 8.1 uses question marks to show that the contents of ptr are
undefined at this stage in the program. Like any other variable that has
been declared but not initialized, the contents of ptr are unknown. You
must take special care not to use pointers that have not been initialized,
since an uninitialized pointer might point anywhere in memory─including
sensitive operating-system addresses.
────────────────────────────────────────────────────────────────────────────
WARNING
Because a pointer can potentially access any memory address, using an
uninitialized pointer can have drastic consequences.
────────────────────────────────────────────────────────────────────────────
How Pointers Are Stored
Figure 8.1 also shows that while a pointer is a special kind of variable, it
is not a mysterious entity floating in limbo. A pointer is a true variable
whose contents are stored at a specific memory address.
In POINTER.C we don't care precisely where the pointer's contents are
stored─the compiler handles that detail for us, as it does so many others.
So Figure 8.1 does not include the address of the storage for ptr. It does
show, however, that the pointer is stored in two bytes, the same amount of
memory needed to store an int value.
────────────────────────────────────────────────────────────────────────────
NOTE
The actual amount of memory needed to store a pointer variable depends on
the current "memory model." In the small memory model─the default for QuickC
version 2.5─a pointer is stored in two bytes. In some larger memory models,
a pointer is stored in four bytes. For purposes of discussion, this chapter
and the following chapter assume the small memory model. Appendix B,
"Working with QuickC Memory Models," in the Microsoft QuickC Tool Kit
discusses memory models.
────────────────────────────────────────────────────────────────────────────
Initializing a Pointer Variable
The next step in the POINTER.C program is to initialize the pointer variable
ptr, making it point to some meaningful address in memory:
ptr = &val;
The "address-of operator" (&) gives the address of the name it precedes. So
in plain English the above statement says, "assign the address of val to
ptr."
After its initialization, the variable ptr points to val in the sense
that it contains the address where val is stored.
The output from POINTER.C shows that ptr contains the address of val.
First it prints the address of val using the address-of operator to
directly obtain the variable's address,
&val = 5308
then it prints the contents of ptr:
ptr = 5308
The two values are identical. Figure 8.2 shows the relationship of val and
ptr at this stage in the POINTER.C program.
(This figure may be found in the printed book.)
Initialization is especially important for pointers because, as noted
earlier, they have the potential to point anywhere in memory. If you forget
to initialize it, or make it point to the wrong place, a pointer can wreak
havoc with your program or even the operating system itself.
The target of a pointer must be present in memory at run time.
The pointer in POINTER.C points to a simple int variable. As a general rule,
pointers can point to any data object that is present in memory at run time.
This category mainly includes objects for which the program allocates (sets
aside) memory. Memory can be allocated implicitly, by defining a variable or
function, or explicitly, by calling a memory-allocating library function
such as malloc.
A pointer can't point to program elements such as expressions or register
variables, which aren't present in addressable memory.
POINTER.C initializes the pointer ptr by assigning it an address constant
(the address of val, obtained with the address-of operator). You can also
assign the value of one pointer to another, as shown here:
ptr = ptr1;
If ptr and ptr1 are both pointers, this statement assigns the address
contained in ptr1 to ptr.
Using a Pointer Variable
Once ptr points to val, we have two ways to access the int value stored
in val. The usual way is direct, using the name of val:
printf( " val = %d\n", val );
The second way to access val is indirect, using the pointer variable ptr
and the indirection operator:
printf( "*ptr = %d\n\n", *ptr );
Both of the preceding statements print the value of val, confirming that
you can access the contents of val indirectly as well as directly. Once
ptr points to val you can use *ptr anywhere that you would use val.
The indirection operator can obtain the value to which a pointer points.
Using the indirection operator to access the contents of val is the second
use of this operator (the first is in declaring pointer variables, as
explained earlier). When the asterisk appears in front of the name ptr, the
expression states that you want to use the value the pointer points to, not
the value of ptr itself.
The second printf statement in POINTER.C uses the expression *ptr to
access the value stored in val.
This use of a pointer is analogous to the PEEK function in QuickBASIC. You
can just as easily use ptr to change the data in val, an operation that
somewhat resembles a QuickBASIC POKE statement.
For instance, if you add the following statements to the end of POINTER.C,
*ptr = 3301;
printf( "%d\n", val );
the program prints 3301.
Figure 8.3 shows the relationship between ptr and val after executing
the previous two statements.
(This figure may be found in the printed book.)
As Figure 8.3 shows, the value stored in val has changed from 25 to 3301.
The contents of val were changed indirectly, through the pointer ptr.
Summary of Pointer Basics
In the preceding sections, you have seen how to do these operations:
■ Declare a pointer to a simple variable
■ Initialize a pointer, making it point to a variable
■ Use a pointer to get the value of a variable
■ Use a pointer to change the contents of a variable
It's important for you to be comfortable with these basic ideas before
reading about the more advanced uses of pointers. If you're not sure you
understand these concepts, you may want to experiment with the POINTER.C
program to reinforce what you know. For instance, you might add some new
variables of different types and create new pointers to access them.
Pointers to Arrays
Pointers and arrays are closely related in C─a major theme we'll elaborate
throughout the rest of this chapter and Chapter 9, "Advanced Pointers." This
section explains one of the simpler ways to use pointers with arrays.
A pointer to an array, or "array pointer," combines two powerful language
features─the pointer's ability to provide indirect access and the
convenience of accessing array elements through numerical subscripts.
An array pointer can point to any element in a given array.
A pointer to an array is not much different than a pointer to a simple
variable. In both cases, the pointer can point only to a single object at
any given time. An array pointer, however, can reference any individual
element within an array (but just one at a time).
The program PARRAY.C shows how to access the elements of an int array
through a pointer:
/* PARRAY.C: Demonstrate pointer to array. */
#include <stdio.h>
int i_array[] = { 25, 300, 2, 12 };
main()
{
int *ptr;
int count;
ptr = &i_array[0];
for( count = 0; count < 4 ; count++ ) {
printf( "i_array[%d] = %d\n", count, *ptr );
ptr++;
}
}
Here is the output from PARRAY.C:
i_array[0] = 25
i_array[1] = 300
i_array[2] = 2
i_array[3] = 12
The PARRAY.C program creates a four-element int array named i_array. Then
it declares a pointer named ptr and uses ptr in a for loop to access
each of the elements in i_array.
Notice the similarity between PARRAY.C and the previous example (POINTER.C).
The pointer is declared in the same way:
int *ptr;
As noted before, this declaration states that ptr can point to any object
of the int type, which includes an element in an int array as well as a
simple int. The initialization of ptr looks similar, too:
ptr = &i_array[0];
This statement assigns ptr the address of the first element of i_array,
which is i_array[0]. (There's a more compact way to initialize this
pointer, but we'll defer that discussion for a moment.) Figure 8.4 shows the
relationship between ptr and i_array immediately after ptr is
initialized.
(This figure may be found in the printed book.)
Arrays and Pointer Arithmetic
Once a pointer points to an array, it can access any of the array's
elements. By adding or subtracting from the pointer's value (using "pointer
arithmetic") you can access any element in the array, just as you can access
it with array subscripts.
So in PARRAY.C, just as in POINTER.C, we can use *ptr to access the int
value that ptr references. The only difference is now ptr points to an
array element instead of a simple variable.
When the for loop in PARRAY.C executes the first time, ptr points to the
first element of i_array, which is i_array[0]. The second statement in the
loop body,
ptr++;
increments the pointer. Now ptr points to the next element in i_array,
which is i_array[1]. Figure 8.5 shows the relationship of ptr and
i_array after the first iteration of the for loop in PARRAY.C.
(This figure may be found in the printed book.)
Figures 8.4 and 8.5 illustrate another important fact about pointers.
Pointer arithmetic is automatically scaled to the size of the object that a
pointer references. As explained above, incrementing ptr with the
statement
ptr++;
Pointer arithmetic is scaled to the size of elements in an array.
moves the pointer forward to the next element in i_array. Since each
element of an int array contains two bytes, this operation actually adds 2
to the address stored in ptr, but you don't have to worry about that
detail. The compiler knows the size of the elements in the array and adjusts
the pointer accordingly.
Incrementing a pointer adds 1 if it points to a char array, 4 if it points
to a float array, and so on.
You can also decrement an array pointer. If ptr points to i_array[2],
this statement moves the pointer back one element, to i_array[1]:
ptr--;
Although the previous expressions increment and decrement ptr by 1, you
can add or subtract any integer value from a pointer. For instance, the
following statement moves ptr forward three elements in i_array:
ptr += 3;
Be careful not to overrun the bounds of an array when accessing its elements
with a pointer. As noted in Chapter 4, "Data Types," the C language doesn't
check array subscripts. This rule applies equally when you access an array
with a pointer, which can potentially reference any address in memory.
────────────────────────────────────────────────────────────────────────────
WARNING
The C language does not check array pointer references. If you increment or
decrement a pointer past the limits of an array, you can corrupt other parts
of your program or cause other unexpected results.
────────────────────────────────────────────────────────────────────────────
It's your job to make sure an increment or decrement doesn't move a pointer
outside the memory where an array is stored. For instance, if you decrement
ptr when it points to i_array[0], it will point to whatever happens to be
stored in the int-size memory area below the element i_array[0].
Most pointer arithmetic occurs in connection with arrays, where a numerical
index has obvious utility. It's not illegal to do pointer arithmetic on
nonarray pointers, but such operations normally serve no purpose. For
instance, if you increment a pointer to a simple variable, the pointer no
longer points to the variable and becomes useless.
Comparing Pointers
The special nature of a pointer variable─the fact that it contains an
address─ precludes most operations that are legal for other variables.
There's no such thing as a fractional memory address, for example. So it
wouldn't make sense to divide a pointer, or add a floating-point number to
it. The most common pointer operations are assignment, incrementing, and
decrementing, as described earlier. You can also compare one pointer to
another.
If a program allocates memory for a stack, for instance, you might create
two pointers that point to different parts of the stack. One pointer can
show where the stack begins and the other where it ends. To see how much of
the stack is in use, you can subtract the pointers. (A "stack" is a memory
area used for temporary storage.)
You can compare pointer variables with relational operators or by
subtraction.
Pointer comparisons can be done with relational operators (such as ) or by
subtracting one pointer from another. Of course, pointer comparisons are
meaningful only for pointers that point to the same data object or related
objects of the same type.
PARRAY.C Revisited
Before leaving the PARRAY.C program, we should note that most C programmers
would write it more compactly (PARRAY1.C):
/* PARRAY1.C: Compact version of PARRAY.C. */
#include <stdio.h>
int i_array[] = { 25, 300, 2, 12 };
main()
{
int count;
int *ptr = i_array;
for( count = 0; count < 4 ; count++ )
printf( "i_array[%d] = %d\n", count, *ptr++ );
}
You can declare and initialize a pointer variable in one statement.
The PARRAY1.C program uses several shorthand techniques you can expect to
see in C programs. Like other variables, pointers can be initialized at the
same time they are declared. The following statement in PARRAY1.C performs
both operations:
int *ptr = i_array;
The statement above is equivalent to these statements:
int *ptr;
ptr = i_array;
You may have noticed another difference in the way ptr is initialized. The
PARRAY1.C program omits the address-of operator and array subscript that
PARRAY.C used to signify the address of the first element of i_array.
Instead of
&i_array[0]
the program uses
i_array
An array name is a pointer.
In fact, the two expressions are equivalent. In the C language, the name of
an array is actually a pointer. Any array name that doesn't have a subscript
is interpreted as a pointer to the base address of the array. (The "base
address" is the address of the array's first element.) We'll explore this
equivalence further in the following sections and in Chapter 9, "Advanced
Pointers."
Finally, PARRAY1.C uses the expression *ptr++ to perform two jobs:
accessing the value ptr points to and incrementing ptr. Note the order in
which the two operators in this expression take effect. The indirection
operator takes effect first, accessing the value of the array element that
ptr currently points to. Then the increment operator (++) adds 1 to ptr,
making it point to the next element in i_array.
Pointers and Strings
String pointers are handled like other array pointers.
Because a string is an array of characters, pointers to strings are handled
much like other array pointers. The program PSTRING.C is similar to the
examples that demonstrated array pointers (PARRAY.C and PARRAY1.C). It uses
a pointer to access a char array:
/* PSTRING.C: Demonstrate pointer to a string. */
#include <stdio.h>
main()
{
int count;
char name[] = "john";
char *ptr = name;
for( count = 0; count < 4; count++ )
{
printf( "name[%d]: %c\n", count, *ptr++ );
}
}
The PSTRING.C program steps through the name array, printing each
character in turn:
name[0]: j
name[1]: o
name[2]: h
name[3]: n
The notable difference between PARRAY.C and PSTRING.C is that PSTRING.C has
a char array instead of an int array. Again, incrementing an array pointer
moves the pointer to the next array element. So in PSTRING.C each iteration
of the for loop moves the pointer to the next char in the string.
The first time through the loop, ptr points to name[0]. The second time
it points to name[1], and so on.
As mentioned in Chapter 4, "Data Types," one difference between strings and
noncharacter arrays is that strings end with a null character. The string in
PSTRING.C actually contains five characters: four letters and a null
character. We can exploit this fact to simplify the program, as we do below
in PSTRING.C.
/* PSTRING1.C: Look for null at string's end. */
#include <stdio.h>
main()
{
char name[] = "john";
char *ptr = name;
while( *ptr )
printf( "*ptr = %c\n", *ptr++ );
}
Here is the output from PSTRING1.C:
*ptr = j
*ptr = o
*ptr = h
*ptr = n
Like PSTRING.C, the PSTRING1.C program steps through the array one character
at a time. However, it replaces the for loop with a simpler while loop. The
test expression in the while loop,
while( *ptr )
is evaluated as true until ptr points to the null character that
terminates the string. It's a more compact way of writing this expression:
while( *ptr != 0 )
Any operation done with array subscripts can also be done with pointer
notation.
This is an ideal time to elaborate on the relationship between arrays and
pointers. Any operation you can do with conventional array notation
(subscripts) can also be done with pointers. This is possible because an
array name, as we noted earlier, is itself a pointer.
To illustrate, the PSTRING2.C program uses only array notation:
/* PSTRING2.C: Demonstrate strings and array notation. */
#include <stdio.h>
#include <string.h>
main()
{
int count;
char name[] = "john";
for( count = 0; count < strlen( name ); count++ )
printf( "name[%d]: %c\n", count, name[count] );
}
PSTRING2.C gives the same output as PSTRING.C. In this program, the
expression
name[count]
uses count as in an index to the name array.
PSTRING3.C is the same program written with pointer notation:
/* PSTRING3.C: Strings and pointer notation. */
#include <stdio.h>
#include <string.h>
main()
{
int count;
char name[] = "john";
for( count = 0; count < strlen( name ); count++ )
printf( "*(name+%d) = %c\n", count,*(name+count) );
}
Here is the output from PSTRING3.C:
*(name+0) = j
*(name+1) = o
*(name+2) = h
*(name+3) = n
Notice how PSTRING3.C replaces the expression
name[count]
with the expression:
*(name+count)
Both expressions use the variable count as an offset from the base address
of the array. The parentheses in the second expression are important. They
are necessary because the indirection operator takes effect before the
addition operator. If you omit the parentheses, as in
*name+count
the expression has the same effect as
(*name)+count
which adds the value of count to the object name references.
In summary, the examples in this section show three alternative ways to
access a character inside a string. In the printf statements in the
examples, these expressions are equivalent:
*ptr
name[count]
*(name+count)
Many C programmers prefer pointer notation to array notation because
pointers are faster for some operations. In other cases─including the one
above─the choice is entirely one of taste. There's more to say about the
relationship between pointers and arrays. We'll return to this topic later
in this chapter and in Chapter 9, "Advanced Pointers."
Passing Pointers to Functions
A function that receives pointers can access variables that are local to
other functions.
One of the most common uses of pointers is to pass them as arguments to
functions. Functions that receive variables as parameters get local copies
of those variables, not the originals. In contrast, functions that receive
pointers to variables gain access to the original variables associated with
the pointers. This allows the functions to
■ Return more than one value
■ Read and change values in variables─including arrays and
structures─that otherwise aren't visible to the function
The first item listed above relates to the return statement. As we noted in
Chapter 2, "Functions," a function can return only one value through return.
How-ever, it's not difficult to imagine a useful function─a sort, for
instance─that would return more than one value. Pointers offer an elegant
solution.
The second item involves visibility. Most variables in C programs are local
to the functions where they are defined, and a function normally can't
access local variables in other functions. There are times, however, when
you want a function to have access to a local variable defined elsewhere in
the program. By passing the function a pointer to the local variable, you
can give it access to the variable itself.
The PFUNC.C program illustrates both ideas. It has a function that returns
more than one value and uses pointers to alter variables that aren't visible
within the function:
/* PFUNC.C: Pass pointers to a function. */
#include <stdio.h>
void swap( int *ptr1, int *ptr2 );
main()
{
int first = 1, second = 3;
int *ptr = &second;
printf( "first: %d second: %d\n", first, *ptr );
swap( &first, ptr );
printf( "first: %d second: %d\n", first, *ptr );
}
void swap( int *ptr1, int *ptr2 )
{
int temp;
temp = *ptr1;
*ptr1 = *ptr2;
*ptr2 = temp;
}
Here is the output from PFUNC.C:
first: 1 second: 3
first: 3 second: 1
Pointers can eliminate the need for external variables.
The PFUNC.C program swaps the values of two int variables named first and
second, using a function named swap. Since the exchange involves two
values, the swap function can't use return to communicate its results.
More-over, the variables first and second are defined only in the main
function, and as good C programmers, we want to exchange their values
without making them externally visible.
The prototype for the swap function shows that swap expects to receive
two pointers to int variables:
void swap( int *ptr1, int *ptr2 );
Notice the use of void in the prototype and function definition. The void
specifier shows that the swap function doesn't return any value through a
return statement. Instead, swap returns its results indirectly, through the
action of pointers.
The variables we want to exchange are defined only in main:
int first = 1, second = 3;
No other function in the program can access these variables directly by
using the variable names first and second. We must pass these variables
as arguments; but since the C language passes arguments by value, we need to
pass pointers to the variables.
The main function calls swap with the following statement:
swap( &first, ptr );
This statement shows two different ways to pass a pointer to a function. The
first argument in the function call,
&first
passes the address of first as a constant, using the address-of operator.
The second argument,
ptr
passes the address of second with a pointer variable. Earlier in PFUNC.C
we declared ptr as a pointer to an int and assigned it the address of
second:
int *ptr = &second;
Both arguments pass the same kind of data─the address of a local variable─to
the function. We'll return to this idea after we see how the rest of PFUNC.C
works.
When the swap function executes, it creates two int pointers named ptr1
and ptr2 and assigns the passed addresses to them:
void swap( int *ptr1, int *ptr2 )
Since there's a one-to-one correspondence between arguments and parameters,
the pointer ptr1 receives the address of first and ptr2 receives the
address of second. The swap function exchanges the values of first and
second, using the two pointers and a temporary int variable named temp:
int temp;
temp = *ptr1;
*ptr1 = *ptr2;
*ptr2 = temp;
Within the swap function, PFUNC.C uses the indirection operator to access
the values that ptr1 and ptr2 reference. The expression *ptr1 accesses
the value stored in first. Likewise, the expression *ptr2 accesses the
value stored in second.
Through the addresses contained in the pointers, the swap function can
indirectly access variables that are local to the main function.
Passing Address Constants Versus Passing Pointer Variables
Now that you know how the swap function works, we can elaborate on the two
methods that PFUNC.C uses to pass the address of first and second to
swap.
When you pass a pointer to a function, the function actually receives an
address.
Earlier, we said the swap function expects to receive two pointers as
parameters. While it's common to say pointers in this context, it would be
more accurate to say the function expects addresses, since that's what it
actually receives.
To work correctly, swap only needs the addresses of two variables. Once it
has the addresses, it assigns them to its own local pointers and proceeds to
do its work─modifying the original variables at long distance, as it were.
The swap function doesn't care whether you pass the addresses as constants
or pointer variables, since it receives the same kind of value in either
case. The address is all the function needs to change the value of a
variable defined elsewhere.
The first argument in the function call to swap shows a straightforward
way to pass an address. Inside the main function of PFUNC.C, the expression
&first equals the address of first. When you pass this argument to swap,
the function clearly receives an address.
The second argument is an address, too. Since main assigns the address of
second to the pointer variable ptr, the expression ptr equals the
address of second. When you pass this argument to swap, the function also
receives an address. (Remember, the value contained in a pointer variable is
an address.)
Some beginning programmers get confused by functions that expect to receive
pointers, thinking they must always pass pointer variables to such
functions. As PFUNC.C shows, if the function expects an address you can
simply pass the address as a constant, using the address-of operator.
────────────────────────────────────────────────────────────────────────────
NOTE
When a function expects to receive an address as a parameter, you can pass
either an address constant or a pointer variable, whichever is more
suitable.
────────────────────────────────────────────────────────────────────────────
Why, then, would you ever go to the trouble of passing a pointer variable to
this kind of function? In a real program, the function that calls swap
might well use pointers to process first and second for some other
purpose. In such a case you might prefer to use pointers in the function
call, too.
Arrays of Pointers
Pointers, like other variables, can be stored in arrays. This feature allows
you to create a variety of useful data structures.
In an array of pointers, each array element is a pointer variable.
If you find an array of pointers hard to picture, begin with the idea that
an array is a group of variables of the same type. An "array of pointers" is
also a group of variables, but instead of simple variables, it contains a
group of pointer variables.
Each element in an array of pointers, then, is a pointer that contains an
address. Like other array elements, each element can be accessed with a
numerical subscript.
Pointer arrays are often used to speed up sorts. The QCSORT.C program shows
the basic idea behind such a sort:
/* QCSORT.C: Demonstrate sorting array of pointers. */
#include <stdio.h>
#define SIZE 4
void sort( int size, double *p[] );
void show( int size, double *p[], double dd[] );
main()
{
int x;
double d[] = { 3.333, 1.111, 2.222, 4.444 };
double *d_ptr[SIZE];
for( x = 0; x < SIZE; x++ )
d_ptr[x] = &d[x];
show( SIZE, d_ptr, d );
sort( SIZE, d_ptr );
show( SIZE, d_ptr, d );
}
void sort( int size, double *p[] )
{
int x, x1;
double *temp;
for( x = 0; x < size - 1; x++ )
for( x1 = x + 1; x1 < size; x1++ )
{
if( *p[x] > *p[x1] )
{
temp = p[x1];
p[x1] = p[x];
p[x] = temp;
}
}
}
void show( int size, double *p[], double dd[] )
{
int x;
printf( "------------------------" );
printf( "------------------------\n" );
for( x = 0; x < size; x++ )
{
printf( "*d_ptr[%d] = %1.3f ", x, *p[x]);
printf( "d_ptr[%d] = %u ", x, p[x]);
printf( " d[%d] = %1.3f\n", x, dd[x] );
}
}
Here is the output from QCSORT.C:
------------------------------------------------
*d_ptr[0] = 3.333 d_ptr[0] = 66 d[0] = 3.333
*d_ptr[1] = 1.111 d_ptr[1] = 74 d[1] = 1.111
*d_ptr[2] = 2.222 d_ptr[2] = 82 d[2] = 2.222
*d_ptr[3] = 4.444 d_ptr[3] = 90 d[3] = 4.444
------------------------------------------------
*d_ptr[0] = 1.111 d_ptr[0] = 74 d[0] = 3.333
*d_ptr[1] = 2.222 d_ptr[1] = 82 d[1] = 1.111
*d_ptr[2] = 3.333 d_ptr[2] = 66 d[2] = 2.222
*d_ptr[3] = 4.444 d_ptr[3] = 90 d[3] = 4.444
Since the purpose of QCSORT.C is to demonstrate pointers, not sorting
methods, it uses a simple bubble sort. This method isn't efficient but has
the advantage of being short and easy to follow.
The QCSORT.C program creates a double array named d and an array of
pointers named d_ptr. Each array has four elements. To illustrate the sort,
the elements of d are initialized out of order.
The goal of QCSORT.C is to display a sorted list of the values in d. You
could do this by sorting the elements of d itself, but that solution is
not efficient. Every double value contains eight bytes, and sorting a large
number of double values requires that you move a lot of memory.
Instead of moving the double values themselves, QCSORT.C creates an array of
pointers that point to the elements of the d array, then sorts the
pointers. This saves time because a pointer is stored in only two bytes.
Figure 8.6 shows the relationship between the d and d_ptr arrays
immediately after both are initialized.
(This figure may be found in the printed book.)
At the stage shown in Figure 8.6, the pointers in the d_ptr array have
been initialized to point to the double elements in the d array. (The
array element d_ptr[0] points to d[0], d_ptr[1] points to d[1], and so
on.) The function show displays three sets of data:
■ The value each pointer references
■ The address assigned to each pointer
■ The value of each element in the d array
After calling the show function, QCSORT.C calls the sort function, which
sorts the pointers in d_ptr.
The declaration of sort contains something new. In the declaration
void sort( int size, double *p[] );
the expression *p[] shows that the sort function expects to receive a
pointer to an array of pointers. When the program calls sort, it passes the
size of the array to be sorted (first argument) and a pointer to the array
of pointers (second argument):
sort( SIZE, d_ptr );
Now the sort function has all the information it needs to sort the
pointers in the d_ptr array, making each pointer point to the correct
element in the d array.
After the sort is complete, QCSORT.C calls show again to display the
results of the sort. Now that the pointers have been sorted, they can be
used to display a sorted list of double values. Figure 8.7 shows the
relationship between the d and d_ptr arrays after the sort is complete.
(This figure may be found in the printed book.)
Of course, the array in QCSORT.C is so small that the time savings from
using pointers is negligible. In a real program, however, which might sort
thousands of values instead of four, the difference between moving eight
bytes and two bytes can be dramatic. The advantage of sorting pointers is
even greater when sorting large data objects such as strings or structures.
The elements in a pointer array can point to any type of data.
The QCSORT.C example section uses a fairly simple array of pointers. But you
can use such arrays to create quite complex data structures. The basic form
of the array is always the same─it is a group of pointer variables, each
pointer accessible through a subscript─but the pointers in an array can
point to any kind of data object. You can have an array of pointers to
structures, an array of pointers to strings, and so on. The only difference
is in what the pointers reference.
Don't confuse an array of pointers with a pointer to an array. A pointer to
an array (or "array pointer") is a single pointer variable that points to an
array element. The single pointer can access any element of the array, but
only one pointer is involved.
In contrast, an array of pointers is a group of related pointer variables
stored in an array. Each element in the array is a pointer, and you can
access individual pointers with the array name and subscript. Each pointer
in the array points, in turn, to some other object.
A Pause for Reflection
If this is your first exposure to pointers, you may want to reflect on what
you have learned before reading the next chapter. This chapter has explained
the basic uses of pointers, and you can write a great many useful programs
using only these techniques. If you're not comfortable with all these ideas,
you may want to experiment with them before reading more about pointers.
The next chapter, "Advanced Pointers," examines further uses of pointers,
including multiple indirection and pointers to structures.
Chapter 9 Advanced Pointers
────────────────────────────────────────────────────────────────────────────
The preceding chapter, "Pointers," explained the basics of using pointers─
how to declare and initialize pointer variables and use them to access basic
data types. This chapter explores more advanced pointer techniques,
including multiple indirection, pointers to structures, and pointers to
functions.
Pointers to Pointers
In Chapter 8, "Pointers," we stated a pointer can point to any kind of
variable. Since a pointer is a variable, you can make it the target of
another pointer, creating a pointer to a pointer. This concept is useful in
itself and is also important for understanding the equivalence of array
notation and pointer notation, which is explained in the next section.
The program PTRPTR.C demonstrates a pointer to a pointer insimple terms:
/* PTRPTR.C: Demonstrate a pointer to a pointer. */
#include <stdio.h>
main()
{
int val = 501;
int *ptr = &val;
int **ptr_ptr = &ptr;
printf( "val = %d\n", **ptr_ptr );
}
Here is the output from PTRPTR.C:
val = 501
The first two statements in PTRPTR.C should look familiar by now. They
create an int variable named val and an int pointer named ptr. The third
line, however, requires some explanation:
int **ptr_ptr = &ptr;
This statement uses double indirection to create a variable named ptr_ptr,
which is a pointer to a pointer. This pointer is assigned the address of the
first pointer, ptr. The pointer ptr references val, and the pointer
ptr_ptr references ptr. Figure 9.1 illustrates the relationship between
ptr and ptr_ptr.
(This figure may be found in the printed book.)
Once we have initialized both pointers, we can use ptr_ptr to access val:
**ptr_ptr
The double indirection operator ( ** ) is used with a pointer to a pointer.
The double indirection operator (**) in front of ptr_ptr tells two things
about ptr_ptr: that ptr_ptr is itself a pointer and it points to a second
pointer. Both asterisks are needed to access the contents of val. If you
use only one, as in
*ptr_ptr
then ptr_ptr accesses the contents of ptr, which is the address of val.
This statement, for instance, prints the address stored in ptr:
printf( "ptr = %u", *ptr_ptr );
Using pointers to pointers is known as "multiple indirection." One pointer
points to a second pointer, which in turn accesses a third data object. In
theory, there's no limit to how far you can take multiple indirection. You
can create pointers to pointers, pointers to pointers to pointers, and so
on. However, there's rarely any practical reason to carry indirection beyond
two levels (a pointer to a pointer).
Equivalence of Array and Pointer Notation
In previous sections we noted, more or less in passing, two important facts
about arrays and pointers:
1. An array name is actually a pointer.
2. Array notation (subscripts) and pointer notation are interchangeable.
These ideas are significant enough to warrant an explicit demonstration.
Let's rewrite the QCSORT.C program using pointer notation:
/* QCSORT1.C: Demonstrate sort with pointer notation. */
#include <stdio.h>
#define SIZE 4
void sort( int size, double **p );
void show( int size, double **p, double dd[] );
main()
{
int x;
double d[] = { 3.333, 1.111, 2.222, 4.444 };
double *d_ptr[SIZE];
for( x = 0; x < SIZE; x++ )
d_ptr[x] = &d[x];
show( SIZE, d_ptr, d );
sort( SIZE, d_ptr );
show( SIZE, d_ptr, d );
}
void sort( int size, double **p )
{
int x, x1;
double *temp;
for( x = 0; x < size - 1; x++ )
for( x1 = x + 1; x1 < size; x1++ )
{
if( **(p+x) > **(p+x1) )
{
temp = *(p+x1);
*(p+x1) = *(p+x);
*(p+x) = temp;
}
}
}
void show( int size, double **p, double dd[] )
{
int x;
printf( "------------------------" );
printf( "------------------------\n" );
for( x = 0; x < size; x++ )
{
printf( "*d_ptr[%d] = %1.3f ", x, **(p+x) );
printf( "d_ptr[%d] = %u ", x, *(p+x) );
printf( " d[%d] = %1.3f\n", x, dd[x] );
}
}
The QCSORT1.C program works like its predecessor, QCSORT.C. (It sorts an
array of pointers that point to elements in an int array.) The only
difference is QCSORT1.C uses pointer notation instead of array notation.
Let's look at how the change affects the sort function, beginning with its
prototype. In the previous program, QCSORT.C, the prototype
void sort( int size, double *p[] );
uses array notation to show we'll pass the name of an array of pointers to
sort. Since an array name is a pointer, we can rewrite the prototype using
pointer notation, as in QCSORT1.C:
void sort( int size, double **p );
The sort function definition is rewritten in the same way. Here is the
definition of sort in the original program (QCSORT.C):
void sort( int size, double *p[] )
{
int x, x1;
double *temp;
for( x = 0; x < size - 1; x++ )
for( x1 = x + 1; x1 < size; x1++ )
{
if( *p[x] > *p[x1] )
{
temp = p[x1];
p[x1] = p[x];
p[x] = temp;
}
}
}
The same function using pointers looks like this in QCSORT1.C:
void sort( int size, double **p )
{
int x, x1;
double *temp;
for( x = 0; x < size - 1; x++ )
for( x1 = x + 1; x1 < size; x1++ )
{
if( **(p+x) > **(p+x1) )
{
temp = *(p+x1);
*(p+x1) = *(p+x);
*(p+x) = temp;
}
}
}
Within the sort function, the variable p is a pointer to a pointer. When
we use a single asterisk, as in,
*(p+x1)
we access the contents of the x1 pointer, which is an address. When we
place a double asterisk in front of an address value, as in,
**(p+x)
we access the contents of this address.
Using pointer notation in place of array notation, QCSORT1.C achieves the
same result as QCSORT.C. In many cases─including this one─it doesn't really
matter which notation you use. If you're still more comfortable with array
notation, you may prefer to use it sometimes. Since many C programs use
pointers to manipulate arrays, however, it's worth taking the time to learn
pointer notation, too.
Getting Command-Line Arguments
Command-line arguments are passed to programs through argv, an array of
pointers.
Arrays of pointers have one very common use─accessing command-line
arguments. When a C program begins execution, DOS passes two arguments to
it. The first argument, normally called argc, is an int variable that
indicates the number of command-line arguments. The second, normally called
argv, is a pointer to an array of strings. Each string in the array contains
one of the command-line arguments.
Even if you don't plan to use argc and argv in your programs, you can
expect to see them often in other C programs, so it's useful to know how
they're used. The ARGV.C program uses argc and argv.
/* ARGV.C: Demonstrate accessing command-line arguments. */
#include <stdio.h>
void show_args( char *argument );
int main( int argc, char *argv[] )
{
int count;
for( count=0; count < argc; count++ )
show_args( argv[count] );
return 0;
}
void show_args( char *argument )
{
printf( "%s\n", argument );
}
To make ARGV.C produce output, you must give it some command-line arguments.
(If you run ARGV.C in the QuickC environment, select Run/Debug from the
Options menu and type the command-line arguments at the Command Line
prompt.) The program prints each argument on the screen.
If you use this command line, for instance,
argv harpo chico groucho zeppo
then ARGV.C gives this output:
C:\SOURCES\ARGV.EXE
harpo
chico
groucho
zeppo
The first argument may have surprised you. In DOS versions 3.0 and higher,
the first string in the argv array ( argv[0] ) contains the drive
specification and full pathname to the program that is executing. The drive
and path you see will depend on how your system is configured. In the
example the ARGV.EXE program is located in the SOURCES directory of drive C.
Thus, the value of argc actually is one greater than the number of
command-line arguments, and the first argument typed on the command line is
the second string in the array ( argv[1] ). If you type the arguments shown
above, the value of argc is 5 and argv[1] contains the argument harpo.
Null Pointers
We can use the ARGV.C program to illustrate another handy property of
pointers: null pointers. Consider this modification (ARGV1.C):
/* ARGV1.C: Demonstrate null pointers. */
#include <stdio.h>
void show_args( char *argument );
int main( int argc, char **argv )
{
while( *argv )
show_args( *(argv++) );
return 0;
}
void show_args( char *argument )
{
printf( "%s\n", argument );
}
The ARGV1.C program gives the same output as the previous program but it
uses a while loop instead of a for loop. The test expression in this loop,
while( *argv )
is equivalent to this test expression:
while( *argv != 0 )
The loop in ARGV1.C continues until it finds a "null pointer," a pointer
that contains 0. In this case, the null pointer means we have reached the
end of the array: no more strings are available.
Null pointers can be used to show success or failure and as markers in a
series.
Many C library functions use null pointers to signal the success or failure
of an operation that returns a pointer. For instance, the library function
malloc normally returns a pointer to the beginning address of the memory
area it allocates. If no memory is available, malloc returns a null pointer
to show the operation failed. Similarly, the fopen function usually returns
a pointer to a FILE structure, but returns a null pointer when it fails.
Null pointers can also be used to mark the end of a list of pointers, such
as the argv array or a linked list.
Pointers to Structures
A structure pointer can access any member of a structure.
A pointer to a structure, or "structure pointer," is conceptually similar to
an array pointer. Just as an array pointer can point to any element in an
array, a structure pointer can reference any member in a structure. The
major difference is one of notation.
In case you're not yet an expert on structure notation, let's review it very
briefly. First recall that each element in an array has the same type, so
you refer to individual array elements with subscripts:
i_array[3]
Because members of a structure can have different types, you can't use
numerical subscripts to refer to them based on their order. Instead, each
structure member has a symbolic name. You refer to a member with a structure
name and member name, separating the two names with the member-of operator
(.):
jones.name
The notation for structure pointers follows the same pattern, with only two
differences. You must
1. Replace the structure name with the name of the pointer
2. Replace the member-of operator with a two-character operator called
the "pointer-member" operator (->)
The pointer-member operator is formed by a dash and a right-angle bracket.
The following name uses the pointer-member operator:
jones_ptr->name
Here jones_ptr is the name of a pointer to a structure, and name is a
member of the structure that jones_ptr points to.
The EMPLOY1.C program is a revision of the EMPLOYEE.C program that
demonstrates structures in Chapter 4, "Data Types." This program illustrates
how to manipulate a structure through a pointer:
/* EMPLOY1.C: Demonstrate structure pointers. */
#include <stdio.h>
struct employee
{
char name[10];
int months;
float wage;
};
void display( struct employee *e_ptr );
main()
{
struct employee jones =
{
"Jones, J",
77,
13.68
};
display( &jones );
}
void display( struct employee *e_ptr )
{
printf( "Name: %s\n", e_ptr->name );
printf( "Months of service: %d\n", e_ptr->months );
printf( "Hourly wage: %6.2f\n", e_ptr->wage );
}
Structure pointers allow functions to access structures that are local
to other functions.
The EMPLOY1.C program gives the same output as the earlier version. But
instead of passing the entire structure to the display function, this
program passes a structure pointer. This method conserves memory, since the
display function doesn't create a local copy of the structure. It also
allows display to change members in the original structure, which is local
to the main function.
The header of the display function shows that the function expects to
receive a structure pointer:
void display( struct employee *e_ptr )
The expression in parentheses specifies what type of value the function
expects. This expression is a bit complex, so let's look at each part
individually. The expression *e_ptr indicates the function expects to
receive a pointer, which it names e_ptr. It is preceded by
struct employee
which states what type of pointer e_ptr is. The struct keyword indicates
e_ptr is a pointer to a structure, and the tag employee specifies the
structure type.
The next item of interest in EMPLOY1.C is the function call that passes the
structure pointer:
display( &jones );
This statement uses the address-of operator to pass the address of the
jones structure to the display function. The address-of operator is not
optional. Since we want the function to access the original structure─not a
local copy─we must pass the structure's address.
When the display function executes, it creates a pointer variable named
e_ptr and assigns to it the address passed in the function call. Now the
display function can refer to any member of the structure indirectly
through the pointer e_ptr. Within the display function, the statement
printf( "%s\n", e_ptr->name );
has the same effect that the statement
printf( "%s\n", jones.name );
has in the main function. Figure 9.2 illustrates the relationship between
the structure pointer and structure members in EMPLOY1.C.
(This figure may be found in the printed book.)
Just to confirm that the display function can access the original
structure in EMPLOY1.C, try adding this statement to the end of the display
function:
strcpy( e_ptr->name, "King, M" );
and this statement to the end of the main function:
printf( "%s\n", jones.name );
These changes cause EMPLOY1.C to print:
King, M
Acting indirectly through a structure pointer, the display function was
able to change a structure defined elsewhere in the program.
Pointers to Functions
At the beginning of the previous chapter we stated that a pointer can point
to any object present in memory at run time. Since functions themselves are
located in memory, you can assign the address of a function to a pointer,
creating a "function pointer."
A function pointer makes it possible to pass a function as a function
argument.
Function pointers provide a way─in fact, the only practical way─to pass a
function as an argument to another function. This permits the second
function to call the first function indirectly through the pointer.
While function pointers may sound rather obscure, they have some common
practical uses:
■ Some QuickC run-time library functions, such as qsort, expect to
receive a pointer to a user-defined function in your program. (Online
help includes an example program that uses qsort.)
■ Function pointers are used extensively in Windows and OS/2
Presentation Manager programs.
■ Using an array of function pointers, you can create a "dispatch
table." A dispatch table is a list of related functions that can be
called based on some choice made at run time. It is similar to an ON
GOSUB statement in BASIC or a call table in assembly language.
The syntax for function pointers is a bit complex, so let's start with a
simple example. The FUNCPTR.C program creates a pointer to our old friend,
printf, and calls printf through the pointer:
/* FUNCPTR.C: Demonstrate function pointers. */
#include <stdio.h>
main()
{
int (*func_ptr) ();
func_ptr = printf;
(*func_ptr) ( "Curiouser and curiouser...\n" );
}
Here is the output from FUNCPTR.C:
Curiouser and curiouser...
This line from FUNCPTR.C declares func_ptr as a pointer to a function:
int (*func_ptr) ();
The declaration of a function pointer must use the same type specifier as
the function it references. If the function returns a float value, the
pointer uses type float, and so on. Since the printf function returns an int
value showing how many characters it displays, the declaration of func_ptr
uses the type int.
A function-pointer declaration must have two pairs of parentheses.
Function-pointer declarations may look complex, but all the parentheses are
essential. The empty parentheses at the end of the declaration are needed to
show the pointer points to a function.
The parentheses enclosing the function name itself are mandatory, too.
Notice what happens if you omit them:
void *func_ptr(); /* Error! Not a function pointer. */
Instead of declaring a pointer to a function, this statement declares a
function that returns a pointer─not at all what we want in FUNCPTR.C.
The next program line initializes the function pointer, assigning it the
address of the printf function:
func_ptr = printf;
This line has two important features. First, notice the name printf isn't
followed by parentheses, as it would be when you call printf directly. We
want to obtain the address of printf, not call it.
Second, note that it's not necessary to place the address-of operator before
the name printf. Because func_ptr was declared as a function pointer, the
compiler knows it should use the address of printf here. If you like,
however, you can add the address-of operator to make the statement a little
more readable:
func_ptr = &printf;
The next line calls the printf function indirectly through the pointer
func_ptr:
(*func_ptr) ( "Curiouser and curiouser...\n" );
Note the similarity between this statement and a normal call to printf. It's
equivalent to this line:
printf( "Curiouser and curiouser...\n" );
To call printf indirectly through func_ptr, you supply the same arguments
as when you call printf directly.
Passing Function Pointers as Arguments
Function pointers are usually passed as function arguments.
Like other pointers, function pointers can be passed as arguments to
functions. Normally, in fact, this is the only reason to use a function
pointer.
The FUNCPTR.C program in the previous section is easy to follow but not very
practical. In a real program, you wouldn't go to the trouble of creating a
function pointer just to call printf from the main function.
The FUNCPTR1.C program demonstrates how to pass a function pointer as an
argument. It has a function named gimme_func that expects to be passed a
function pointer:
/* FUNCPTR1.C: Passing function pointers as arguments. */
#include <stdio.h>
void gimme_func( void (*func_ptr) () );
main()
{
gimme_func( puts );
gimme_func( printf );
}
void gimme_func( void (*func_ptr) () )
{
(*func_ptr) ( "Ausgezeichnet!" );
}
Here is the output from FUNCPTR1.C:
Ausgezeichnet!
Ausgezeichnet!
In the interests of brevity, the function gimme_func does a very simple
job. It expects to receive a pointer to a function that can display a string
and uses that pointer to print the string. The first call to gimme_func
passes a pointer to the library function puts, and the second passes a
pointer to printf.
Since the declaration of gimme_func states it takes a pointer to a
function, the address-of operator is optional in a call to gimme_func. The
following statements are equivalent:
gimme_func( puts );
gimme_func( &puts );
A Parting Word on Pointers
If you have read the previous two chapters from beginning to end, you may be
suffering from a mild─or perhaps not so mild─case of information overload.
Pointers have so many different uses that it's difficult to learn everything
about them at once.
Don't be discouraged if some uses of pointers still aren't clear to you. The
latter parts of this chapter cover some rather esoteric techniques, which
you probably won't use often. When needed, however, these techniques offer
some very powerful capabilities.
Like other programming concepts, pointers are best learned through practice,
so use them at every sensible opportunity. Remember, you don't need to know
everything about pointers in order to do something with them. The more you
use pointers in everyday programming, the sooner all the pieces of the
puzzle will fall into place.
Chapter 10 Programming Pitfalls
────────────────────────────────────────────────────────────────────────────
In C, as in every language, it's rare for any program to work perfectly the
first time. An important part of knowing a language is recognizing what not
to do and why certain problems occur.
This chapter describes common C programming pitfalls and how to avoid them.
It is organized under broad topics, such as "Pointer Problems," with a
category for miscellaneous problems at the end. The description of each
error gives a code example, explains why the error occurs, and offers a
solution.
Operator Problems
The most common operator problems involve operators unique to C. Others
involve questions of precedence, which can cause problems in any language.
Confusing Assignment and Equality Operators
A common error is to confuse the assignment operator (=) with the equality
operator (==). The mistake often occurs in decision-making statements:
int val = 555;
if( val = 20 ) /* Error! */
printf( "val equals 20\n" );
The above code prints val equals 20 even though it's clear val doesn't
equal 20 when the if statement begins. Instead of testing whether x equals
20, the expression val = 20 assigns the value 20 to val.
Remember, the single equal sign (=) performs an assignment in C. This
particular assignment results in a nonzero value, so the if test is
evaluated as true, causing the printf statement to execute.
To correct the problem, use the double equal sign (==) to test equality:
if( x == 20 )
printf( "x equals 20\n" );
Once you're in the habit of using the equality operator, you might make the
opposite mistake of using two equal signs where you should use only one:
main()
{
int val;
for( val == 0; val < 5; val++ ) /* Error! */
printf( "val = %d\n", val );
}
Here the error appears in the initializing expression of the for statement.
It's the reverse of what happened in the first example. Instead of assigning
the value 0 to val, the expression val == 0 evaluates whether or not val
equals 0. The expression doesn't change the value of val at all. Since
val is an uninitialized variable, the for loop is unpredictable.
Confusing Operator Precedence
Peculiar things can happen if you ignore operator precedence:
main()
{
int ch;
while( ch = getch() != '\r' )
printf( "%d\n", ch );
}
Instead of assigning the result of the getch library-function call to ch,
the above code assigns the value 0 to ch when you press the ENTER key and
the value 1 when you press any other key. (The values 1 and 0 represent true
and false.)
The error occurs because the inequality operator (!=) has higher precedence
than the assignment operator (=). The expression
ch = getch() != '\r'
is the same as
ch = (getch() != '\r')
Both expressions compare the result of the getch call to the character
constant \r. The result of that comparison is then assigned to ch.
For the program to work correctly, these operations must happen in the
reverse order. The result of the function call must be assigned to the
variable before the variable is compared to the constant. We can solve the
problem by adding parentheses:
main()
{
int ch;
while( (ch = getch()) != '\r')
printf( "%d\n", ch );
}
Parentheses have the highest precedence of any operator, so the expression
(ch = getch()) != '\r'
works correctly. It assigns the result of the getch call to ch before
comparing ch to the constant.
The list of precedence-related errors is almost endless. Fortunately, QuickC
makes it unnecessary to memorize precedence rules. To view a complete table
of operator precedences, see Appendix A, "C Language Guide," and online help
in the QuickC environment.
Use parentheses to avoid operator precedence problems.
When in doubt, use extra parentheses to make the order of operations
absolutely clear. Extra parentheses don't degrade performance, and they can
improve readability as well as minimize precedence problems.
Confusing Structure-Member Operators
Two different operators are used to access the members of a structure. Use
the structure-member operator (.) to access a structure member directly, and
the pointer-member operator (->) to access a structure member indirectly
through a pointer.
For instance, you may create a pointer to a structure of the employee
type,
struct employee *p_ptr;
and initialize the pointer to point to the jones structure:
p_ptr = &jones;
If you use the structure-member operator to access a structure member
through the pointer,
p_ptr.months = 78; /* Error! */
QuickC issues this error message:
C2040: '.' requires struct/union name
Use the pointer-member operator to access a structure member through a
pointer:
p_ptr->months = 78;
Array Problems
The most common errors associated with arrays involve indexing errors. The
problems described in this section all concern indexing errors of one form
or another.
Array Indexing Errors
The first C array subscript is 0.
If you're used to a language that has different subscripting rules, it's
easy to forget that the first subscript of a C array is 0 and the last
subscript is 1 less than the number used to declare the array. Here's an
example:
int i_array[4] = { 3, 55, 600, 12 };
main()
{
int count;
for( count = 1; count < 5; count++ ) /* Error! */
printf( "i_array[%d] = %d\n", i_array[count] );
}
The for loop in the above program starts at i_array[1] and ends at
i_array[4]. It should begin with the first element, i_array[0] and end at
the last, i_array[3]. The following corrects the error.
for( count = 0; count < 4; count++ )
printf( "i_array[%d] = %d\n", i_array[count] );
Omitting an Array Subscript in Multidimensional Arrays
Enclose each subscript in its own set of brackets.
Programmers who know QuickBASIC, QuickPascal, or FORTRAN may be tempted to
place more than one array subscript in the same pair of brackets. In C, each
subscript of a multidimensional array is enclosed in its own pair of
brackets:
int i_array[2][2] = { { 12, 2 }, { 6, 55 } };
main()
{
printf( "%d\n", i_array[ 0, 1 ] ); /* Error! */
}
In the preceding example, the expression
i_array[ 0, 1 ]
does not access element 0,1 of i_array . Here is the correct way to refer
to that array element:
i_array[0][1]
Interestingly, the deviant array reference doesn't cause a syntax error. As
mentioned in Chapter 6, "Operators," it's legal to separate multiple
expressions with a comma operator, and the final value of such a series is
the value of the rightmost expression in the group. Thus, the expression
i_array[ 0, 1 ]
is equivalent to this one:
i_array[ 1 ];
Both expressions give an address, not the value of an array element.
Overrunning Array Boundaries
Since C doesn't check array subscripts for validity, you must keep track of
array boundaries on your own. For instance, if you initialize a
five-character array,
char sample[] = "ABCD";
and refer to a nonexistent array element,
sample[9] = 'X';
QuickC doesn't signal an error, although the second statement overwrites
memory outside the array. It stores a character in element 9 of an array
that contains only 5 elements.
The same problem can occur when accessing an array through a pointer:
char sample[] = "ABCD";
char *ptr = sample;
*--ptr = 'X'; /* Error! */
The code overwrites the byte in memory below the array. To avoid such
problems, confine all array operations within the range used to declare the
array.
String Problems
Strings are handled a little differently in C than most languages─a fact
that can cause problems. The following errors are common to programs that
use strings.
Confusing Character Constants and Character Strings
Remember the difference between a character constant, which has one byte,
and a character string, which is a series of characters ending with a null
character:
char ch = 'Y';
if( ch == "Y" ) /* Error! */
printf( "The ayes have it..." );
The example above mistakenly compares the char variable ch to a
twocharacter string ( "Y" ) instead of a single character constant ( 'Y' ).
Since the comparison is false, the printf statement never executes─no matter
what ch equals.
The if statement needs to use single quotes. This code correctly tests
whether ch equals the character 'Y':
char ch = 'Y';
if( ch == 'Y' )
printf( "The ayes have it..." );
Forgetting the Null Character That Terminates Strings
Remember that strings end with a null character in C. If you declare this
five-character array,
char sample[5];
the compiler allocates five bytes of memory for the array. If you try to
store the string "Hello" in the array like this,
strcpy( sample, "Hello" );
you'll overrun the array's bounds. The string "Hello" contains six
characters (five letters and a null character), so it's one byte too big to
fit in the sample array. The strcpy overwrites one byte of memory outside
the array's storage.
It's easy to make this error when allocating memory for a string, too:
char str[] = "Hello";
char *ptr;
ptr = malloc( strlen( str ) ); /* Error! */
if( ptr == NULL )
exit( 1 );
else
strcpy( ptr, str );
This time the error occurs in the call to the malloc function, which
allocates memory to a pointer prior to a string copy. The strlen function
returns the length of a string not including the null character that ends
the string. Since the amount of memory allocated is one byte too small, the
strcpy operation overwrites memory, just as in the previous example.
To avoid the problem, add 1 to the value returned by strlen:
ptr = malloc( strlen( str ) + 1 );
Forgetting to Allocate Memory for a String
If you declare a string as a pointer, don't forget to allocate memory for
it. This example tries to create a char pointer named ptr and initialize
it with a string:
main()
{
char *ptr;
strcpy( ptr, "Ashby" ); /* Error! */
}
The pointer declaration char *ptr; creates a pointer variable but nothing
else. It allocates enough memory for the pointer to store an address but
doesn't allocate any memory to store the object to which ptr will point.
The strcpy operation in the next line overwrites memory by copying the
string into an area not used by the program.
One way to allocate memory is by declaring a char array large enough to hold
the string:
main()
{
char c_array[10];
strcpy( c_array, "Randleman" );
}
You can also call the malloc library function to allocate memory at run
time:
#define BUFFER_SIZE 30
#include <malloc.h>
main()
{
char *ptr;
if( ptr = (char *) malloc( BUFFER_SIZE ) )
{
strcpy( ptr, "Duvall" );
printf( ptr );
free( ptr );
}
}
Pointer Problems
Every experienced C programmer has a collection of favorite pointer-induced
bugs. Pointer errors can wreak havoc because pointers can change the
contents of any addressable memory location. If a pointer writes to an
unexpected address, the results can be disastrous.
Using the Wrong Address Operator to Initialize a Pointer
If you're still learning about pointers, it's easy to forget which address
operator to use when initializing a pointer variable. For example, you might
want to create a pointer to a simple int variable:
int val = 25;
int *ptr;
ptr = val; /* Error! */
The code above doesn't initialize ptr correctly. Instead of assigning to
ptr the address of val, the statement
ptr = val;
tries to assign ptr the contents of val, causing an error message:
warning C4047: '=' : different levels of indirection
Because val is an int variable, its contents can't form a meaningful
address for ptr. You must use the address-of operator to initialize ptr:
ptr = &val;
Here's another pointer initialization error:
int val = 25;
int *ptr;
*ptr = &val; /* Error! */
The last line doesn't initialize ptr to point to the variable val. The
expression to the left of the equal sign, *ptr, stands for the object ptr
points to. Instead of assigning ptr the address of val, the line tries to
assign the address of val to the place where ptr points. Because ptr
has never been initialized, the assignment triggers a run-time error:
run-time error R6001
-null pointer assignment
Here is the correct way to initialize this pointer:
ptr = &val;
Declaring a Pointer with the Wrong Type
You should make sure the type used to declare a pointer matches the type of
data object it points to:
main()
{
int *ptr;
.
.
.
float val = 3.333333;
ptr = val; /* Error! */
printf( "val = %f\n", *ptr );
}
The program declares ptr as a pointer to an int. Later on, forgetting what
type we used when declaring ptr, we assign it the address of the
floating-point variable val.
Declaring a pointer with the wrong type can cause unwanted type
conversions.
Since C allows you to assign any address to a pointer, the assignment
doesn't cause an error. But accessing val through ptr creates problems.
Because ptr is declared as a pointer to an int, the compiler does a type
conversion on the float it points to, converting the float value to an int.
The output is garbage:
val = 11242989923343410000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000.000000
The following program cures the error by declaring ptr as a pointer to a
float data type:
main()
{
float *ptr;
float val = 3.333333;
ptr = &val;
printf( "%f\n", *ptr );
}
Now it gives the correct output:
val = 3.333333
Using Dangling Pointers
A "dangling pointer" is one that points to a memory area no longer in use by
your program. Dangling pointers, like uninitialized pointers, can be very
dangerous to use.
For instance, say you allocate a block of memory with the malloc library
function:
#define BUFSIZE 1000
char *ptr;
if( ptr = (char *) malloc( BUFSIZE ) )
/* do something */ ;
After the memory block has been allocated with malloc, the pointer ptr
points to a valid data object. Once you're done using allocated memory, you
normally return it to the heap:
free( ptr );
After you free the memory it points to, ptr is a dangling pointer. It
still points to a valid machine address, but that address is no longer in
use by the program. You shouldn't use the pointer at this stage, just as you
shouldn't use it before it has been initialized.
Dangling pointers can also be created by a function that returns a pointer
to a local variable:
int *boo_boo( void )
{
int object;
.
.
.
return &object; /* Error! */
}
The boo_boo function returns the address of the local variable object,
forgetting the storage for object is no longer part of the program after
the function ends.
Here's a variant of the previous example involving a string pointer:
char *boo_boo( void )
{
char *c_ptr;
c_ptr = "Hello";
.
.
.
return c_ptr; /* Error! */
}
Since the string constant "Hello" is local to the function, it evaporates
when the function ends, leaving the pointer c_ptr dangling.
Library-Function Problems
Once you've learned enough about C to write practical programs, you can
begin to explore the rich function library supplied with QuickC. This
section outlines a few common problems related to using library functions.
Again, you can use online help to get information about specific library
functions.
Failing to Check Return Values from Library Functions
Always check library function return values.
Almost all library functions return some value─either the result of
processing or an error code showing success or failure. You should always
check libraryfunction return values, even if you're confident of the result.
This rule is critical when calling a library function such as malloc, which
allocates memory at run time:
char *ptr;
ptr = (char *) malloc( BUFSIZE ); /* Error! */
If the call to malloc fails, the pointer ptr is assigned a null (0) value.
Using ptr under these circumstances can overwrite unexpected memory
addresses or cause a run-time error. The following code checks the return
value from malloc:
#define NULL 0
#define BUFSIZE 32768
.
.
.
char *ptr;
if( (ptr = (char *) malloc( BUFSIZE ) ) != NULL )
{
printf( "Copacetic.\n" );
/* Do something useful... */
}
else
printf( "Not enough memory!\n" );
Duplicating Library-Function Names
There are so many functions in the QuickC run-time library that it's
sometimes difficult to avoid duplicating function names. For instance, if
you write a function that reads data from a buffer, the name read may
strike you as short and descriptive.
The only problem is that read is the name of a QuickC library function. A
program that defines its own read function may work correctly at first,
but if you later include the header file that declares the read library
function,
#include <io.h>
then redefinition errors occur. You can't use the same name for two
different functions. The solution here is to rename the user-defined
function.
Use online help to check for function-name conflicts.
QuickC's online help lets you check for such name conflicts on the spot. Put
the cursor on the function name you wish to use, then press F1. If the name
is already used for a library function, online help displays information
about the function. If the name isn't in online help, it's not used in the
QuickC function library and is a safe choice.
Unless you're writing your own library functions, it's a good rule to avoid
declaring names that begin with an underscore ( _ ), since many of the
system-defined names in QuickC start with that character. (Non-ANSI library
functions begin with a single underscore. Predefined identifiers such as
TIME start with two underscores, and routines internal to the C run-time
library can begin with either one or two underscores.)
Forgetting to Include Header Files for Library Functions
Because they contain needed function prototypes, it's important to include
the correct header files when using QuickC library functions:
main()
{
double val = sqrt( (double) 10 );
printf( "square root of 10 = %le\n", val );
}
The program above calls the library function sqrt, which calculates a square
root. Most of the program is correct. When passing the value 10 to sqrt, it
casts the argument as a double, the type sqrt expects. The return value from
sqrt is assigned to a double variable, too.
Unfortunately, the program still gives the wrong output. The square root of
10 is not 171 (1.710000e+002 in exponential notation):
square root of 10 = 1.710000e+002
Function prototypes can prevent unexpected type conversions.
Because the program has no prototype for the sqrt function, sqrt has the int
return type by default. The value returned by sqrt undergoes an unexpected
type conversion─from type double to int─and becomes garbage.
This problem is easily solved. Simply include the standard header file that
contains the prototype for sqrt:
#include <stdio.h>
#include <math.h>
main()
{
double val = sqrt( (double) 10 );
printf( "square root of 10 = %le\n", val );
}
Now the program works correctly:
square root of 10 = 3.162278e+000
If you're not sure which header file a library function needs, take
advantage of QuickC's online help. (Put the cursor on the function name and
press F1.) If the function needs a header file, the file name appears in an
#include directive above the function prototype.
Omitting the Address-Of Operator When Calling scanf
Don't forget to put the address-of operator in front of arguments when using
the scanf library function (the scanf function accesses keyboard input; see
Chapter 11, "Input and Output"):
main()
{
int val;
printf( "Type a number: " );
scanf( "%d", val ); /* Error! */
printf( "%d", val );
}
When the program calls scanf, it omits the address-of operator that should
precede the second argument:
scanf( "%d", val ); /* Error! */
The scanf function expects to be passed a pointer to a variable (in this
case, a pointer to val ) so it can assign an input value to the variable.
But because the address-of operator is missing, the program passes the value
of val, not its address.
Instead of storing an input value in val as intended, scanf uses the
uninitialized value of val as a pointer and assigns the input value to an
unpredictable address. As a result, val remains uninitialized and the
program overwrites memory elsewhere─two very undesirable events.
Here is the correct way to call scanf in this program:
scanf( "%d", &val );
Macro Problems
Function-like macros─macro definitions that take arguments─share many of the
advantages of functions. They can cause unwanted side effects, however, if
you fail to put parentheses around their arguments or carelessly supply an
argument that uses an increment or decrement operator.
Omitting Parentheses from Macro Arguments
A macro definition that doesn't enclose its arguments in parentheses can
create precedence problems:
#include <stdio.h>
#define FOURX(arg) ( arg * 4 )
main()
{
int val;
val = FOURX( 2 + 3 );
printf( "val = %d\n", val );
}
The FOURX macro in the program multiplies its argument by 4. The macro
works fine if you pass it a single value, as in
val = FOURX( 2 );
but returns the wrong result if you pass it this expression:
val = FOURX( 2 + 3 );
QuickC expands the above line to this line:
val = 2 + 3 * 4;
Use parentheses to avoid precedence problems in macros.
Because the multiplication operator has higher precedence than the addition
operator, this line assigns val the value 14 (or 2 + 12) rather than the
correct value 20 (or 5 * 4).
You can avoid the problem by enclosing the macro argument in parentheses
each time it appears in the macro definition:
#include <stdio.h>
#define FOURX(arg) ( (arg) * 4 )
main()
{
int val;
val = FOURX(2 + 3);
printf( "val = %d\n", val );
}
Now the program expands this line
val = FOURX(2 + 3);
into this one:
val = (2 + 3) * 4;
The extra parentheses assure that the addition is performed before the
multiplication, giving the desired result.
Using Increment and Decrement Operators in Macro Arguments
If a function-like macro evaluates an argument more than once, you should
avoid passing it an expression that contains an increment or decrement
operator:
#include <stdio.h>
#define ABS(value) ( (value) >= 0 ? (value) : -(value) )
main()
{
int array[4] = {3, -20, -555, 6};
int *ptr = array;
int val, count;
for( count = 0; count < 4; count++ )
{
val = ABS(*ptr++); /* Error! */
printf( "abs of array[%d] = %d\n", count, val );
}
}
The program uses the ABS macro that was used to explain macros in Chapter
7, "Preprocessor Directives." The macro returns the absolute value of the
argument you pass to it.
The goal in this program is to display the absolute value of every element
in array. It uses a for loop to step through the array and a pointer named
ptr to access each array element in turn. Instead of the output you would
expect,
abs of array[0] = 3
abs of array[1] = 20
abs of array[2] = 555
abs of array[3] = 6
the program gives this output:
abs of array[0] = -20
abs of array[1] = -6
abs of array[2] = 8307
abs of array[3] = 24864
(The last two array values may differ if you run the program. They are the
contents of memory not used by the program.)
The error occurs in this line,
val = ABS(*ptr++); /* Error! */
which QuickC expands as shown here:
val = ( (*ptr++) >= 0 ? (*ptr++) : -(*ptr++) ); /* Error! */
Because it uses the conditional operator, the ABS macro always evaluates
its argument at least twice. This isn't a problem when the argument is a
constant or simple variable. In the example, however, the argument is the
expression *ptr++. Each time the macro evaluates this expression, the
increment operator takes effect, causing ptr to point to the next element
of array.
The first time the program invokes the macro, ptr points to the first
array element, array[0]. Since this element contains a nonnegative value
(3) the macro evaluates the argument twice. The first evaluation takes the
value that ptr points to and then increments ptr. Now ptr points to the
second element, array[1]. The second evaluation takes the value of
array[1] and increments ptr again.
The first macro invocation not only returns an incorrect value (-20, the
value of array[1] ). It also leaves ptr pointing to the third array
element, making the results of later invocations unpredictable. (The pointer
eventually moves past the last element of array and points to unknown
data.)
To avoid the problem, don't use the increment or decrement operators in
arguments you pass to a macro. This revision removes the error by
incrementing ptr in the for statement instead of the macro invocation:
#include <stdio.h>
#define ABS(value) ( (value) >= 0 ? (value) : -(value) )
main()
{
int array[4] = {3, -20, -555, 6};
int *ptr = array;
int val, count;
for( count = 0; count < 4; count++, ptr++ )
{
val = ABS(*ptr);
printf( "abs of array[%d] = %d\n", count, val );
}
}
This advice applies generally to QuickC library routines as well as macros
you write. Remember, some run-time library routines are implemented as
macros rather than C functions. If you're not sure whether a library routine
is actually a macro, look it up in online help.
Miscellaneous Problems
This section describes C programming problems that don't fit into any
convenient category.
Mismatching if and else Statements
In nested if statements, each else is associated with the closest preceding
if statement that does not have an else. Although indentation can make
nested constructs more readable, it has no syntactical effect:
if( val > 5 )
if( count == 10 )
val = sample;
else
val = 0;
The indentation suggests that the else associates with the first if. In
fact, the else is part of the second if, as shown more clearly here:
if( val > 5 )
if( count == 10 )
val = sample;
else
val = 0;
The else is part of the second if statement─the closest preceding if that
doesn't have a matching else. To tie the else to the first if, you must use
braces:
if( val > 5 )
{
if( count == 10 )
val = sample;
}
else
val = 0;
Indentation makes programs easier to read, but is ignored by the
compiler.
Now the else belongs with the outermost if. Remember, indentation is
meaningful only to humans. The compiler relies strictly on punctuation when
it translates the source file.
Misplacing Semicolons
Misplaced semicolons can cause subtle bugs:
#include <stdio.h>
main()
{
int count;
for( count = 0; count < 500; count++ ); /* Error! */
{
printf( "count = %d\n", count );
printf( "And the beat goes on...\n" );
}
}
You might expect the program to print the value of count 500 times, but
this is all it prints:
count = 500
And the beat goes on...
The culprit is the extra semicolon immediately after the parentheses of the
for statement. Its effect is more evident if we reformat the statement:
#include <stdio.h>
main()
{
int count;
for( count = 0; count < 500; count++ )
; /* Null statement */
{
printf( "count = %d\n", count );
printf( "And the beat goes on...\n" );
}
}
Instead of printing the value of count 500 times, the program executes the
null statement (;) 500 times. Null statements are perfectly legal in C, so
the compiler has no way to tell this is a mistake.
Since the null statement is interpreted as the loop body, the printf
statements inside curly braces are interpreted as a statement block and
executed once. Statement blocks usually appear as part of a loop, function
definition, or decision- making statement, but it's legal to enclose any
series of statements in braces.
The program works as intended if you remove the extra semicolon:
#include <stdio.h>
main()
{
int count;
for( count = 0; count < 500; count++ )
{
printf( "count = %d\n", count );
printf( "And the beat goes on...\n" );
}
}
Here's another one. If you know QuickPascal, you might be tempted to put a
semicolon after the parentheses of a function definition:
void func( void );
void func( void ); /* Error! No semicolon here. */
{
printf( "C is not Pascal\n" );
}
The function header causes a syntax error. While a function declaration
requires a semicolon after its parentheses, a function definition does not.
This code corrects the error:
void func( void );
void func( void )
{
printf( "C is not Pascal\n" );
}
Omitting Double Backslashes in DOS Path Specifications
Because C uses the backslash (\) as an escape character, it's easy to create
garbled path specifications:
fp = fopen( "c:\temp\bodkin.txt", "w" );
At first glance, the path specification in the string
"c:\temp\bodkin.txt"
looks good because that's how you would type it on the DOS command line. In
a quoted string, however, the backslash is interpreted as an escape
character. In this string the sequences \t and \b are interpreted as the tab
and backspace character, respectively, garbling the path and file name. Even
if the indicated file exists, this call to fopen is sure to fail.
In a quoted string the escape sequence for a backslash character is a double
backslash (\\). This statement solves the problem:
fp = fopen( "c:\\temp\\bodkin.txt", "w" );
Omitting break Statements from a switch Statement
Don't forget to include break statements when using the switch statement:
switch( ch )
{
case 'e':
printf( "Bye bye\n" );
break;
case 'l':
printf( "Loading the file\n" );
load_file( fp );
break;
case 's':
printf( "Saving the file\n" );
write_file( fp ); /* Error! Missing break. */
case 'd':
printf( "Deleting the file\n" );
kill_file( fp );
break;
default:
break;
}
In this code a break statement is missing from the statements following the
third case label (the statements that print Saving the file ). After those
statements execute, execution falls through to the next case label, deleting
the newly saved file.
To avoid this problem, place a break at the end of every case item:
case 's':
printf( "Saving the file.\n" );
write_file( fp );
break;
It's legal, of course, to write a program in which execution deliberately
falls through from one case label to the next. In such cases you may want to
add a comment to prevent confusion.
Mixing Signed and Unsigned Values
If you explicitly compare two values of different types, the compiler
normally catches the error. Some type mismatches aren't easy to spot,
however, even for humans:
#define CHARVAL '\xff'
main()
{
unsigned char uc;
uc = CHARVAL;
if( uc == CHARVAL )
printf( "Eureka!" );
else
printf( "Oops..." );
}
The program prints Oops... which probably wasn't expected. The comparison
between CHARVAL and uc is false even though both are clearly char
values.
The answer lies in the way the compiler converts signed and unsigned char
values into int values for internal use. The #define directive,
#define CHARVAL '\xff'
defines CHARVAL as the constant 0xff. Since no sign is specified, the
compiler treats the constant as a signed char value by default. When it
converts the char to an int for internal use, as it does all character
values, the compiler extends the value's sign. The result is an int with the
value 0xffff.
The variable uc undergoes the same internal conversion, with an important
difference. Since uc is explicitly declared as unsigned, its value is
converted to an int value of 0x00ff.
When the two int values are compared, the result is false (0xffff does not
equal 0x00ff). One solution is to explicitly cast CHARVAL to the desired
type:
#define CHARVAL (unsigned char)'\xff'
Now the compiler compares two unsigned char values, giving the desired
result. Another solution is to make CHARVAL an int instead of a char
constant:
#define CHARVAL 0xff
Both solutions give the desired result, although the second is slightly less
efficient. It creates word-size, rather than byte-size, machine-code
instructions.
PART II Using C
────────────────────────────────────────────────────────────────────────────
Part 2 of C for Yourself is called "Using C" and should be read after you
are familiar with basic C concepts. It covers practical topics that make it
possible for you to write real programs. The features discussed in these
chapters are provided in the QuickC run-time library, which, as you may
recall from Part 1, is not part of the C language itself.
While Part 1 was designed to be read sequentially, Part 2 is topical. So you
don't need to read its chapters in any particular order. If you are new to
C, however, it is recommended that you begin with Chapter 11, "Input and
Output," which describes how to read and write data, and process files.
Similarly, if you're not familiar with QuickC graphics, you should read
Chapters 13-15 in order.
Chapter 11 Input and Output
────────────────────────────────────────────────────────────────────────────
The first part of this book explored the fundamentals of the C language. In
the second part (starting with this chapter), the topics include more
complex and powerful functions: accessing disk files, creating
high-resolution graphics, creating graphs, manipulating fonts, and adding
assembly-language routines to your C programs.
Program examples in previous chapters used printf to print to the screen. In
this chapter, we'll cover printf in more detail, moving on to other I/O
functions such as fprintf, which prints to a file, instead of to the screen.
This chapter covers three broad topics: keyboard and screen input/output
(I/O), reading and writing standard disk files, and low-level disk access.
It also introduces several common string-handling functions.
Input and Output Streams
Books about C often refer to "input streams" and "output streams." A stream
is a sequence of bytes flowing into the program (input) or flowing out
(output). The data might have originally come from the keyboard, a modem, a
disk file, or some other peripheral device. The outgoing data might be sent
out to the screen, a modem, or a disk file.
Thus, when you see a phrase such as "opening a stream," it means opening a
line of communication to the disk drive or to some other peripheral.
Peripherals and files are called "streams" in C.
The five streams always open and available for input or output are shown in
Table 11.1.
Table 11.1 Standard I/O Streams
╓┌───────────────┌───────────────────────────────────────────────────────────╖
Name Stream
────────────────────────────────────────────────────────────────────────────
stdin Standard input (keyboard)
stdout Standard output (screen)
stderr Standard error channel (screen)
stdprn Standard printer (parallel port)
stdaux Standard auxiliary device (serial port)
────────────────────────────────────────────────────────────────────────────
Screen and Keyboard I/O
Imagine an application program that doesn't ever send output to the screen
or accept input from the keyboard. It's possible to write such a program,
but it's unlikely you'd ever want to.
In most situations, you need to display various kinds of data on the screen
and to accept input from the keyboard. "Manipulating and Printing Strings"
introduces the functions commonly used to communicate back and forth.
Manipulating and Printing Strings
Always pass at least one string to the printf function.
Previous chapters have used the printf function to display results on the
screen. By now you should be accustomed to how it works. There's one rule
you must always follow when using printf: pass it at least one format
string, which may be a literal string or a pointer to a string. The string
may or may not include format specifiers, which are defined below.
The printf function always prints to the stdout device. Unless the output
has been redirected, the standard output device is the screen.
The following program illustrates some typical ways to manipulate strings
and to print them:
/* PRTSTR.C: Print strings. */
#include <stdio.h>
#include <string.h>
main()
{
char aline[80], more[80];
char *strptr;
/* aline = "Another line."; */
/* Note: This causes a compiler error */
strcpy( aline, "Another line." );
strcpy( more, aline );
strptr = aline;
strcat( aline, "dog" );
printf( "A line of text." );
printf( aline );
printf( more );
printf( strptr );
}
The declarations come first:
char aline[80], more[80];
char *strptr;
The variables aline and more are arrays of characters. In this program,
they act as strings. Although these arrays have 80 characters each (numbered
0-79), the maximum string length is 79 characters, because strings must end
with a null character. The variable strptr is a pointer to a string.
If you've previously programmed in BASIC, you might expect to use the equal
sign to assign a value to a string variable. The program won't compile if
you remove the comment symbols from the following line:
/* aline = "Another line."; */
Faced with this line, QuickC prints the error message:
2106: '=' : Left operand must be lvalue
(an "lvalue" is a value allowed on the left side of an equal sign).
Use the strcpy function─ not the equal sign─ to copy a string.
You can use the equal sign to assign a value to a numeric variable. When
you're using strings, however, you almost always use the library function
strcpy, which copies a string to a character array from either a string
constant or another array:
strcpy( aline, "Another line." );
strcpy( more, aline );
The strcpy function makes an exact copy of a string. The first argument is
the address of the destination string. The second is the address of the
source string. The first strcpy above copies " Another line." to the aline
string. The second copies aline to more.
Note that the first argument must be the address of an array, but the second
is either a string constant (enclosed in quotation marks) or the address of
a character array.
The 80-character arrays have more than enough room for the 13 characters of
"Another line." and a null character. In your own programs, you should be
aware of the declared size of an array and avoid overrunning the bounds of
the array. See Chapter 10, "Programming Pitfalls," for more information
about this programming mistake.
It is possible to assign the address of a string to a pointer:
strptr = aline;
Notice that both strptr and aline point to the same string. There's one
object in memory, but it has two different names. If aline changes, the
same change occurs in the string referenced by strptr, because they're the
same string. Below, the word "dog" is added to the end of the string
aline:
strcat( aline, "dog" );
The strcat function concatenates one string to the end of a second string.
In the line above, both aline and the string referenced by strptr have
been changed from "Another line." to "Another line.dog".
Now four printf statements execute:
printf( "A line of text." );
printf( aline );
printf( more );
printf( strptr );
The screen should look like this:
A line of text.Another line.dogAnother line.Another line.dog
To the first printf we passed a string constant. To the other three we
passed names of strings. Concatenating aline and "dog" also affected the
string referenced by strptr, because they both point to the same string in
memory. The contents of more weren't affected, however, because the strcpy
function makes a complete and unique copy of the source string at the memory
location referenced by more.
Unfortunately, the strings ran together. As we saw in Chapter 1, "Anatomy of
a C Program," printf is unlike QuickBASIC's PRINT command or Pascal's
Writeln procedure in one respect: it does not automatically move the cursor
to the beginning of the next line. You need to include the newline character
(\n), which is one of a series of available escape codes discussed in
Chapter 4, "Data Types." The program below includes a few examples of escape
codes, each of which begins with the backslash character:
/* PRTESC.C: Print escape characters \",\n, and \t. */
#include <stdio.h>
#include <string.h>
main()
{
char b[80];
int i,j;
strcpy( b, "and seven years ago\n" );
printf( "\"Four score\n" );
printf( b );
printf( "\tone tab\n\t\ttwo tabs\n\t\t\tthree tabs\n" );
i = sizeof( b );
j = strlen( b );
printf( "Size is %d\nLength is %d.\n", i, j );
}
If you compile and run the PRTESC.C program, the following text prints on
the screen:
"Four score
and seven years ago
one tab
two tabs
three tabs
Size is 80
Length is 20.
To print a newline character in a string, type a backslash and the letter n
(\n). For a quotation mark, use \". For tabs, use \t. Escape sequences can
appear anywhere within a string:
printf( "\tone tab\n\t\ttwo tabs\n\t\t\tthree tabs\n" );
You'll find complete lists of escape characters in Appendix A, "C Language
Guide," and in online help.
Finding the Size
The last call to printf in PRTESC.C provides two pieces of information: the
size of the character array and the length of the string inside the array.
The variable b was declared to be an 80-character array, but the string
inside b contains only 20 characters; it holds 19 letters plus one newline
character. Although typing \n takes two characters, it's stored in memory as
one character─ the ASCII value 10. As we'll see later in this chapter, the
newline character is sometimes expanded to two characters (a carriage return
and a linefeed) when it is written to disk. But while it's in memory, it's a
single character.
The sizeof operator examines array size; the strlen function returns the
length of a string.
There are two methods available to find the size of arrays and strings. The
sizeof operator returns the size (in bytes) of an identifier or type. The
string-handling function strlen counts the number of characters in a string,
up to but not including the null that marks the end of the string:
i = sizeof( b );
j = strlen( b );
printf( "Size is %d\nLength is %d.\n", i, j );
The final line of the program PRTESC.C prints out two integer values, which
follow the format string. When printf evaluates the format string, it
substitutes the two values for the %d specifiers:
Size is 80
Length is 20
The sizeof operator is part of the C language. In this example, it evaluates
to the value 80, which is the size of the array. The strlen function is a
library function for measuring strings (up to, but not including the null at
the end). It returns a 20 because that's the length of the string.
Printing Numeric Values
The printf format string may hold one or more format specifiers.
We've seen how printf requires at least one string (or a pointer to a
string). To print variables and values, place a comma and the name of the
variable or value after the format string. Then, within the format string,
include a format specification. See Table 11.2.
Table 11.2 Common Format Specifications
╓┌─────────────────────┌─────────────────────────────────────────────────────╖
Specification Format
────────────────────────────────────────────────────────────────────────────
%c Print a character
%d Print a decimal integer
%f Print a floating-point number
Specification Format
────────────────────────────────────────────────────────────────────────────
%f Print a floating-point number
%i Print a decimal integer (same as %d)
%s Print a string
%u Print an unsigned integer
%x Print in hexadecimal format
────────────────────────────────────────────────────────────────────────────
The percent sign (%) always marks the beginning of a format specification.
The letters c, d, f, i, s, u, and x are called the "type." Between the
percent sign and the type, you may include optional specifications for
flags, width, or precision values.
At the very least, you must include the type, as in the program below:
/* NFORMAT.C: Print numbers and a string. */
#include <stdio.h>
#include <string.h>
main()
{
int a = -765,
b = 1,
c = 44000,
d = 33;
float e = 1.33E8,
f = -0.1234567,
g = 12345.6789,
h = 1.0;
char i[80];
strcpy( i, "word 1, word 2, word 3, word 4, word 5" );
printf( "Unformatted:\n%d %d %d %d\n", a, b, c, d );
printf( "%f %f %f %f\n", e, f, g, h );
printf( "%s\n", i );
}
The output looks like this:
Unformatted:
-765 1 -21536 33
133000000.000000 -0.123457 12345.678711 1.000000
word 1, word 2, word 3, word 4, word 5
If you carefully compare NFORMAT.C with its output, you'll notice some
unexpected results. For example, the variable c, which was initialized to
44000, has somehow changed to -21536.
The %d format specification applies to signed integers in the range -32768
to +32767. The value of c (44000) is outside that range, but still within
the realm of unsigned integers, which can hold values up to +65535. The
proper format specification would be %u (where u represents the unsigned
type).
Two of the floating-point values have changed, too. The %f specification
defaults to 6 digits of precision to the right of the decimal point. The
value of f (.1234567) is therefore rounded off to a precision of 6 digits:
.123457. Also, the limitations of floating-point accuracy transform the
value of g from 12345.6789 to 12345.678711. If you modify the program,
changing the float declarations to double, the second problem disappears.
The variable g prints correctly as 12345.67.
Between the% and the type character, you may include two numbers separated
by a period. The first number is called the "width;" the second is the
"precision." The width and precision affect integers, floating-point
numbers, and strings in different ways. For example, we could specify a
width of 2 and precision of 3 for each of the above variables:
printf( "\nWidth 2, Precision 3:\n" );
printf( "%2.3d %2.3d %2.3u %2.3d\n", a, b, c, d );
printf( "%2.3f %2.3f %2.3f %2.3f\n", e, f, g, h );
printf( "%2.3s\n", i );
(Note that the variable c has a format specifier of %2.3u instead of
%2.3d.) The screen displays the following lines:
Width 2, Precision 3:
-765 001 44000 033
133000000.000 -0.123 12345.679 1.000
wor
For integers, the precision of 3 causes at least 3 digits to print, preceded
by leading zeros. For floating-point numbers, the precision of 3 truncates
fractions to 3 digits to the right of the decimal point. For strings, the
precision of 3 causes only 3 characters to print. The string output is
truncated to the right. Numbers are never truncated, however.
We can change the width to 8 and the precision to 1:
printf( "\nWidth 8, Precision 1:\n" );
printf( "%8.1d %8.1d %8.1u %8.1d\n", a, b, c, d );
printf( "%8.1e %8.1f %8.1f %8.1f\n", e, f, g, h );
printf( "%8.1s\n", i );
We made an additional modification by printing the variable e as an %e
type instead of an %f type. This prints the value of e (1.33E8) in
exponential format:
Width 8, Precision 1:
-765 1 44000 33
1.3e+008 -0.1 12345.7 1.0
w
The width controls the printing area: all 3 variable types are printed in
fields 8 characters wide. The precision of 1 affects different data types in
different ways: the integers print at least 1 digit; the floating-point
numbers print only the first number to the right of the decimal point; and
the string prints as the first character only. Each value prints flush right
in its field.
Between the % and the width, you may also insert a flag. The plus flag (+),
for example, forces numbers to print with a leading sign:
printf( "\nForced signs, Width 10, Precision 2:\n" );
printf( "%+10.2d %+10.2d %+10.2u %+10.2d\n", a, b, c, d );
printf( "%+10.2e %+10.2f %+10.2f %+10.2f\n", e, f, g, h );
printf( "%+10.2s\n", i );
Note that the plus flag has no effect on strings or on unsigned integers:
Forced signs, Width 10, Precision 2:
-765 +01 44000 +33
+1.33e+008 -0.12 +12345.68 +1.00
wo
Another flag is the number 0, which forces leading zeros to print within the
limits of the width. If you only specify the width, the system default is
used for the precision. You can use the type %x to represent hexadecimal; it
displays the letters a-f in lowercase. If you prefer uppercase, you can use
%X instead.
printf( "\nHexadecimal, Forced Zeros, Width 6:\n" );
printf( "%06x %06x %06x %06x\n", a, b, c, d );
The printf statements above display these lines:
Hexadecimal, Forced Zeros, Width 6:
00fd03 000001 00abe0 000021
For strings, the width and precision specifiers describe the field width and
the number of characters printed. Note the minus sign in the final line,
which forces the truncated string to print from the left:
printf( "\nWidth 40, Precision 10:\n" );
printf( "%40.10s\n", i );
printf( "\nWidth 40, Precision 20:\n" );
printf( "%40.20s\n", i );
printf( "\nFlush left, Width 40, Precision 20:\n" );
printf( "%-40.20s\n", i );
The lines are displayed on the screen as follows:
Width 40, Precision 10:
word 1, wo
Width 40, Precision 20:
word 1, word 2, word
Flush left, Width 40, Precision 20:
word 1, word 2, word
Using scanf for Keyboard Input
Pass a variable address to scanf, not a variable value.
While printf is the most widely used output function, scanf is the most
popular for input. The arguments and format strings passed to scanf resemble
the arguments for printf, except for one requirement: the scanf function
always takes pointers. You never pass a variable value to scanf, you always
pass the variable address so that scanf can store data in the memory
location that contains the input variable.
The first argument for scanf is always a format string. Additional arguments
include the addresses of variables to which values will be assigned.
The program below demonstrates several ways to use scanf and various other
I/O functions:
/* INPUT.C: Read keyboard. */
#include <stdio.h>
#include <conio.h>
#include <ctype.h>
main()
{
int num;
char c;
char name[80];
float rb;
puts( "** Type \"Name:\" and your name" );
scanf( "Name: %40s", name );
printf( "** You typed this:\n%s", name );
puts( "\n\n** Try again, with the gets function." );
fflush( stdin );
gets( name );
printf( "** You typed this:\n%s\n", name );
printf( "\n** Now type an integer.\n" );
scanf( "%i", &num );
sprintf( name, "** You typed this number: %i\n", num );
puts( name );
fflush( stdin );
printf( "** Enter a floating-point value.\n" );
scanf( "%f", &rb );
printf( "** The answer is %f or %e\n", rb, rb );
printf( "** Continue? Y or N\n" );
do
{
c = getch();
c = tolower( c );
} while( c != 'y' && c != 'n' );
}
First, the puts function prints a string that requests input from the user.
Then scanf reads the input:
puts( "** Type \"Name:\" and your name" );
scanf( "Name: %40s", name );
Unfortunately, the use of scanf for string input creates some difficulties.
For one thing, you're forced to type Name: before typing the rest of the
string. (If you don't type Name:, scanf won't put a value into the name
variable.)
A second problem is that scanf reads the input stream until it finds a
white-space character: a SPACE, TAB, or ENTER.
The prompt below appears on the screen:
** Type "Name:" and your name
You might type this (you must begin the line with "Name:" ):
Name: F. Scott Fitzgerald
The next line takes effect:
printf( "** You typed this:\n%s", name );
Which prints the following line:
** You typed this:
F.
The string passed to the scanf function told it to expect "Name:" and then
to read a string, storing it in the name variable.
Since the scanf function reads strings until it finds a white-space
character, the value of name is " F." In addition, the words Scott
Fitzgerald are waiting in the input stream. To clear any stream, use the
fflush function:
puts( "\n\n** Now try it again, with the gets function." );
fflush( stdin );
gets( name );
To clear the buffer associated with a stream (including disk files), call
fflush, passing the pointer to the file or stream. In the example above,
stdin is the standard input device, the keyboard.
The puts function acts like a limited version of printf. It prints a string
to the standard output device, but can't insert formatted variable values.
You pass it a string constant or the name of a string. Also, it always adds
a newline to the end of the string it prints.
It is usually preferable to use gets when working with string input.
The gets function receives an entire line from the standard input device and
places the line in an array of characters. It does not include the newline
character typed by the user. It does, however, add a null to the end of the
line, to make the series of characters into a string. When you're working
with string input, gets is generally preferable to scanf.
For numeric values, scanf is the function of choice:
printf( "\n** Now type an integer.\n" );
scanf( "%i", &num );
sprintf( name, "** You typed the number: %i\n", num );
puts( name );
The format string %i forces scanf to treat the input as an integer. The
second argument is the address of the variable num.
The letter s in sprintf marks it as a string function. (There is also a
sscanf function that handles strings, but we won't discuss it here.) Instead
of printing the format string to the screen, as printf would do, sprintf
prints the results to another string. Note that scanf requires the address
of num, but sprintf uses its value.
The next scanf in program INPUT.C treats the input as a floating-point
number:
scanf( "%f", &rb );
printf( "** The answer is %f or %e\n", rb, rb );
If you enter -555.12, the computer responds:
** The answer is -555.119995 or -5.551200e+002
Finally, the program uses getch to receive a character from the input
stream:
printf( "** Continue? Y or N\n" );
do
{
c = getch();
c = tolower( c );
} while( c != 'y' && c != 'n' );
The getch function returns a character. That value, in turn, is passed to
tolower, which converts any uppercase characters to lowercase (in case the
CAPS LOCK key is on). Then, the byte is assigned to the variable c. The do
loop continues processing characters until you press y or n. The program
then ends. This simple example ends no matter which key ( y or n ) you
press. A real program would take some action based on the value returned by
the getch function.
Standard Disk I/O
If you can read input from the keyboard and write output to the screen,
you'll find standard disk files relatively easy to manipulate. There are
three rules to remember:
1. You can't do anything with a disk file until you open it. The act of
opening a file gives you a FILE pointer through which you can access
the file.
2. While the file is open, you can use most of the screen and keyboard
I/O functions if you precede them with the letter f (fprintf instead
of printf, for example). The file-handling functions work the same as
their counterparts, but you must add the FILE pointer.
3. When you're finished with a file, it's good programming practice to
close it. When exit ends the execution of a program, all previously
open files are closed (if you'd rather leave them open, use _exit
instead of exit).
Creating and Writing to a Text File
The WRFILE.C program opens a text file, writes a string to it, and closes
the file.
/* WRFILE.C: Create and write to a disk file. */
#include <stdio.h>
main()
{
FILE *fp;
if( (fp = fopen( "c:\\testfile.asc","w" )) != NULL )
{
fputs( "Example string", fp );
fputc( '\n', fp );
fclose( fp );
}
else
printf( "error message\n" );
}
You must include the standard I/O header file ( #include <stdio.h> )
whenever you plan to call input or output functions. It contains essential
definitions and prototypes that you need.
The only variable in this program is fp which is declared as a pointer to
a FILE. FILE is defined in STDIO.H as a structure of _iobuf type, but we
don't need to know the specifics. We will refer to the variable fp as a
"FILE pointer."
The first statement combines several operations in one line:
if( (fp = fopen( "c:\\testfile.asc", "w" )) != NULL )
The fopen function opens a file. It expects two parameters, both of which
are literal strings or pointers to strings. You provide the name of the file
to be opened and the type (read, write, or append). The six types of files
are listed in Table 11.3.
Table 11.3 Disk File Types
╓┌─────────────────────────────────┌─────────────────────────────────────────╖
Type Action
────────────────────────────────────────────────────────────────────────────
Type Action
────────────────────────────────────────────────────────────────────────────
r Open an existing file for reading.
w Create and open a file for writing. Any
existing file is replaced. If the file
doesn't exist, a new file is created.
a Open a file for appending. Data is added
to the end of an existing file or
a new file is created.
r+ Open an existing file for reading and
writing.
w+ Create and open a file for reading and
writing. An existing file is replaced.
a+ Open a file for reading and appending.
Data is added to the end of an
existing file or a new file is created.
Type Action
────────────────────────────────────────────────────────────────────────────
existing file or a new file is created.
────────────────────────────────────────────────────────────────────────────
In WRFILE.C, the file called c:\testfile.asc is opened for writing with
type "w" (a string, not the character 'w' in single quotes). We plan to
write to it.
Notice that the file-name string as it appears in the fopen statement
contains two backslashes: "c:\\testfile.asc". If you tried to use the string
"c:\testfile. asc", which looks correct, the character sequence \t would
be incorrectly interpreted as a tab character. C automatically converts the
two backslashes in the string to a single backslash.
Getting a FILE Pointer
The fopen function returns the address of a FILE. This value is assigned to
fp, which is the FILE pointer used in all subsequent file operations.
If something goes wrong─if the disk is full or not in the drive or
write-protected or whatever─fopen doesn't return a FILE pointer. When fopen
fails, it returns a null value.
What we're looking for is any FILE pointer that's not null:
if( (fp = fopen( "c:\\testfile.asc","w" )) != NULL )
Writing to the File
As we saw earlier, puts displays a string on the screen. Add an f to it and
the result is fputs, which works similarly. It sends the string to a
specified stream (a file) instead of to the standard output device:
fputs( "Example string", fp );
The function fputs takes two parameters: a pointer to the string and the
FILE pointer. In this and other I/O functions, you refer to the file by name
only once (when you use fopen). Thereafter, you use its FILE pointer.
The fputs function writes the entire string to the file but does not include
the null that marks the end of the string. Nor does it write a newline
character─unless the string already contains a newline.
The fputc function writes a character to a file. In the following line, the
newline character is sent to the file:
fputc( '\n', fp );
Closing the File
When the writing is done, fclose closes the file:
fclose( fp );
Conceptually, you can imagine that file I/O functions such as fputs and
fputc write directly to the disk file. In reality, they're storing strings
and characters in an intermediate area (called a "memory buffer"). When the
buffer fills up, the entire chunk of memory is sent to the file. The process
of emptying the buffer is called "flushing." You may forcibly flush the
buffer with the fflush and fclose functions. If you do not close the file
before exiting the program, the buffer is not flushed and you may lose data
that might remain there.
The else clause in WRFILE.C should execute only if something has gone wrong
with the fopen function:
else
printf( "error message\n" );
This line executes if an error occurs when fopen tries to create the file.
Handling errors is covered in more detail later in this chapter.
Reading a Text File in Binary Mode
The WRFILE.C program that created and wrote to a file was fairly simple.
Here's an equally simple program to read the file just created:
/* RDFILE.C: Read a file and print characters to the screen. */
#include <stdio.h>
main()
{
int c;
FILE *fp;
if( fp = fopen( "c:\\testfile.asc", "rb" ) )
{
while( (c = fgetc( fp )) != EOF )
printf( " %c\t%d\n", c, c );
printf( "\nEnd of file marker: %d", c );
fclose( fp );
}
else
printf( "Error in opening file\n" );
}
Although we plan to read the characters as eight-bit entities, the variable
c should be declared as an int instead of a char. All of the incoming
characters will be bytes the size of a char, except one.
When the file has been read from beginning to end, the end-of-file (EOF)
marker appears on the stream. Within QuickC, an integer value of -1 (0xFFFF)
represents EOF. To correctly identify this value, the variable c must be
an integer.
Opening a File for Binary Reading
In the line below, the fopen function attempts to open a file. The first
argument is the file name; the second is the type and mode, both of which
may be literal strings or pointers to strings:
if( fp = fopen( "c:\\testfile.asc", "rb" ) )
The single backslash character used in path specifications must once again
be represented by two backslashes. The file type is r for read-only. The
additional b character forces the file to be read in binary mode instead of
text mode. The differences between binary and text files are discussed later
in this chapter.
The fopen function returns a pointer to a FILE. If fopen fails, it returns a
NULL pointer.
Finally, the if expression tests for a null value. The original WRFILE.C
program included the != NULL test for inequality. Within the test
expression of an if or a while, a 0 value is always false and any other
value is considered true. In other words, should fp receive a valid
nonzero address from fopen, the program continues. If something goes wrong,
the remaining lines don't execute and the program drops through to the else.
Note that the expression above uses an assignment operator (=), not an
equality operator (==). The value returned by fopen is always assigned to
fp ; they aren't being compared to each other. Then the if expression tests
that value for truth or falsity.
Getting a Character
The key to the next line in RDFILE.C is the fgetc function, to which you
pass a FILE pointer. It returns the next character from the given file:
while( (c = fgetc( fp )) != EOF )
The character is assigned to the integer variable c. As long as the
character doesn't equal EOF, the while loop continues.
The end-of-file marker equals -1, but it's preferable to use the symbolic
constant EOF. If the program is transported to another computer, you might
find EOF has another value. Using the symbolic constant allows you to
maintain compatibility between computers and operating systems.
For the same reason, it's preferable to test for NULL instead of assuming
that NULL will always equal 0.
Viewing the File
Since there's only one line inside the while loop, it's not necessary to
enclose it in curly braces. The variable c contains the character read
from the file. It then can be printed:
printf( " %c\t%d\n", c, c );
The characters from the file print twice, once as a character (%c) and once
as a decimal number (%d), separated by a tab stop. This printf statement
repeats until fgetc (inside the while loop) finds no more characters in the
file.
Binary and Text Files
Normally, you wouldn't write a file in text mode and then read it in binary
mode. As a general rule, you pick whichever mode is more appropriate (text
mode for text or binary mode for data) and stick with it.
A somewhat baffling thing happened in the example above, however. The
WRFILE.C program wrote "Example string" to a disk file and then added a
newline character. That should be a total of 15 characters. But if you
examine the directory, you'll see the file uses 16 bytes.
Where did the extra byte come from?
Testing Text Mode
If you ran the RDFILE.C program, you probably noticed two characters
followed the line: a carriage return (ASCII 13) and a linefeed (ASCII 10).
If you make the following change to the program, the output of RDFILE.C is
different:
if( (fp = fopen( "c:\\testfile.asc","rt" )) != NULL )
The only modification is that the second string is "rt" instead of "rb".
The t represents text mode; the b is binary mode. If you don't specify a
mode, the fopen function defaults to text mode.
The list below shows the output of the two programs.
╓┌────────────────────────────────┌──────────────────────────────────────────╖
RDFILE.C (binary mode) RDFILE.C (text mode)
RDFILE.C (binary mode) RDFILE.C (text mode)
────────────────────────────────────────────────────────────────────────────
E69 E69
x120 x120
a97 a97
m109 m109
p112 p112
l108 l108
e101 e101
32 32
s115 s115
t116 t116
r114 r114
i105 i105
n110 n110
g103 g103
13 10
10 End-of-file marker: -1
End-of-file marker: -1
────────────────────────────────────────────────────────────────────────────
In binary mode there seems to be two characters after the string. In text
mode there's only one.
Ends-of-Lines, Ends-of-Files
The two modes─binary and text─treat end-of-line (EOL) characters and
end-of-file (EOF) characters in different ways.
In DOS, a line of text ends with a carriage return (CR) and a linefeed (LF),
which appear above as ASCII 13 plus ASCII 10. In the UNIX operating system,
which has close ties to the C language, a single ASCII 10 (the newline
character) marks the end of a line.
The once-popular CP/M operating system signals the end of files with a
CTRL+Z character (ASCII 26, 0x1A)─a tradition that carried forward to DOS.
This is not the case with UNIX (and C), which don't use a unique EOF
character.
Text Mode Translations
It's important to understand the differences between text mode and binary
mode when writing and reading disk files. No translations are made in binary
mode. In text mode, however, the end-of-line and end-of-file characters are
translated.
When you read a file in text mode and a CR-LF combination appears in the
stream, the two characters are translated to one newline character. The
opposite translation occurs when you write a file in text mode: each CR-LF
combination is translated to one newline character. In other words, the
newline is represented by two characters on disk and one character in
memory. These translations do not occur when you read and write a file in
binary mode.
When you read a file in text mode and a CTRL+Z (0x1A) character appears in
the stream, the character is interpreted as the end-of-file character.
However, when you're in text mode and you close a file to which you've been
writing, a CTRL+Z is not placed in the file as the last character. In binary
mode, the CTRL+Z character has no special meaning (it is not interpreted as
the end-of-file character).
The difference between text mode and binary mode is relatively minor when
you're handling strings, but it's important when you're writing numeric
values to disk files.
Text Format for Numeric Variables
Many programs, of course, use numeric as well as character data. When you
wish to save numbers, you have two choices: text mode or binary mode. The
SVTEXT.C program below illustrates the less desirable way of creating files
for numeric variables.
/* SVTEXT.C: Save integer variables as text. */
#include <stdio.h>
int list[] = { 53, -23456, 50, 500, 5000, -99 };
extern int errno;
char fname[] = "numtext";
char temp[81];
main()
{
FILE *fptr;
int i;
if( (fptr = fopen( "numtext","wt" )) != NULL )
{
for( i=0; i<6; i++ )
fprintf( fptr, "Item %d: %6d \n", i, list[i] );
fclose( fptr );
}
else
printf( "Error: Couldn't create file.\n" );
if( (fptr = fopen( "badname", "rt" )) != NULL )
{
/* do nothing */
}
else
{
printf( "Error number: %d\n\t", errno );
perror( "Couldn't open file BADNAME\n\t" );
}
if( (fptr = fopen( fname, "rt" )) != NULL )
{
list[0] = 0;
fscanf( fptr, "Item %d: %d \n", &i, &list[0] );
printf( "Values read from file:\t %d %d\n", i, list[0] );
fgets( temp, 80, fptr );
printf( "String from file: \t%s\n", temp );
while( (i = fgetc( fptr )) != '\n' )
printf( "char: %c \t ASCII: %d \n", i, i );
rewind( fptr );
printf( "Rewind to start -->\t%s", fgets( temp, 80, fptr ) );
fclose( fptr );
}
else printf( "Trouble opening %s \n", fname );
}
The SVTEXT.C program does three things:
1. First, it creates a text file called NUMTEXT. If you TYPE NUMTEXT from
the DOS prompt or load the NUMTEXT file into a word processor, it
looks like this:
Item 0: 53
Item 1: -23456
Item 2: 50
Item 3: 500
Item 4: 5000
Item 5: -99
2. Next, SVTEXT.C deliberately attempts to open a nonexistent file called
BADNAME, to cause a disk error. This section serves no purpose except
to illustrate error handling.
3. Finally, it reads parts of NUMTEXT, using several file-input
functions.
Opening the File for Writing
By now, the fopen function should look familiar to you. The only change in
the block below is the "wt" mode. The fopen function returns a NULL if any
errors occur, so the block after the if should execute if fopen succeeds.
if( (fptr = fopen( "numtext","wt" )) != NULL )
{
for( i=0; i<6; i++ )
fprintf( fptr, "Item %d: %6d \n", i, list[i] );
fclose( fptr );
}
else
printf( "Error: Couldn't create file.\n" );
The for loop counts from 0 to 5, printing 6 strings to the file. The fprintf
function works the same as printf with one change. You must place the FILE
pointer before the format string.
Error Handling
To illustrate what happens when something goes wrong, the next line creates
a disk error (as long as you don't have a file called BADNAME in your
working directory).
if( (fptr = fopen( "badname", "rt" )) != NULL )
The if block is empty, because we expect the program to drop through to the
else clause that handles errors:
else
{
printf( "Error number: %d\n\t", errno );
perror( "Couldn't open file BADNAME\n\t" );
}
The else block shows two ways you can deal with errors. Note that the errno
variable, which was declared as an external integer, has never been assigned
a value. QuickC automatically puts error numbers into errno. In this
program, the error number is printed to the screen. In your own programs,
you might wish to branch to various error-handling routines, based on the
value in the system variable errno. For a list of values for errno, see the
individual online help entries for file-handling functions.
It's important to remember that the standard output device is the screen and
that printf sends messages to stdout. However, if you redirect output to a
disk file, using a command line such as SVTEXT > MYFILE, the printf
statement prints the error message to MYFILE. In most cases, you'd prefer to
see the error message on the screen.
The second, and better, way to handle I/O errors is the perror function,
which prints two strings: one that you pass to it and one that spells out─in
English─the error message. This message goes to the standard error stream
(stderr), which is always the screen, regardless of whether you've
redirected output or not. For this reason, perror is preferable to printf
for printing error messages.
The error messages should look like this on your screen:
Error number: 2
Couldn't open file BADNAME
: No such file or directory
Reading Text with fscanf
The final fopen in SVTEXT.C opens the file created earlier:
if( (fptr = fopen( fname, "rt" )) != NULL )
Note that we passed the name of a string rather than a literal string.
Below, fscanf reads in two numeric variables from the first string in the
file. Note that it works the same as scanf, but you add the FILE pointer as
the first argument:
fscanf( fptr, "Item %d: %d \n", &i, &list[0] );
printf( "Values read from file:\t %d %d\n", i, list[0] );
Reading Text with fgets and fgetc
At this point, the first line in the file has been read and converted to two
integer values. The file is straight text, so you can treat the second line
as a string:
fgets( temp, 80, fptr );
printf( "String from file: \t%s\n", temp );
The fgets function requires three arguments: a pointer to a string, the
maximum number of characters to read, and the FILE pointer. The function
stops reading characters when it encounters a newline character or when it
reaches the maximum number of characters or the end of the file.
If you prefer, you can input the characters one by one:
while( (i = fgetc( fptr )) != '\n' )
printf( "char: %c \t ASCII: %d \n", i, i );
The printf inside the while loop prints each character as a character (%c)
and also as a decimal value (%d). The while loop continues reading
characters until it finds the end of the line.
Back to the Beginning
The rewind function resets the position pointer to the beginning of the
file. In the program line below, the first line from the file is printed:
rewind( fptr );
printf( "Rewind to start -->\t%s", fgets( temp, 80, fptr ) );
The screen output looks like this:
Error number: 2
Couldn't open file BADNAME
: No such file or directory
Values read from file: 0 53
String from file: Item 1: -23456
char: I ASCII: 73
char: t ASCII: 116
char: e ASCII: 101
char: m ASCII: 109
char: ASCII: 32
char: 2 ASCII: 50
char: : ASCII: 58
char: ASCII: 32
char: ASCII: 32
char: ASCII: 32
char: ASCII: 32
char: ASCII: 32
char: 5 ASCII: 53
char: 0 ASCII: 48
char: ASCII: 32
Rewind to start --> Item 0: 53
It is inefficient to store numeric data in text format.
There seem to be quite a few white-space characters in the text file. Text
files are great for text, but they store numeric values in a wasteful way.
Binary format offers several advantages.
Using Binary Format
When you're processing strings of ASCII characters and writing them to disk
files, it matters little whether you use text mode or binary mode, as long
as you're consistent. The advantage of text mode is that it translates
newlines to the carriage-return-line-feed combination, making it possible to
use the DOS TYPE command to view the file.
When you're processing numeric values (integers and floating-point numbers),
however, you may wish to save your variables in binary mode files, in binary
format, for the following reasons:
■ Binary format almost always saves disk space. In text mode, the number
12345.678 would require eight bytes for the ASCII numerals, one byte
for the decimal point, and one or more bytes for a separator between
variables. In binary format, a floating-point number uses four bytes,
regardless of its value. Short integers use only two bytes.
■ Binary format generally saves computer time. When you use fprintf to
print a numeric value to disk, the computer must translate the
internal binary representation to a series of characters. Likewise,
when fscanf reads characters into memory, the ASCII values must be
translated to the internal binary format. In binary format, none of
these translations takes place.
■ Binary format preserves the precision of floating-point numbers. The
translation from binary to decimal ASCII and back to binary affects
the precision of the value.
■ A binary save of arrays or structures is fast. It's not necessary to
read through an array of 100 items and print each one to the disk
file. Instead, you call the fwrite function (discussed below) once,
passing it the size of the array to be saved.
NOTE Binary mode is separate from binary format. The modes (binary
and text) are parameters you pass to the fopen function. They affect
the translation of newlines and the placing of EOF markers. The
formats (binary and text) are ways of representing numeric values. An
integer in binary format always occupies two bytes on disk. An integer
in text format uses a variable number of bytes: it might contain one
character (5) or six (-10186).
Opening a Binary File
The SVBIN.C program below creates two binary mode files with the variables
saved in binary format:
/* SVBIN.C: Save integer variables in binary format. */
#include <stdio.h>
#define ASIZE 10
main()
{
FILE *ap;
int zebra[ASIZE], acopy[ASIZE], bcopy[ASIZE];
int i;
for( i = 0; i < ASIZE; i++ )
zebra[i] = 7700 + i;
if( (ap = fopen( "binfile", "wb" )) != NULL )
{
fwrite( zebra, sizeof(zebra), 1, ap );
fclose( ap );
}
else
perror( "Write error" );
if( (ap = fopen( "morebin", "wb" )) != NULL )
{
fwrite( &zebra[0], sizeof(zebra[0]), ASIZE, ap );
fclose( ap );
}
else
perror( "Write error" );
if( (ap = fopen( "binfile", "rb" )) != NULL )
{
printf( "Hexadecimal values in binfile:\n" );
while( (i = fgetc( ap )) != EOF )
printf( "%02X ", i );
rewind( ap );
fread( acopy, sizeof(acopy), 1, ap );
rewind( ap );
fread( &bcopy[0], sizeof( bcopy[0] ), ASIZE, ap);
for( i=0; i<ASIZE; i++ )
printf( "\nItem %d = %d\t%d", i, acopy[i], bcopy[i] );
fclose( ap );
}
else
perror( "Read error" );
}
Focus your attention on the zebra array. It contains 10 integers, because
the array size ASIZE was defined as 10. First, some values are stored in
zebra (in a moment, we'll see why 7700-7709 are significant):
for( i = 0; i < ASIZE; i++ )
zebra[i] = 7700 + i;
Next, we open a file and use fwrite to write the entire array to disk:
if( (ap = fopen( "binfile", "wb" )) != NULL )
{
fwrite( zebra, sizeof(zebra), 1, ap );
fclose( ap );
}
Writing an Array in One Line
The fwrite function requires four pieces of information:
1. The address of the item (a variable, array, or structure)
2. The size of the item in bytes
3. The number of items to be written
4. The FILE pointer for a previously opened binary mode file
In this example, the first argument, zebra is an array and, as you may
remember from Chapter 8, "Pointers," the name of an array is the address of
the array.
To provide the second argument for fwrite, SVBIN.C uses the sizeof operator,
which returns the number of bytes a variable requires. Because zebra is an
array of 10 integers and integers use 2 bytes each, the size of zebra
should be 20. If you view a directory of your disk after running this
program, you'll notice that the file BINFILE is exactly 20 bytes long.
The third argument tells fwrite how many items to write to the file. We have
1 array, so this parameter is 1.
The fourth argument is the FILE pointer returned by fopen.
There's another way to copy the 20 bytes of zebra to the file. After
writing to BINFILE, the program uses the fopen function to create a second
file called MOREBIN. The following fwrite line writes 10 integers instead of
1 array:
fwrite( &zebra[0], sizeof(zebra[0]), ASIZE, ap );
The second and third arguments have changed. Instead of passing the size of
the array (20) and writing 1 copy of the array, we're accessing the size of
1 element (2 bytes) and writing 10 of them (using the symbolic constant
ASIZE). The contents of this disk file should match, byte for byte, the
contents of BINFILE.
Examining the Binary Contents
Finally, we look at what's inside the file BINFILE. It is opened for reading
as a binary file:
if( (ap = fopen( "binfile", "rb" )) != NULL )
A short while loop reads the bytes from BINFILE and displays them in
hexadecimal notation:
printf( "Hexadecimal values in binfile:\n" );
while( (i = fgetc( ap )) != EOF )
printf( "%02X ", i );
After running SVBIN.C, the screen displays these values:
14 1E 15 1E 16 1E 17 1E 18 1E
19 1E 1A 1E 1B 1E 1C 1E 1D 1E
The low byte precedes the high byte, so the first two bytes represent the
number 0x1E14, which is 7700 in decimal. The next two bytes equal 7701, and
so on.
A curious thing happens when you run SVBIN.C and then try to treat the
20-byte file as text. If you TYPE BINFILE from the DOS command line, the
file appears as gibberish (of course), and you see only 12 of the 20
characters on the screen. Where did the other characters go? Recall the
previous discussion of binary and text files. In DOS, a CTRL+Z (0x1A) marks
the end of a text file. And in the midst of our binary file is one of those
EOF characters. It's not acting as an EOF; it's part of the number 0x1E1A.
But if you ever open this file in text mode, you'll be unable to read past
the twelfth byte.
Retrieving the Values from Disk
Most of the time, you won't want to read a binary file one byte at a time.
Instead, you call fread, which reads a disk file and stores the values in a
variable, an array, or a structure. The fread function complements fwrite.
It takes four parameters:
1. The address of the variable
2. The size of the variable in bytes
3. The number of values to read
4. The FILE pointer that references a binary file opened for reading
Here's one way to read values into an array:
rewind( ap );
fread( acopy, sizeof( acopy ), 1, ap );
The rewind command is necessary because we've already read through the file
once. The acopy and bcopy arrays are the same size as our original
zebra array. To fill an array with this technique, pass the address, the
size of the entire array, a number 1, and the FILE pointer.
A second way to fill an array is to pass the size of a single element and
the number of elements you want to read:
rewind( ap );
fread( &bcopy[0], sizeof( bcopy[0] ), ASIZE, ap );
In the first example of fread, we pass the information that the array acopy
is 20 bytes long and we want to read it once. In the second example, we
pass the size of an integer (2 bytes) and ask for 10 of them. In either
case, 20 bytes are transferred.
Just to make sure both arrays are equal, we can print them out:
for( i = 0; i < ASIZE; i++ )
printf( "\nItem %d = %d\t%d", i, acopy[i], bcopy[i] );
fclose( ap );
The screen displays the values 7700 through 7709, which survived the trek
from zebra to BINFILE and back again. These values were stored in the
zebra array, written to a binary file, then read back into the acopy and
bcopy arrays.
Low-Level Input and Output
The file-handling routines such as fopen, fprintf, and fclose are called
"standard" because they're defined in the ANSI standard. Code that uses the
standard routines will generally be portable from one machine to another.
In addition to the standard file-handling functions, the QuickC library
includes some low-level I/O functions, which allow more direct access to
disk files.
The low-level I/O routines (also called "system-level") are generally not
portable. They work in DOS and OS/2, but they may not work elsewhere.
They're also a little more difficult to use. Instead of declaring a pointer
to a FILE structure, you must allocate your own buffer and manage transfer
of the bytes yourself. You move values into the buffer, then send the
contents of the buffer to disk.
Low-level routines can be more efficient, but they usually aren't
portable.
Low-level routines have some advantages, though. One is that you have more
control over the machine. Another is that low-level I/O can be faster than
standard I/O, if you know what you're doing. The choice is up to you:
portability versus efficiency. You should choose one or the other; it's not
a good idea to mix standard and system-level routines.
Low-Level Reading and Writing
The program RWFILE.C illustrates some of the low-level, file-handling
commands. It creates a file, writes to it, and closes it. Then the file is
opened for reading and the contents of the file are displayed on the screen.
/* RWFILE.C: Read and write a file. */
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <sys\types.h>
#include <sys\stat.h>
#include <io.h>
#define BUFF 512
main()
{
char inbuffer[BUFF];
char outbuffer[BUFF];
int infile, outfile, length, num;
strcpy( outbuffer, "Happy Birthday." );
length = strlen( outbuffer );
length++;
if( (outfile = open( "testfile.bin",
O_CREAT | O_WRONLY | O_BINARY, S_IWRITE )) != -1 )
{
if( (num = write( outfile, outbuffer, length )) == -1 )
perror( "Error in writing" );
printf( "\nBytes written to file: %d\n", num );
close( outfile );
}
else
perror( "Error opening outfile" );
if( (infile = open( "testfile.bin", O_RDONLY | O_BINARY )) != -1 )
{
while( length = read( infile, inbuffer, BUFF ) )
printf( "%d bytes received so far.\n", length );
close( infile );
printf( "%s\n", inbuffer );
}
else
perror( "Error opening infile" );
}
Several header files must be included:
#include <stdio.h>
#include <fcntl.h>
#include <sys\types.h>
#include <sys\stat.h>
#include <io.h>
The symbolic constant BUFF is defined as 512. This value is used
immediately in the declaration of two buffers:
char inbuffer[BUFF];
char outbuffer[BUFF];
Note that we don't need FILE structures anywhere in the program. The
standard I/O routines automatically allocated space for a buffer. Since
we're operating closer to the DOS level, we must allocate our own buffers,
instead of relying on the system. If you set the buffer size to a
sufficiently large value, QuickC will run out of stack space. When this
happens, you may either make the buffers global variables or use the malloc
function to allocate an additional chunk of memory.
The open function takes three parameters:
if( (outfile = open( "testfile.bin",
O_CREAT | O_WRONLY | O_BINARY, S_IWRITE )) != -1 )
The first parameter is the file name. The second is a sequence of "oflags"
that are combined with the OR operator. The oflags determine which type of
file will be opened: it will be created ( O_CREAT ), it will be write-only (
O_WRONLY ), and it will be a binary─not a text─file ( O_BINARY ). When you
create a new file, you must include the third parameter: S_IWRITE.
The open function returns a file handle, which is assigned to the integer
variable outfile. Note that this is an integer, not a pointer to a FILE
structure. If anything goes wrong, a value of -1 is returned by open, and we
should test for this.
Table 11.4 summarizes the differences between fopen and open.
Table 11.4 Standard vs. Low-Level
╓┌─────────┌──────────────────────────┌─────────────────┌────────────────────╖
Function Error Parameters Returns Condition
────────────────────────────────────────────────────────────────────────────
fopen File name, type Pointer to FILE NULL
(r, w, a), and mode
(t, b)
open File name, oflags File handle -1
(integer)
────────────────────────────────────────────────────────────────────────────
Low-Level Writing
The write function takes three parameters:
1. The file handle returned by open
2. The address of the buffer
3. The number of bytes to write
You, the programmer, are responsible for filling up the buffer. The write
function returns the number of bytes actually written to the file.
if( (num = write( outfile, outbuffer, length )) == -1 )
perror( "Error in writing" );
printf( "\nBytes written to file: %d\n", num );
close( outfile );
Low-Level Reading
Next, we open the file for reading. Again, the oflags are required:
if( (infile = open( "testfile.bin", O_RDONLY | O_BINARY )) != -1 )
{
while( length = read( infile, inbuffer, BUFF ) )
printf( "%d bytes received so far.\n", length );
close( infile );
printf( "%s\n", inbuffer );
}
The read function takes three parameters:
1. The file handle
2. The address of the buffer
3. The size of the buffer
The value returned is the number of bytes read. The while loop continues as
long as there are characters in the stream. In a real application, you'll
have to handle the bytes stored in the buffer.
The low-level file functions are unbuffered. When you call write, the bytes
are written directly to the disk file. The standard file function fwrite
doesn't write data to disk; it writes to a buffer. The buffer is transferred
to disk when the buffer fills up, when the fflush function is called, or
when the file is closed. As a gen-eral rule, you should not mix buffered and
unbuffered routines. Use the standard routines or the low-level routines,
but not both.
This chapter started with keyboard input and screen output and led into
discussions of file I/O. The following chapters cover in greater depth
assembly language routines and some specialized types of screen output,
including high-resolution graphics, fonts, and presentation graphics.
Chapter 12 Dynamic Memory Allocation
────────────────────────────────────────────────────────────────────────────
A program that allocates memory "dynamically" (as it runs) can respond
flexibly to a user's needs, creating new data structures when the need
arises and discarding them when their job is done.
As you read this chapter you'll learn how to allocate memory with the malloc
library function and free memory with the free function. We'll also look at
two related functions, calloc and realloc.
Memory allocation requires the use of pointers. If you're not familiar with
pointers, read Chapter 8, "Pointers," before tackling this chapter.
Why Allocate?
The malloc family of library functions can allocate memory during run time.
The malloc (memory allocate) family of library functions enables you to
allocate blocks of memory dynamically. The capability to create new data
structures on the fly lets you tailor a program's behavior precisely to the
user's needs.
For simple programs, such as the examples in Part 1, memory allocation is
largely automatic. When you declare variables, as in the lines
int count;
char buffer[160];
QuickC allocates enough memory to store each variable (2 bytes for the first
variable and 160 for the second). This method works fine if you know each
vari-able's size in advance. Some program memory needs aren't easy to
predict, however.
To take a simple example, say you write an address-book program that stores
addresses in an array of structures. A novice programmer might begin by
declaring an array of, say, 100 structures, in the following manner,
struct address list[100];
but this approach is needlessly limiting. If your list contains only a few
addresses, most of the memory in the array is wasted. And if you want to
enter more than 100 addresses, you're out of luck.
A better approach is to allocate memory for the array dynamically. This way,
the program can use only as much memory as needed for the current address
list. Each time you add an address, or delete one, the program can expand or
shrink the array as needed.
Memory Allocation Basics
We'll use a simple example program, COPYFILE.C, to demonstrate the basics of
dynamic memory allocation─how to allocate a memory block, access its
contents, and free the block when its purpose is served.
The COPYFILE.C program, shown below, dynamically allocates a buffer that it
uses to store file data.
/* COPYFILE.C: Demonstrate malloc and free functions. */
#include <stdio.h> /* printf function and NULL */
#include <io.h> /* low-level I/O functions */
#include <conio.h> /* getch function */
#include <sys\types.h> /* struct members used in stat.h */
#include <sys\stat.h> /* S_ constants */
#include <fcntl.h> /* O_ constants */
#include <malloc.h> /* malloc function */
#include <errno.h> /* errno global variable */
int copyfile( char *source, char *destin );
main( int argc, char *argv[] )
{
if( argc == 3 )
if( copyfile( argv[1], argv[2] ) )
printf( "Copy failed\n" );
else
printf( "Copy successful\n" );
else
printf( " SYNTAX: COPYFILE <source> <target>\n" );
return 0;
}
int copyfile( char *source, char *target )
{
char *buf;
int hsource, htarget, ch;
unsigned count = 50000;
if( (hsource = open( source, O_BINARY | O_RDONLY )) == - 1 )
return errno;
htarget = open( target, O_BINARY | O_WRONLY | O_CREAT | O_EXCL,
S_IREAD | S_IWRITE );
if( errno == EEXIST )
{
cputs( "Target exists. Overwrite? " );
ch = getch();
if( (ch == 'y') || (ch == 'Y') )
htarget = open( target, O_BINARY | O_WRONLY | O_CREAT | O_TRUNC,
S_IREAD | S_IWRITE );
printf( "\n" );
}
if( htarget == -1 )
return errno;
if( filelength( hsource ) < count )
count = (int)filelength( hsource );
buf = (char *)malloc( (size_t)count );
if( buf == NULL )
{
count = _memmax();
buf = (char *)malloc( (size_t)count );
if( buf == NULL )
return ENOMEM;
}
while( !eof( hsource ) )
{
if( (count = read( hsource, buf, count )) == -1 )
return errno;
if( (count = write( htarget, buf, count )) == - 1 )
return errno;
}
close( hsource );
close( htarget );
free( buf );
return 0;
}
Before we look at how COPYFILE.C works, let's note what it does. Unlike the
DOS COPY command, the COPYFILE.C program asks for confirmation before
overwriting an existing file. The program expects to receive two file names
as command-line parameters: the name of the file to copy and the name of the
new file. For instance, the following command copies the file SAMPLE.EXE to
the new file EXAMPLE.EXE:
copyfile sample.exe example.exe
If the target file already exists, COPYFILE.C displays:
Target exists. Overwrite?
COPYFILE.C overwrites an existing file only if the user presses the Y key in
response.
Preparing to Allocate Memory
The COPYFILE.C program copies the source file in chunks, using an allocated
memory block as a buffer for file data. The following program lines are the
ones involved in allocating and freeing the memory block. (These are taken
from the program in order, but are not consecutive.)
#include <malloc.h> /* malloc function */
char *buf;
unsigned count = 50000;
buf = (char *)malloc( (size_t)count );
free( buf );
The first of these,
#include <malloc.h> /* malloc function */
includes the standard include file MALLOC.H, which contains declarations for
malloc and other memory-allocating functions.
The malloc function, which the program will call to allocate a memory block,
returns the address where the block begins. COPYFILE.C declares the pointer
variable buf to store this address:
char *buf;
As you'll see shortly, the pointer buf will be initialized to point to the
allocated block. Once this is done, the program can access the block's
contents through the pointer.
The COPYFILE.C program declares another variable, count, which is used to
tell malloc how much memory (in bytes) to allocate. The program initially
sets this value to 50,000:
unsigned count = 50000;
If the source file is smaller than 50,000 bytes, COPYFILE.C later resets
count to the smaller value.
Specifying the Size of the Allocated Block
Now we're ready to allocate the block. The statement
buf = (char *)malloc( (size_t)count) );
in COPYFILE.C calls the malloc function, passing the value of count as an
argument. This argument indicates the size of the desired block in bytes. In
COPYFILE.C this value is 50,000 or the size of the source file, whichever is
smaller.
Look at the type cast preceding the function argument:
(size_t)
The cast is performed for ANSI compatibility (malloc is part of the ANSI
standard). Under ANSI, malloc is declared as taking an argument of the type
size_t. To ensure the portability of your programs, the value passed to
malloc should be either declared or cast as type size_t.
A Graphic Illustration
Figures 12.1 and 12.2 show how the COPYFILE.C program allocates a memory
block. The figures are simplified and are not drawn to scale. They represent
the program's "data segment," which is the memory area available for the
program's data storage.
────────────────────────────────────────────────────────────────────────────
NOTE
The details of data storage differ depending on the current "memory model,"
an advanced concept that goes beyond the scope of this book. For purposes of
discussion, this book assumes the small memory model, which is the default
for QuickC.
────────────────────────────────────────────────────────────────────────────
Figure 12.1 represents the program's data segment before COPYFILE.C
allocates a block of memory. The shaded area labeled "Declared data"
contains the program's declared variables and "stack," which is used for
temporary storage. The unshaded area labeled "Heap" contains the memory
available for allocation by COPYFILE.C.
(This figure may be found in the printed book.)
Figure 12.2 shows COPYFILE.C immediately after the program calls the malloc
function to allocate a block of memory. The allocated block is taken from
heap memory and lies directly above the program. If COPYFILE.C allocated a
second memory block, that block would lie above the first, further
diminishing heap memory.
(This figure may be found in the printed book.)
While it's common to say the malloc function "creates" a memory block, that
terminology is a bit misleading. As Figures 12.1 and 12.2 show, malloc
simply gives your program control over memory that's already present.
The malloc function does not initialize the memory it allocates.
Since the allocated block has been present in memory all along, it may
contain random values or values left over from some previous use. The malloc
function doesn't initialize allocated memory. Substitute the calloc function
for malloc if you want to clear an allocated block before use. (See the
section "The calloc Function" below.)
Assigning the Address that malloc Returns
If the call to malloc succeeds, malloc returns the address of the memory
block it allocates. COPYFILE.C assigns that return value to the pointer
variable buf and then accesses the allocated block through buf.
Before assigning the address that malloc returns, COPYFILE.C performs a
type cast on the address
(char *)
The type cast indicates which type of memory you are allocating. Prior to
the ANSI standard, malloc was declared as returning a pointer to type char,
so it was necessary to cast the return value when assigning the value to any
other pointer type.
Under ANSI, malloc returns a pointer to type void. Since a void pointer can
be converted to any pointer type, it's not strictly necessary to cast the
return from malloc. (If you omit the cast, QuickC does a silent type
conversion.) The type cast improves readability, however.
Checking the Return from malloc
If a call to malloc fails─usually because not enough memory is available─the
function returns a null pointer (defined as NULL in the standard include
file STDIO.H). You should always test this return value, even if you're
confident the allocation will succeed. If you ignore the return value and
access memory through a null pointer, your program may stop with a run-time
error or overwrite unpredictable memory addresses.
Thus, before attempting to use the allocated memory block, COPYFILE.C checks
to make sure the call to malloc succeeded:
if( buf == NULL )
{
.
.
.
}
The if statement tests whether the pointer buf has been set to NULL,
which would signal failure. In that case, the program executes the code
within the braces of the if statement.
Sometimes there may be enough free memory to satisfy only part of your
memory request. Look at how COPYFILE.C handles this situation:
buf = (char *)malloc( (size_t)count );
if( buf == NULL )
{
count = _memmax();
buf = (char *)malloc( (size_t)count );
if( buf == NULL )
return ENOMEM;
}
If fewer than count bytes of memory are available, the initial call to
malloc returns NULL, indicating failure. In that event, COPYFILE.C calls
the _memmax library routine to find how much memory is available and assigns
that value to the variable count:
count = _memmax();
Then COPYFILE.C calls malloc again, requesting a smaller amount of memory.
This request is bound to succeed unless no memory is available.
Accessing an Allocated Memory Block
Once you have allocated a block of memory, you can access it through its
pointer ( buf, in this example). COPYFILE.C uses its allocated block as a
file buffer, alternately reading in data from the source file, through the
statement
if( (count = read( hsource, buf, count )) == -1 )
return errno;
and writing it to the target file, through the statement
if( (count = write( htarget, buf, count )) == - 1 )
return errno;
The read and write function calls occur within if statements that compare
the function return values to -1, which would indicate failure.
COPYFILE.C treats its allocated block as a single chunk of memory. To access
individual data items in an allocated block, you can use either pointer or
array notation. Both of the following statements, for instance, refer to the
third byte in the block that buf points to:
buf[2] = 'x';
*(buf+2) = 'x';
Allocating Memory for Different Data Types
Since COPYFILE.C accesses its allocated block through a char pointer, the
program must treat the items in that block as char types. If you need to use
a different type of memory, simply change the pointer declaration and cast
the return from malloc accordingly. For instance, you could use the
following statements to allocate a block large enough to store 30 int
values:
int *buf;
buf = (int *)malloc( (size_t)sizeof( int ) * 30 );
Here, the sizeof operator eliminates the need to calculate how many bytes of
storage 30 integers require. The expression
sizeof( int )
returns the size of an int type, which we then multiply by the desired
number of int items.
If the above call to malloc succeeds, you have, in effect, a 30-element
array of integers. And since pointer notation and array notation are
interchangeable, you can access any element of the array using the pointer
name and array notation. For instance, the expression
ptr[2] = 50;
assigns the value 50 to the third element of the array. Note that this
statement accesses the third int element in the array, not the third byte.
Pointer references, as explained in Chapter 8, "Pointers," are always scaled
by the size of the type used to declare the pointer.
Allocating memory for structures is equally straightforward. Say that you
want to allocate memory to store 10 structures of the type employee, which
is declared in the EMPLOYEE.C program in Chapter 4, "Data Types." The
EMPLOYEE.C program uses the following structure type:
struct employee
{
char name[10];
int months;
float wage;
};
You could use the statement
struct employee *e_ptr;
to declare a pointer to an item of the employee type. Once you have a
suitable pointer, you could use the following statement to allocate enough
memory to store 10 structures of the same type:
e_ptr = (struct employee *) malloc( (size_t)sizeof( struct employee ) * 10
);
Here, the sizeof operator
sizeof( struct employee )
returns the size of a structure of the employee type.
If the allocation succeeds, you have, in effect, an array of structures of
type employee. Using structure notation, you can access any structure member
in the block. The following statements, for instance, initialize the members
of the third structure in the array:
strcpy( e_ptr[2].name, "Isaac, N." );
e_ptr[2].months = 54;
e_ptr[2].wage = (float) 12.21;
Deallocating Memory with the free Function
The free function deallocates an allocated memory block.
When you have finished using an allocated memory block, you should free
(deallocate) the block with the free library function. The free function
takes one argument: the address of the block you wish to free. The
COPYFILE.C program frees its allocated block with the statement:
free( buf );
It's your responsibility to pass a valid address to free. Unlike most
library functions, free doesn't return any value to indicate success or
failure. If you pass an invalid address, the memory block remains allocated
and can't be used for any other purpose.
Figure 12.3 shows COPYFILE.C after the program frees its allocated block.
The free function releases the block from the program's control, returning
it to the heap. The same memory is still present, of course. But since your
program no longer has control of that memory, you shouldn't attempt to use
it. (See "Using Dangling Pointers" in Chapter 10, "Programming Pitfalls,"
for more information on this point.)
(This figure may be found in the printed book.)
Specialized Memory-Allocating Functions
The C library contains two specialized versions of malloc that you may find
useful. The calloc function allocates memory for an array and sets the
block's contents to 0. The realloc function can expand or shrink an existing
memory block.
The calloc Function
The calloc (calculated allocate) function is especially useful for
allocating memory for an array. It works like malloc but takes two
arguments:
■ The number of data items for which you wish to allocate memory
■ The size of each data item
This scheme eliminates the need for you to calculate the number of bytes
needed to store the desired array. For instance, the statement
ptr = (int *) calloc( (size_t)30, (size_t)sizeof( int ) );
allocates enough memory for a 30-element integer array, and
e_ptr = (struct employee *) calloc( (size_t)30, sizeof( struct employee )
);
allocates enough memory for a 30-element array of structures of type
employee.
The calloc library function allocates memory and sets every byte in the
block to 0.
The calloc function also sets every byte in the requested block to 0. The
malloc function, as we noted earlier, doesn't change the contents of an
allocated block. If the block contained garbage values before allocation, it
contains garbage after allocation, too.
The realloc Function
Sometimes you may need to adjust the size of an allocated memory block. The
realloc (reallocate) function can expand or shrink an existing memory block.
The function takes two arguments:
■ The address of an existing allocated block
■ The size (in bytes) you want to give the block
The realloc library function expands or shrinks an existing allocated
block.
If enough memory is available to accommodate the resized block, realloc
allocates sufficient memory and copies as much of the existing block as the
new block will hold. If the new block is smaller than the original, data is
truncated.
For instance, if you had allocated a 30-element int array with the statement
ptr = (int *) calloc( (size_t)30, (size_t)sizeof( int ) );
the following statement would expand the block to contain 20 extra elements,
for a total of 50:
ptr = (int *)realloc( ptr, (size_t)sizeof( int ) * 50 );
The address you pass to realloc can be the address returned from a previous
call to any memory-allocation function: malloc, calloc, or realloc itself.
Like malloc, both calloc and realloc return a null address if they fail.
Remember to check the return value whenever you call a memory-allocating
function.
Keeping Out of Trouble
Here are a few rules to help you avoid trouble when allocating memory
dynamically:
■ Always check the return value when allocating memory.
■ Be careful not to index past the boundaries of an allocated memory
block.
■ Free allocated memory as soon as you have finished using it.
■ Make sure that the address you pass to the free function is valid.
■ Don't use a pointer to an allocated block after freeing the block.
Most of these points were mentioned earlier, but the second deserves a
little elaboration. As you may recall from earlier chapters, the C language
doesn't check array subscripts or pointer references for validity. It's
important to remember this rule when using a pointer to access an allocated
block.
For instance, suppose that you allocate a 30-element integer array with the
statement
ptr = (int *) malloc( (size_t)sizeof( int ) * 30 );
and then execute either of these statements:
ptr[32] = 80;
*(ptr+32) = 80;
Since the array has only 30 elements, both of the latter statements
overwrite memory outside the allocated memory block. The statements store
the value 80 in the address four bytes (two int elements) above the highest
element in the array.
While uncontrolled pointer operations always carry the potential for
disaster, they can create especially tricky program bugs if you write just
beyond an allocated memory block.
Near the beginning of each allocated block is a tiny "link" containing
information about the block. The memory-allocating functions use these links
to keep track of allocated memory, and the more blocks you have allocated,
the more important it is to keep all the links intact. If a bad pointer
reference overwrites a link, it can cause problems in an entirely unexpected
part of your program.
Chapter 13 Graphics
────────────────────────────────────────────────────────────────────────────
This chapter explains how to call graphics functions that set points, draw
lines, change colors, and draw shapes such as rectangles and circles. The
first section lists the three steps to using high-resolution graphics,
defines important graphics terms, and works through an example program step
by step, showing how to use the basic functions. The next sections explain
coordinate systems and show how to display graphics inside viewports and
windows.
Graphics Mode
There are three steps to displaying graphics in QuickC:
1. Use the _getvideoconfig function to determine which video adapter is
installed. (See the section "Checking the Current Video Mode.")
2. Use the _setvideomode function to set the desired graphics mode for
the installed video adapter. (See the section "Setting the Video
Mode.")
3. Draw the graphics on the screen. (See the section "Writing a Graphics
Program.")
There are several definitions you need to know before you can create
graphics programs. The following list explains the most useful terms:
■ The "x axis" determines the horizontal position on the screen. The
"origin" (point 0, 0) is in the upper left corner. The maximum number
of horizontal "pixels" (picture elements) varies from 320 to 640 to
720, depending on the graphics card installed and the graphics mode in
effect.
■ The "y axis" is the vertical position. The origin is the upper left
corner. The number of vertical pixels ranges from 200 to 480.
■ Each graphics mode offers a "palette" from which you may choose the
colors to be displayed. You may have access to 2, 4, 8, 16, or 256
"color indexes," depending on the graphics card in the computer and
the graphics mode in effect.
■ The CGA (Color Graphics Adapter) modes offer four fixed palettes
containing predefined colors that may not be changed. In EGA (Enhanced
Graphics Adapter), MCGA (Multicolor Graphics Array), and VGA (Video
Graphics Array) graphics modes, you may change any of the color
indexes by providing a color value that describes the mix of colors
you wish to use.
■ A color index is always a short integer. A color value is always a
long integer. When you're calling graphics functions that require
color-related parameters, you should be aware of the difference
between color indexes and color values.
Checking the Current Video Mode
Before or after entering graphics mode, you may inquire about the current
video configuration. This requires a special structure type called
videoconfig, which is defined in the GRAPH.H header file. You pass the
address of the structure to the function _getvideoconfig, which returns the
current video configuration information.
All graphics programs should include the graphics header file and declare a
structure of type videoconfig. The structure contains the following
elements:
short numxpixels; /*number of pixels on x axis*/
short numypixels; /*number of pixels on y axis*/
short numtextcols; /*number of text columns available*/
short numtextrows; /*number of text rows available*/
short numcolors; /*number of color indexes*/
short bitsperpixel; /*number of bits per pixel*/
short numvideopages;/*number of available video pages*/
short mode; /*current video mode*/
short adapter; /*active display adapter*/
short monitor; /*active display monitor*/
short memory; /*adapter video memory in K bytes*/
These variables within the videoconfig structure are initialized when you
call _getvideoconfig.
Setting the Video Mode
Before you can start drawing pictures on the screen, your program must tell
the graphics adapter to switch from video text mode to graphics mode. To do
this, call _setvideomode, passing it a single integer that tells it which
mode to display. The following constants are defined in the GRAPH.H file.
The dimensions are listed in pixels for graphics mode and in columns for
video text modes.
╓┌───────────────┌─────────────────────────────┌─────────────────────────────╖
Constant Video Mode Mode Type/Hardware
────────────────────────────────────────────────────────────────────────────
_DEFAULTMODE Restores to original mode Both/All
_ERESCOLOR 640 x 350, 4 or 16 color Graphics/EGA
_ERESNOCOLOR 640 x 350, BW Graphics/EGA
Constant Video Mode Mode Type/Hardware
────────────────────────────────────────────────────────────────────────────
_ERESNOCOLOR 640 x 350, BW Graphics/EGA
_HERCMONO 720 x 348, BW for HGC Graphics/HGC
_HRES16COLOR 640 x 200, 16 color Graphics/EGA
_HRESBW 640 x 200, BW Graphics/CGA
_MRES4COLOR 320 x 200, 4 color Graphics/CGA
_MRES16COLOR 320 x 200, 16 color Graphics/EGA
_MRES256COLOR 320 x 200, 256 color Graphics/VGA/
MCGA
_MRESNOCOLOR 320 x 200, 4 gray Graphics/CGA
_ORESCOLOR 640 x 400, 1 of 16 colors Graphics/
Olivetti(R)
Constant Video Mode Mode Type/Hardware
────────────────────────────────────────────────────────────────────────────
Olivetti(R)
_TEXTBW40 40-column text, 16 gray Text/CGA
_TEXTBW80 80-column text, 16 gray Text/CGA
_TEXTC40 40-column text, 16/8 color Text/CGA
_TEXTC80 80-column text, 16/8 color Text/CGA
_TEXTMONO 80-column text, BW Text/MDA
_VRES2COLOR 640 x 480, BW Graphics/VGA/
MCGA
_VRES16COLOR 640 x 480, 16 color Graphics/VGA
────────────────────────────────────────────────────────────────────────────
Constant Video Mode Mode Type/Hardware
────────────────────────────────────────────────────────────────────────────
If _setvideomode returns a 0, it means the hardware does not support the
selected mode. You may continue to select alternate video modes until a
nonzero value is returned. If the hardware configuration doesn't support any
of the selected video modes, take the appropriate exit action.
Writing a Graphics Program
The SINE.C program below graphs a sine curve. The program illustrates how to
call many of the important graphics functions. The main function calls five
other functions, which are defined later in this chapter. To view the
complete program, use online help.
────────────────────────────────────────────────────────────────────────────
WARNING
When you installed QuickC on your system, you may have chosen not to include
the graphics library. If this is the case, the programs in this chapter
won't compile unless you explicitly link the graphics library. See the
Microsoft QuickC Tool Kit for information about linking libraries.
────────────────────────────────────────────────────────────────────────────
/* SINE.C: Basic graphics commands. */
#include <stdio.h>
#include <stdlib.h>
#include <graph.h>
#include <math.h>
#include <conio.h>
#define PI 3.14159
void graphics_mode( void );
void draw_lines( void );
void sine_wave( void );
void draw_shapes( void );
void end_program( void );
int newx( int );
int newy( int );
struct videoconfig myscreen;
int maxx, maxy;
unsigned char diagmask[8] =
{ 0x93, 0xC9, 0x64, 0xB2, 0x59, 0x2C, 0x96, 0x4B };
unsigned char linemask[8] =
{ 0xFF, 0x00, 0x7F, 0xFE, 0x00, 0x00, 0x00, 0xCC };
main()
{
graphics_mode();
draw_lines();
sine_wave();
draw_shapes();
end_program();
}
/*
Definitions of functions go here
*/
The SINE.C program's output is shown in Figure 13.1.
(This figure may be found in the printed book.)
Turning on Graphics Mode
Before you can display graphics, you must put the graphics adapter into a
graphics mode. The _setvideomode function performs this task. Before calling
_setvideomode, you must decide which graphics modes are acceptable for your
purposes. The first function in SINE.C is named graphics_mode. It selects
the highest possible resolution available, based on the graphics card
currently in use.
Four header files are included in the SINE.C program:
#include <stdio.h>
#include <stdlib.h>
#include <graph.h>
#include <math.h>
Although the MATH.H file is not required for graphics programs, we include
it in the SINE.C program because it contains floating-point math functions
such as sin.
Later in the program we'll need to get information about the screen size, so
the videoconfig structure called myscreen is declared:
struct videoconfig myscreen;
The functions called by main aren't in the standard library; they're defined
within SINE.C.
The first function is graphics_mode, which turns on graphics capabilities:
void graphics_mode( void )
{
_getvideoconfig( &myscreen );
switch( myscreen.adapter )
{
case _CGA:
_setvideomode( _HRESBW );
break;
case _OCGA:
_setvideomode( _ORESCOLOR );
break;
case _EGA:
case _OEGA:
if( myscreen.monitor == _MONO )
_setvideomode( _ERESNOCOLOR );
else
_setvideomode( _ERESCOLOR );
break;
case _VGA:
case _OVGA:
case _MCGA:
_setvideomode( _VRES2COLOR );
break;
case _HGC:
_setvideomode( _HERCMONO );
break;
default:
printf( "This program requires a CGA, EGA, VGA, or Hercules
card\n" );
exit( 0 );
}
_getvideoconfig( &myscreen );
maxx = myscreen.numxpixels - 1;
maxy = myscreen.numypixels - 1;
}
────────────────────────────────────────────────────────────────────────────
NOTE
If you use a Hercules(R) adapter, you must run the MSHERC.COM program before
attempting to display any graphics. Always run MSHERC.COM before running
QuickC (do not run it from QuickC's DOS shell).
────────────────────────────────────────────────────────────────────────────
The function begins by calling _getvideoconfig, passing the address of the
videoconfig structure. Within the structure a member called adapter tells us
the type of adapter currently in use. With that knowledge, and a switch
statement, we can enter the appropriate graphics mode.
But how much screen do we have to work with? The screen might be 720 x 348,
640 x 480, 640 x 400, 640 x 350, or 640 x 200. Whenever you call
_setvideomode, you can ask for information about the currently displayed
screen with _getvideoconfig. Just pass it the address of the videoconfig
structure that was declared earlier:
_getvideoconfig( &myscreen );
maxx = myscreen.numxpixels - 1;
maxy = myscreen.numypixels - 1;
Let's say your computer has an EGA card, which means that at this point,
_ERESNOCOLOR is in effect. The horizontal screen size is 640 pixels and
vertical screen size is 350. The two assignments above assign these values
to maxx and maxy, less 1. The horizontal resolution might be 640, but the
pixels are numbered 0-639. Thus, the maxx variable─the highest available
pixel number─must be 1 less than the total number of pixels:
myscreen.numxpixels - 1
Two short functions perform conversions from an imaginary 1000 x 1000 screen
to whatever graphics mode is in effect. From this point forward, the program
will assume it has 1000 pixels in each direction, passing the values to
newx and newy for conversion to actual coordinates:
int newx( int xcoord )
{
int nx;
float tempx;
tempx = ((float)maxx)/ 1000.0;
tempx = ((float)xcoord) * tempx + 0.5;
return( (int)tempx );
}
int newy( int ycoord )
{
int ny;
float tempy;
tempy = ((float)maxy)/ 1000.0;
tempy = ((float)ycoord) * tempy + 0.5;
return( (int)tempy );
}
Drawing Rectangles and Lines
The next function called in SINE.C is draw_lines. As the name implies, the
draw_lines function draws several lines on the screen: a rectangle around
the outer edges of the screen and three horizontal lines that cut the screen
into quarters.
void draw_lines( void )
{
_rectangle( _GBORDER, 0, 0, maxx, maxy );
/* _setcliprgn( 20, 20, maxx - 20, maxy - 20 ); */
_setvieworg( 0, newy( 500 ) );
_moveto( 0, 0 );
_lineto( newx( 1000 ), 0 );
_setlinestyle( 0xAA3C );
_moveto( 0, newy( -250) );
_lineto( newx( 1000 ), newy( -250 ) );
_setlinestyle( 0x8888 );
_moveto( 0, newy( 250 ) );
_lineto( newx( 1000 ), newy( 250 ) );
}
The call to the _rectangle function has five arguments. The first argument
is the fill flag, which may be either _GBORDER or _GFILLINTERIOR. Choose
_GBORDER if you want a rectangle of four lines (a border only, in the
current line style). Or you can choose _GFILLINTERIOR if you want a solid
rectangle (filled in with the current color and fill pattern). We will
discuss how to choose the color and fill pattern later in this chapter.
The second and third arguments are the x and y coordinates of one corner of
the rectangle. The fourth and fifth arguments are the coordinates of the
opposite corner. Since the coordinates for the two corners are ( 0, 0 ) and
( maxx, maxy ) , the call to _rectangle frames the screen.
_rectangle( _GBORDER, 0, 0, maxx, maxy );
Drawing lines is a two-step process. Move to one location on the screen and
draw the line to another location, using the _moveto and _lineto functions:
_setlinestyle( 0xAA3C );
_moveto( 0, newy(-250) );
_lineto( newx(1000), newy(-250) );
Use the _setlinestyle function to change from a solid line to a dashed line
by passing it one integer value. In the example above, the number 0xAA3C
causes the line to become the graphics equivalent of binary 1010 1010 0011
1100.
The _moveto function positions an imaginary pixel cursor at a spot on the
screen. Nothing visible appears on the screen. The _lineto function draws a
line. The negative value -250 might seem to be an impossible screen
coordinate. It would be, but the program has changed the viewport
organization of the screen with the _setvieworg function. The top half of
the screen now contains negative y coordinates, and the bottom half contains
positive y coordinates. Viewports are explained in more detail later in this
chapter.
Setting a Pixel
The next step in the SINE.C program is to draw the sine curve. This requires
the sine_wave function which is shown below. This function calculates
positions for two sine waves and plots them on the screen:
void sine_wave( void )
{
int locx, locy;
double i, rad;
for( i = 0; i < 1000; i += 3 )
{
rad = -sin( (PI * (float) i) / 250.0 );
locx = newx( (int) i );
locy = newy( (int) (rad * 250.0) );
_setpixel( locx, locy );
}
}
The only graphics function called is _setpixel, which takes two parameters,
an x and a y coordinate. The function turns on the pixel at that location.
Drawing Shapes
After the sine curve is drawn, the SINE.C program calls the draw_shapes
function to draw two rectangles and two ellipses on the screen. The fill
mask alternates between _GBORDER and _GFILLINTERIOR:
void draw_shapes( void )
{
_setlinestyle( 0xFFFF );
_setfillmask( diagmask );
_rectangle( _GBORDER, newx( 50 ), newy( -325 ), newx( 200 ), newy( -425
) );
_rectangle(_GFILLINTERIOR,newx(550),newy(-325),newx(700),newy(-425));
_setfillmask( linemask );
_ellipse( _GBORDER, newx( 50 ), newy( 325 ), newx( 200 ), newy( 425 )
);
_ellipse( _GFILLINTERIOR,newx( 550 ),newy( 325 ),newx( 700 ),newy( 425
) );
}
Note that _setlinestyle resets the line pattern to solid. If you omit this
function (or comment it out), the first rectangle would be drawn with dashes
instead of a solid line.
The _ellipse function draws an ellipse on the screen. Its parameters
resemble the parameters for _rectangle. Both functions require a fill flag
and two corners of a "bounding rectangle." When the ellipse is drawn, four
points touch the edges of the bounding rectangle.
The _GFILLINTERIOR flag fills the shape with the current fill pattern. To
select a pattern, you must first use the _setfillmask function, passing the
address of an eight-byte array of unsigned characters. Earlier in the
program diagmask was defined as the shape shown in Table 13.1 below.
Table 13.1 Fill Patterns
╓┌─────────────────────────────────┌─────────────────────────────────────────╖
Bit Pattern Value in diagmask
────────────────────────────────────────────────────────────────────────────
☼ ∙ ∙ ☼ ∙ ∙ ☼ ☼ diagmask[0] = 0x93
☼ ☼ ∙ ∙ ☼ ∙ ∙ ☼ diagmask[1] = 0xC9
∙ ☼ ☼ ∙ ∙ ☼ ∙ ∙ diagmask[2] = 0x64
☼ ∙ ☼ ☼ ∙ ∙ ☼ ∙ diagmask[3] = 0xB2
∙ ☼ ∙ ☼ ☼ ∙ ∙ ☼ diagmask[4] = 0x59
∙ ∙ ☼ ∙ ☼ ☼ ∙ ∙ diagmask[5] = 0x2C
☼ ∙ ∙ ☼ ∙ ☼ ☼ ∙ diagmask[6] = 0x96
∙ ☼ ∙ ∙ ☼ ∙ ☼ ☼ diagmask[7] = 0x4B
────────────────────────────────────────────────────────────────────────────
Exiting Graphics Mode
The final function to be called by the SINE.C program is end_program, which
waits for a key press and then sets the screen back to normal:
void end_program( void )
{
getch();
_setvideomode( _DEFAULTMODE );
}
Using Color Graphics Modes
In this example, the program COLOR.C sets a mode with as many color choices
as possible for the available hardware:
/* COLOR.C: Sets a medium resolution mode
with maximum color choices. */
#include <stdio.h>
#include <stdlib.h>
#include <graph.h>
#include <conio.h>
struct videoconfig vc;
main()
{
if( _setvideomode( _MRES256COLOR ) );
else if( _setvideomode( _MRES16COLOR ) );
else if( _setvideomode( _MRES4COLOR ) );
else
{
printf( "Error: No color graphics capability\n" );
exit( 0 );
}
_getvideoconfig( &vc );
printf( "%d available colors\n", vc.numcolors );
printf( "%d horizontal pixels\n", vc.numxpixels );
printf( "%d vertical pixels\n", vc.numypixels );
getch();
_clearscreen( _GCLEARSCREEN );
_setvideomode( _DEFAULTMODE );
}
Although color graphics are an improvement over black and white, if you use
color you must make a compromise. When you request the maximum number of
colors, you sacrifice some resolution─a 320 x 200 screen instead of a higher
resolution. Thus, the COLORS.C program always creates a screen 320 pixels
wide and 200 pixels high. Note also the use of the function _clearscreen,
which clears the screen in any video mode (text or graphics).
To view every possible graphics mode, you can run the program GRAPHIC.C
shown below. Explanations of the various color graphics modes─CGA, EGA, and
VGA─follow.
/* GRAPHIC.C: Display every graphics mode. */
#include <stdio.h>
#include <graph.h>
#include <conio.h>
struct videoconfig screen;
int modes[12] =
{
_MRES4COLOR, _MRESNOCOLOR, _HRESBW, _HERCMONO,
_MRES16COLOR, _HRES16COLOR, _ERESNOCOLOR, _ERESCOLOR,
_VRES2COLOR, _VRES16COLOR, _MRES256COLOR, _ORESCOLOR
};
void print_menu( void );
void show_mode( char );
main()
{
char key;
print_menu();
while( (key = getch()) != 'x' )
show_mode( key );
}
void print_menu( void )
{
_setvideomode( _DEFAULTMODE );
_clearscreen( _GCLEARSCREEN );
printf( "Please choose a graphics mode\nType 'x' to exit.\n\n" );
printf( "0 _MRES4COLOR\n1 _MRESNOCOLOR\n2 _HRESBW\n" );
printf( "3 _HERCMONO\n4 _MRES16COLOR\n5 _HRES16COLOR\n" );
printf( "6 _ERESNOCOLOR\n7 _ERESCOLOR\n" );
printf( "8 _VRES2COLOR\n9 _VRES16COLOR\na _MRES256COLOR\n" );
printf( "b _ORESCOLOR\n" );
}
void show_mode( char which )
{
int nc, i;
int height, width;
int mode = which;
if( mode < '0' || mode > '9' )
if( mode == 'a' )
mode = '9' + 1;
else if( mode == 'b' )
mode = '9' + 2;
else
return;
if( _setvideomode( modes[mode - '0'] ) )
{
_getvideoconfig( &screen );
nc = screen.numcolors;
width = screen.numxpixels/nc;
height = screen.numypixels/2;
for( i = 0; i < nc; i++ )
{
_setcolor( i );
_rectangle( _GFILLINTERIOR, i * width, 0, (i + 1) * width, height );
}
}
else
{
printf( " \nVideo mode %c is not available.\n", which );
printf( "Please press a key.\n" );
}
getch();
_setvideomode( _DEFAULTMODE );
print_menu();
}
CGA Color Graphics Modes
The CGA color graphics modes _MRES4COLOR and _MRESNOCOLOR display four
colors selected from one of several predefined palettes of colors. They
display these foreground colors against a background color which can be any
one of the 16 available colors. With the CGA hardware, the palette of
foreground colors is predefined and cannot be changed. Each palette number
is an integer as shown in Table 13.2.
Table 13.2 Available CGA Colors
╓┌─────────────────┌─────────────┌───────────────┌───────────────────────────╖
Palette Number Color Index
1 2 3
────────────────────────────────────────────────────────────────────────────
0 Green Red Brown
1 Cyan Magenta Light gray
2 Light green Light red Yellow
Palette Number Color Index
1 2 3
────────────────────────────────────────────────────────────────────────────
2 Light green Light red Yellow
3 Light cyan Light magenta White
────────────────────────────────────────────────────────────────────────────
The _MRESNOCOLOR graphics mode produces palettes containing various shades
of gray on black-and-white monitors. The _MRESNOCOLOR mode displays colors
when used with a color display. However, only two palettes
are available with a color display. You can use the _selectpalette function
to select one of these predefined palettes. Table 13.3 shows the
correspondence between the color indexes and the palettes.
Table 13.3 CGA Colors: _MRESNOCOLOR Mode
╓┌─────────────────┌────────────┌───────────┌────────────────────────────────╖
Palette Number Color Index
1 2 3
Palette Number Color Index
1 2 3
────────────────────────────────────────────────────────────────────────────
0 Blue Red Light gray
1 Light blue Light red White
────────────────────────────────────────────────────────────────────────────
You may use the _selectpalette function only with the _MRES4COLOR and
_MRESNOCOLOR graphics modes. To change palettes in other graphics modes, use
the _remappalette or _remapallpalette functions.
The following program sets the video mode to _MRES4COLOR and then cycles
through background colors and palette combinations. It works on computers
equipped with CGA, EGA, MCGA, or VGA cards. A color monitor is required.
/* CGA.C: Demonstrate CGA colors. */
#include <stdio.h>
#include <graph.h>
#include <conio.h>
long bkcolor[8] =
{
_BLACK, _BLUE, _GREEN, _CYAN,
_RED, _MAGENTA, _BROWN, _WHITE
};
char *bkcolor_name[] =
{
"_BLACK", "_BLUE", "_GREEN", "_CYAN",
"_RED", "_MAGENTA", "_BROWN", "_WHITE"
};
main()
{
int i, j, k;
_setvideomode( _MRES4COLOR );
for( i=0; i<= 3; i++ )
{
_selectpalette( i );
for( k=0; k <= 7; k++ )
{
_setbkcolor( bkcolor[k] );
for( j=0; j<=3; j++ )
{
_settextposition( 1, 1 );
printf( "background color: %8s\n", bkcolor_name[k] );
printf( "palette: %d\ncolor: %d\n", i, j );
_setcolor( j );
_rectangle( _GFILLINTERIOR, 160, 100, 320, 200 );
getch();
}
}
}
_setvideomode( _DEFAULTMODE );
}
EGA, MCGA, and VGA Palettes
At the beginning of this chapter, we mentioned the difference between color
indexes and color values. An analogy might make things clearer. Imagine a
painter who owns 64 tubes of paint and a painter's palette that has room for
only 16 globs of paint at any one time. A painting created under these
constraints could contain only 16 colors (selected from a total of 64). One
of the EGA graphics modes (_ERESCOLOR) is similar: 16 color indexes chosen
from a total of 64 color values. (Color indexes are sometimes called "color
attributes," or "pixel values." Color values are sometimes called "actual
colors.")
VGA Color Mixing - VGA offers the widest variety of color values: 262,144
(256K). Depending on the graphics mode, the VGA palette size may be 2, 16,
or 256. When you select a color value, you specify a level of intensity
ranging from 0-63 for each of the red, green, and blue color values. The
long integer that defines a color value consists of four bytes (32 bits):
MSB LSB
zzzzzzzz zzBBBBBB zzGGGGGG zzRRRRRR
The most-significant byte must contain all zeros. The two high bits in the
remaining three bytes must also be 0. To mix a light red (pink), turn red
all the way up, and mix in some green and blue:
00000000 00100000 00100000 00111111
To represent this value in hexadecimal, use the number 0x0020203FL (the L
marks it as a long value). You could also use the following macro:
#define RGB ( r, g, b ) (0x3F3F3FL & ((long)(b) << 16 | (g) << 8 | (r)))
To create pure yellow (100% red plus 100% green) and assign it to a variable
y1, use this line:
y1 = RGB( 63, 63, 0 );
For white, turn all the colors on: RGB( 63, 63, 63 ). For black, set all
colors to 0: RGB( 0, 0, 0 ).
EGA Color Mixing - Mixing colors in EGA modes is similar to the mixing
described above, but there are fewer intensities for the red, green, and
blue components. In the modes that offer 64 colors, the R, G, and B values
cover 2 bits and can range from 0 to 3. The long integer that defines an RGB
color looks like this:
MSB LSB
zzzzzzzz zzBB???? zzGG???? zzRR????
The bits marked z must be zeros and the bits marked with question marks
can be any value. To form a pure red color value, you would use the constant
0x00000030L. For cyan (blue plus green), use 0x00303000L. The RGB macro
defined above is easily modified for EGA monitors:
#define EGARGB( r, g, b ) (0x3F3F3FL & ((long)(b) << 20 | (g) << 12 | (r
<< 4)))
In this macro, you would pass values in the range 0-3 instead of 0-63.
EGA Color Graphics Modes
The _MRES16COLOR, _HRES16COLOR, or _ERESCOLOR video modes display the best
color graphics with an EGA adapter. The CGA modes will also display on the
EGA but with the lower CGA resolution and decreased color options.
The _remappalette function assigns a new color value to a color index. For
example, when you first enter an EGA graphics mode, color index 1 equals the
color value blue. To reassign the pure red color value to color index 1, you
could use this line:
_remappalette( 1, 0x000030L );
Or, use the symbolic constant _RED, which is defined in the GRAPH.H file:
_remappalette( 1, _RED );
After this function call, any object currently drawn in color index 1 will
instantly switch from blue to red.
For EGA graphics, the first value is an integer in the range 0-15 and the
second value is a long int defined as a mixture of red, green, and blue (you
may also use the symbolic constants such as _RED).
The _remapallpalette function changes all of the color indexes
simultaneously. You pass it an array of color values. The first color value
in the list becomes the new color associated with the color index 0.
The number in a function call to set the color (such as _setcolor) is an
index into the palette of available colors. In the default text palette, an
index of 1 refers to blue but the palette could be remapped to change index
1 to any other available color. As a result, the color produced by that
pixel value also changes. The number of color indexes depends on the number
of colors supported by the current video mode.
The _remappalette and _remapallpalette functions work in all modes but only
with the EGA, MCGA, or VGA hardware. The _remappalette and _remapallpalette
functions fail and return a value of -1 when you attempt to remap a palette
without the EGA, MCGA, or VGA hardware.
The following program draws a rectangle with a red interior. In the default
EGA palette, color index 4 is red. This color index is changed to _BLUE in
this program.
/* EGA.C: EGA palettes. */
#include <stdio.h>
#include <conio.h>
#include <graph.h>
main()
{
_setvideomode( _ERESCOLOR );
_setcolor( 4 );
_rectangle( _GFILLINTERIOR, 50, 50, 200, 200 );
_settextposition( 1, 1 );
printf( "Normal palette\n" );
printf( "Press a key" );
getch();
_remappalette( 4, _BLUE );
_settextposition( 1, 1 );
printf( "Remapped palette\n" );
printf( "Press a key" );
getch();
_remappalette( 4, _RED );
_settextposition( 1, 1 );
printf( "Restored palette\n" );
printf( "Press a key to clear the screen" );
getch();
_clearscreen( _GCLEARSCREEN );
_setvideomode( _DEFAULTMODE );
}
VGA Color Graphics Modes
The VGA card adds graphics modes _VRES2COLOR, _VRES16COLOR, and
_MRES256COLOR to your repertoire. EGA and CGA modes can also be used with
the VGA hardware, but with either lower resolution or fewer color choices.
The VGA color graphics modes operate with a range of 262,144 (256K) color
values. The _VRES2COLOR graphics mode displays two colors, the _VRES16COLOR
graphics mode displays 16, and the _MRES256COLOR graphics mode displays 256
colors from the available VGA colors.
Changing the Palette - The _remappalette function changes a color index to a
specified color value. The function below remaps the color index 1 to the
color value given by the symbolic constant _RED (which represents red).
After this statement is executed, whatever was displayed as blue will now
appear as red:
_remappalette( 1, _RED ); /*reassign color index 1
to VGA red */
Use the _remapallpalette function to remap all of the available color
indexes simultaneously. The function's argument references an array of color
values that reflects the remapping. The first color number in the list
becomes the new color associated with color index 0.
Symbolic constants for the default color numbers are supplied so that the
remapping of VGA colors is compatible with EGA practice. The names of these
constants are self-explanatory. For example, the color numbers for black,
red, and light yellow are represented by the symbolic constants _BLACK,
_RED, and _LIGHTYELLOW.
All of the VGA display modes operate with any VGA video monitor. Colors are
displayed as shades of gray when the monochrome analog display is connected.
If you have a VGA card, the HORIZON.C program illustrates what can be done
with the range of 256 colors:
/* HORIZON.C: VGA graphics with cycling of 256 colors. */
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <graph.h>
#define RED 0x0000003FL
#define GRN 0x00003F00L
#define BLU 0x003F0000L
#define WHT 0x003F3F3FL
#define STEP 21
struct videoconfig screen;
long int rainbow[512];
main()
{
int i;
long int col, gray;
if( _setvideomode( _MRES256COLOR ) == 0 )
{
printf( "This program requires a VGA card.\n" );
exit( 0 );
}
for( col = 0; col < 64; col++ )
{
gray = col | (col << 8) | (col << 16);
rainbow[col] = rainbow[col + 256] = BLU & gray;
rainbow[col + 64] = rainbow[col + 64 + 256] = BLU | gray;
rainbow[col + 128] = rainbow[col + 128 + 256] = RED | (WHT & ~gray);
rainbow[col + 192] = rainbow[col + 192 + 256] = RED & ~gray;
}
_setvieworg( 160, 85 );
for( i = 0; i < 255; i++ )
{
_setcolor( 255 - i );
_moveto( i, i - 255 );
_lineto( -i, 255 - i );
_moveto( -i, i - 255 );
_lineto( i, 255 - i );
_ellipse( _GBORDER, -i, -i / 2, i, i / 2 );
}
for( i = 0; !kbhit(); i += STEP, i %= 256 )
_remapallpalette( &(rainbow[i]) );
_setvideomode( _DEFAULTMODE );
}
Using the Color Video Text Modes
Two color video text modes, _TEXTC40 and _TEXTC80, can be used with the CGA,
EGA, and VGA displays. These modes display steady or blinking text in any of
16 foreground colors with any one of 8 background colors.
Basics of Text Color Selection
In a video text mode, each displayed character requires two bytes of video
memory. The first byte contains the ASCII code representing the character
and the second byte contains the display attribute. In the CGA color video
text modes, the attribute byte determines the color and whether it will
blink. Sixteen colors are available: the CGA pixel values, and the default
EGA and VGA pixel values. Since the EGA and VGA palette can be remapped,
these values can be made to correspond to any set of 16 colors with the
appropriate palette mapping.
Using Text Colors
Use the _gettextcolor and _getbkcolor functions to find the current text
foreground and background colors.
Values in the range 0-15 are interpreted as normal color. Values in the
range 16-31 are the same colors as those in the range 0-15 but with blinking
text.
Use the _settextcolor and _setbkcolor functions to set foreground and
background colors in video text mode. These functions use a single argument
that specifies the pixel value to be used for text displayed with the
_outtext function. The color indexes for color video text modes are defined
in Table 13.4.
Table 13.4 Text Colors
╓┌───────┌─────────┌───────┌─────────────────────────────────────────────────╖
Number Color Number Color
────────────────────────────────────────────────────────────────────────────
0 Black 8 Dark gray
1 Blue 9 Light blue
2 Green 10 Light green
3 Cyan 11 Light cyan
4 Red 12 Light red
5 Magenta 13 Light magenta
6 Brown 14 Light brown
Number Color Number Color
────────────────────────────────────────────────────────────────────────────
6 Brown 14 Light brown
7 White 15 Light white
────────────────────────────────────────────────────────────────────────────
Displaying Color Text
The _settextposition function moves the cursor to a row and column for
displaying color text. The _outtext function displays the text.
Example: Viewing Text Colors
The following program displays a chart showing all possible combinations of
text and background colors:
/* COLTEXT.C: Display text in color. */
#include <stdio.h>
#include <conio.h>
#include <graph.h>
char buffer [80];
main()
{
int blink,fgd;
long bgd;
_clearscreen( _GCLEARSCREEN );
printf( "Text color attributes:\n" );
for( blink=0; blink<=16; blink+=16 )
{
for( bgd=0; bgd<8; bgd++ )
{
_setbkcolor( bgd );
_settextposition( bgd + ((blink / 16) * 9) + 3, 1 );
_settextcolor( 7 );
sprintf( buffer, "Bgd: %d Fgd:", bgd );
_outtext( buffer );
for( fgd=0; fgd<16; fgd++ )
{
_settextcolor( fgd+blink );
sprintf( buffer, " %2d ", fgd+blink );
_outtext( buffer );
}
}
}
getch();
_setvideomode( _DEFAULTMODE );
}
Text Coordinates
Before you can write a program to print a word over there on the screen, you
need a system that describes to the compiler where there really is. QuickC
divides the text screen into rows and columns. See Figure 13.2.
(This figure may be found in the printed book.)
Two important conventions to keep in mind about video text mode are:
1. Numbering starts at 1, not 0. An 80-column screen contains columns
1-80.
2. The row is always listed before the column.
If the screen is in a video text mode that displays 25 rows and 80 columns
(as in Figure 13.2), the rows are numbered 1-25 and the columns are numbered
1-80. In functions such as _settextposition, which is called in the next
example program, the parameters you pass are row and column (in that order).
Graphics Coordinates
A similar (but slightly different) system is used for locating pixels on a
graphics screen. There are three ways of describing the location of pixels
on the screen:
1. The physical screen coordinates
2. The viewport coordinates
3. The window coordinates
Each method is explained in the following sections.
The Physical Screen
Suppose you write a program that calls _setvideomode and puts the screen
into the VGA graphics mode _VRES16COLOR. This gives you a screen containing
640 horizontal pixels and 480 vertical pixels. The individual pixels are
named by their location relative to the x axis and y axis, as shown in
Figure 13.3.
(This figure may be found in the printed book.)
Two important differences between text coordinates and pixel coordinates
are:
1. Numbering starts at 0, not 1. If there are 640 pixels, they're
numbered 0-639.
2. The x coordinate (equivalent to a text column) is listed before the y
coordinate.
The upper left corner is called the "origin." The x and y coordinates for
the origin are always (0, 0). If you use variables to refer to pixel
locations, declare them as integers.
Changing the Origin with _setvieworg
The _setvieworg function changes the current location of the viewport's
origin. When you first enter graphics mode, the "viewport" is equivalent to
the physical
screen. You pass two integers, which represent the x and y coordinates of a
physical screen location. For example, the following line would move the
origin to the physical screen location (50, 100):
_setvieworg( 50, 100 );
The effect on the screen is illustrated in Figure 13.4.
(This figure may be found in the printed book.)
The number of pixels hasn't changed, but the names given to the points have
changed. The x axis now ranges from -50 to +589 instead of 0 to 639. The y
axis now covers the values -100 to +379. (If you own an adapter other than
the VGA, the numbers are different but the effect is the same.)
All standard graphics functions are affected by the new origin, including
_arc, _ellipse, _lineto, _moveto, _pie, and _rectangle.
For example, if you call the _rectangle function after relocating the
viewport origin, and pass it the values (0, 0) and (40, 40), the rectangle
would be drawn 50 pixels from the left edge of the screen and 100 pixels
from the top. It would not appear in the upper left corner.
The values passed to _setvieworg are always physical screen locations.
Suppose you called the same function twice:
_setvieworg( 50, 100 );
_setvieworg( 50, 100 );
The viewport origin would not move to (100, 200). It would remain at the
phys-ical screen location (50, 100).
Defining a Clipping Region with _setcliprgn
The _setcliprgn function creates an invisible rectangular area on the screen
called a "clipping region." Attempts to draw inside the clipping region are
successful, while attempts to draw outside the region are not.
When you first enter a graphics mode, the default clipping region occupies
the entire screen. QuickC ignores any attempts to draw outside the screen.
Changing the clipping region requires one call to _setcliprgn. Suppose
you've entered the CGA graphics mode _MRES4COLOR, which has a screen
resolution of 320 x 200. If you draw a diagonal line from (0, 0) to (319,
199), from the top left to the bottom right corner, the screen looks like
Figure 13.5.
(This figure may be found in the printed book.)
You could create a clipping region with this line:
_setcliprgn( 10, 10, 309, 189 )
With the clipping region in effect, the same _lineto command would put the
line shown in Figure 13.6 on the screen.
(This figure may be found in the printed book.)
The broken lines don't actually print on the screen. They indicate the outer
bounds of the clipping region.
Viewport Coordinates
The _setviewport function establishes a new viewport within the boundaries
of the physical screen. A standard viewport has two distinguishing features:
1. The origin of a viewport is in the upper left corner.
2. The clipping region matches the outer boundaries of the viewport.
The _setviewport function does the same thing as calling the _setvieworg and
the _setcliprgn functions.
Real Coordinates in a Window
Functions that refer to coordinates on the physical screen and within the
viewport require integer values. In real-life graphing applications, you
might wish to use floating-point values─stock prices, the price of wheat,
average rainfall, and so on. The _setwindow function allows you to scale the
screen to almost any size. In addition, the window-related functions take
double-precision, floating-point values instead of integers.
For example, say you want to graph 12 months of average temperatures that
range from -40 to +100. You could add the following line to your program:
_setwindow( TRUE, 1.0, -40.0, 12.0, 100.0 );
The first argument is the invert flag, which puts the lowest y value in the
bottom left corner. The minimum and maximum Cartesian coordinates follow
(the decimal point marks them as floating-point values). The new
organization of the screen is shown in Figure 13.7.
(This figure may be found in the printed book.)
Note that January and December are plotted on the left and right edges of
the screen. In an application like this, it might be better to number the x
axis from 0.0 to 13.0, to provide some extra space.
If you next plot a point with _setpixel_w or draw a line with _lineto_w, the
values are automatically scaled to the established window.
Follow these four steps to use real-coordinate graphics:
1. Enter a graphics mode with _setvideomode.
2. Use _setviewport to create a viewport area. (This step is optional if
you plan to use the entire screen.)
3. Create a real-coordinate window with _setwindow, passing an int invert
flag and four double x and y coordinates for the minimum and maximum
values.
4. Draw graphics shapes with _rectangle_w and other functions. Do not
confuse _rectangle (the viewport function) with _rectangle_w (the
window function for drawing rectangles). All window functions end with
an underscore and a letter w or an underscore and wxy.
Real-coordinate graphics can give you a lot of flexibility. For example, you
can fit either axis into a small range (such as 151.25 to 151.45) or into a
large range (-50,000 to +80,000), depending on the type of data you're
graphing. In addition, by changing the window coordinates, you can create
the effects of zooming in or panning across a figure.
Example Program
The program below illustrates some ways to use the real-coordinate windowing
functions.
/* REALG.C: Real-coordinate graphics. */
#include <stdio.h>
#include <conio.h>
#include <graph.h>
#define TRUE 1
#define FALSE 0
int four_colors( void );
void three_graphs( void );
void grid_shape( void );
int halfx, halfy;
struct videoconfig screen;
double bananas[] =
{
-0.3, -0.2, -0.224, -0.1, -0.5, +0.21, +2.9,
+0.3, +0.2, 0.0, -0.885, -1.1, -0.3, -0.2,
+.001, +.005, +0.14, 0.0, -0.9, -0.13, +0.3
};
main()
{
if( four_colors() )
three_graphs();
else
printf( "This program requires a CGA, EGA,\
or VGA graphics card.\n" );
}
/*
. Additional functions defined below
.
.
*/
The main function is very short. It calls the four_colors function
(defined below), which attempts to enter a graphics mode where at least four
colors are available. If it succeeds, the three_graphs function is called,
which uses the numbers in the bananas array to draw three graphs. The
REALG.C screen output is shown in Figure 13.8.
(This figure may be found in the printed book.)
It's worth noting that the grid_shape function (defined below) that draws
the graphs is using the same numbers in each case. However, the program uses
three different real-coordinate windows. The two windows in the top half are
the same size in physical coordinates, but they have different window sizes.
In all three cases, the grid is 2 units wide. In the upper left corner, the
window is 4 units wide; in the upper right, the window is 6 units wide,
which makes the graph appear smaller.
In two of the three graphs, one of the lines goes off the edge, outside the
clipping region. The lines do not intrude into the other windows, since
defining a window creates a clipping region.
Finally, note that the graph on the bottom of the screen seems to be upside
down with respect to the two graphs above it.
Checking the Adapter
The first step in any graphics program is to enter a graphics mode. The
four_colors function performs this step:
/* four_colors function from REALG.C. */
int four_colors( void )
{
_getvideoconfig( &screen );
switch( screen.adapter )
{
case _CGA:
case _OCGA:
_setvideomode( _MRES4COLOR );
break;
case _EGA:
case _OEGA:
_setvideomode( _ERESCOLOR );
break;
case _VGA:
case _OVGA:
_setvideomode( _VRES16COLOR );
break;
default:
return( FALSE );
}
_getvideoconfig( &screen );
return( TRUE );
}
The _getvideoconfig function places some information into the videoconfig
structure called screen. Then we use the member screen.adapter in a
switch statement construct to turn on the matching graphics mode. The
symbolic constants _CGA and the rest are defined in the GRAPH.H file. The
modes that begin with the letter O are Olivetti modes.
If the computer is equipped with a color card, _getvideoconfig returns a
TRUE. If it is not, it returns a FALSE, which causes main to skip the
three_graphs function.
Three Windows, Three Graphs
If the four_colors function works properly, main calls the function below,
which prints the three graphs.
/* three_graphs function from REALG.C. */
void three_graphs( void )
{
int xwidth, yheight, cols, rows;
struct _wxycoord upleft, botright;
_clearscreen( _GCLEARSCREEN );
xwidth = screen.numxpixels;
yheight = screen.numypixels;
halfx = xwidth/2;
halfy = yheight/2;
cols = screen.numtextcols;
rows = screen.numtextrows;
/* first window */
_setviewport( 0, 0, halfx-1, halfy-1 );
_settextwindow( 1, 1, rows/2, cols/2 );
_setwindow( FALSE, -2.0, -2.0, 2.0, 2.0 );
grid_shape();
_rectangle( _GBORDER, 0, 0, halfx-1, halfy-1 );
/* second window */
_setviewport( halfx, 0, xwidth-1, halfy-1 );
_settextwindow( 1, cols/2+1, rows/2, cols );
_setwindow( FALSE, -3.0, -3.0, 3.0, 3.0 );
grid_shape();
_rectangle_w( _GBORDER, -3.0, -3.0, 3.0, 3.0 );
/* third window */
_setviewport( 0, halfy, xwidth-1, yheight-1 );
_settextwindow( rows/2+1, 1, rows, cols );
_setwindow( TRUE, -3.0, -1.5, 1.5, 1.5 );
grid_shape();
upleft.wx = -3.0;
upleft.wy = -1.5;
botright.wx = 1.5;
botright.wy = 1.5;
_rectangle_wxy( _GBORDER, &upleft, &botright );
getch();
_setvideomode( _DEFAULTMODE );
}
Clearing the Screen - Although entering a graphics mode automatically clears
the screen, it doesn't hurt to be sure, so three_graphs calls the
_clearscreen function:
_clearscreen( _GCLEARSCREEN );
The _GCLEARSCREEN constant causes the entire physical screen to clear. Other
options include _GVIEWPORT and _GWINDOW, which clear the current viewport
and the current text window, respectively.
The First Window - After assigning values to some variables, the
three_graphs function creates the first window:
_setviewport( 0, 0, halfx - 1, halfy - 1 );
_settextwindow( 1, 1, rows / 2, cols / 2 );
_setwindow( FALSE, -2.0, -2.0, 2.0, 2.0 );
First a viewport is defined to cover the upper left quarter of the screen.
Next, a text window is defined within the boundaries of that border. (Note
the numbering starts at 1 and the row location precedes the column.)
Finally, a window is defined. The FALSE constant forces the y axis to
increase from top to bottom. The corners of the window are (-2.0, -2.0) in
the upper left and (2.0, 2.0) in the bottom right corner.
Next, the function grid_shape is called, and a border is added to the
window:
grid_shape();
_rectangle( _GBORDER, 0, 0, halfx-1, halfy-1 );
Note that this is the standard _rectangle function, which takes coordinates
relative to the viewport (not window coordinates).
Two More Windows - The two other windows are similar to the first. All three
call grid_shape (defined below), which draws a grid from location (-1.0,
-1.0) to (+1.0, +1.0). The grid appears in different sizes because the
coordinates in the windows vary. The second window ranges from (-3.0, -3.0)
to (+3.0, +3.0), so the width of the grid is one-third the width of the
second window, while it is one-half the width of the first.
Note also that the third window contains a TRUE as the first argument. This
causes the y axis to increase from bottom to top, instead of top to bottom.
As a result, this graph appears to be upside down in relation to the other
two.
After calling grid_shape, the program frames each window with one of the
following functions:
_rectangle( _GBORDER, 0, 0, halfx -1, halfy -1 );
_rectangle_w( _GBORDER, -3.0, -3.0, 3.0, 3.0 );
_rectangle_wxy( _GBORDER, &upleft, &botright );
All three functions contain a fill flag as the first argument. The
_rectangle function takes integer arguments that refer to the viewport
screen coordinates. The function _rectangle_w takes four double-precision,
floating-point values referring to window coordinates: upper left x, upper
left y, lower right x, and lower right y. The function _rectangle_wxy takes
two arguments: the addresses of two structures of type _wxycoord, which
contains two double types named wx and wy. The structure is defined in
GRAPH.H. The values are assigned just before _rectangle_wxy is called.
Text, Colors, and Lines - The grid_shape function is shown below:
/* grid_shape from the REALG.C program. */
void grid_shape( void )
{
int i, numc, x1, y1, x2, y2;
double x, y;
char txt[80];
numc = screen.numcolors;
for( i = 1; i <numc; i++ )
{
_settextposition( i, 2 );
_settextcolor( i );
sprintf( txt, "Color %d", i );
_outtext( txt );
}
_setcolor( 1 );
_rectangle_w( _GBORDER, -1.0, -1.0, 1.0, 1.0 );
_rectangle_w( _GBORDER, -1.02, -1.02, 1.02, 1.02 );
for( x = -0.9, i = 0; x <0.9; x += 0.1 )
{
_setcolor( 2 );
_moveto_w( x, -1.0 );
_lineto_w( x, 1.0 );
_moveto_w( -1.0, x );
_lineto_w( 1.0, x );
_setcolor( 3 );
_moveto_w( x - 0.1, bananas[i++] );
_lineto_w( x, bananas[i] );
}
_moveto_w( 0.9, bananas[i++] );
_lineto_w( 1.0, bananas[i] );
}
First, the number of available color indexes is assigned to the numc
variable and a for loop displays all of the available colors:
numc = screen.numcolors;
for( i = 1; i < numc; i++ )
{
_settextposition( i, 2 );
_settextcolor( i );
sprintf( txt, "Color %d", i );
_outtext( txt );
}
The names of the functions are self-explanatory. The advantage of using
_outtext in graphics mode is that, unlike printf, you can control the text
color.
The function names that end with _w work the same as their viewport
equivalents, except you pass double-precision, floating-point values instead
of integers. For example, you pass integers to _lineto but floating-point
values to _lineto_w.
If you're interested in further explorations of graphics, Chapters 14 and 15
introduce Presentation Graphics and fonts, both of which offer even more
graphics options.
Chapter 14 Presentation Graphics
────────────────────────────────────────────────────────────────────────────
Presentation Graphics is the name given to a library of chart-generating
functions included with the QuickC package. With these functions your QuickC
programs can display data as a variety of graphs such as pie charts, bar and
column charts, line graphs, and scatter diagrams. Whole columns of
unintelligible numbers can be reduced to a single expressive picture with
Presentation Graphics.
This chapter shows you how to use the Presentation Graphics library in your
QuickC programs. The first section is an introduction to Presentation
Graphics. It explains terminology and describes some of the library's many
capabilities. The middle sections of this chapter list the steps involved in
writing a charting program and illustrate them with short examples.
The concluding portions of the chapter delve more deeply into Presentation
Graphics. Here you'll learn about the Presentation Graphics default data
structures and how to manipulate them. The final section presents a short
reference list of all the functions that comprise the Presentation Graphics
library.
To use Presentation Graphics you need a graphics adapter and a monitor
capable of bit-mapped display─the same equipment mentioned in Chapter 13,
"Graphics." Support is provided for CGA, EGA, VGA, MCGA, Hercules monochrome
graphics, and Olivetti Color Board.
Terminology
Certain terms and phrases pertaining to Presentation Graphics and its
functions are used throughout this chapter. The following description of
Presentation Graphics terminology will help you better understand this
chapter.
Data Series
Groups or series of data can be graphed on the same chart.
Data that are related by a common idea or purpose constitutes a "series."
For example, the prices of a futures commodity over the course of a year
form a single series of data. The commodity's volume and open interest form
two more series for the same period of time. Presentation Graphics allows
you to plot multiple series on the same graph. In theory only your system's
memory capacity restricts the number of data series that can appear on a
graph. However, there are practical considerations.
Characteristics such as color and pattern help distinguish one series from
an-other. You can more readily differentiate series on a color monitor than
you can on a monochrome monitor. The number of series that can comfortably
appear on the same chart depends on the chart type and the number of
available colors. Only experimentation can tell you what is best for your
system.
Categories
Categories are non-numeric data. A set of categories forms a frame of
reference for the comparisons of numeric data. For example, the months of
the year are categories against which numeric data such as rainfall can be
plotted.
Regional sales provide another example. A chart can show comparisons of a
company's sales in different parts of the country. Each region forms a
category. The sales within each region are numeric data that have meaning
only within the context of a particular category.
Values
Values are numeric data. Sales, stock prices, air temperatures,
populations─all are series of values that can be plotted against categories
or against other values.
Presentation Graphics allows you to overlay different series of value data
on a single graph. For example, average monthly temperatures or monthly
sales of heating oil during different years─or a combination of temperatures
and sales─can be plotted together on the same graph.
Pie Charts
(Please refer to the printed book.)
"Pie charts" are used to represent data by showing the relationship of each
part to the whole. A good example is a company's monthly sales figures. The
sales to the company's various accounts can be represented as slices of the
pie.
Presentation Graphics can display either a standard or an "exploded" pie
chart. The exploded view shows the pie with one or more pieces separated for
emphasis. Presentation Graphics optionally labels each slice of a pie chart
with a percentage figure.
Bar and Column Charts
(Please refer to the printed book.)
As the name implies, a "bar chart" shows data as horizontal bars. Bar charts
show comparisons among items rather than absolute value.
"Column charts"
(Please refer to the printed book.)
are vertical bar charts. Column charts are frequently used to show
variations over a period of time, since they suggest time flow better than a
bar chart
Line Graphs
(Please refer to the printed book.)
"Line graphs" illustrate trends or changes in data. They show how a series
of values varies against some category─for example, average temperatures
throughout a particular year.
Traditionally, line graphs show a collection of data points connected by
lines; hence the name. However, Presentation Graphics can also plot points
that are not connected by lines.
Scatter Diagrams
(Please refer to the printed book.)
A "scatter diagram" is the only type of graph available in Presentation
Graphics that compares values with values. A scatter diagram simply plots
points. One value may correspond to several other values.
Scatter diagrams illustrate the relationship between numeric values in
different groups of data. They graphically show trends and correlations not
easily detected from rows and columns of raw numbers. This explains why
scatter diagrams are a favorite tool of statisticians and forecasters.
They are most useful with relatively large populations of data. Consider,
for example, the relationship between personal income and family size. If
you poll one thousand wage earners for their income and family size, you
have a scatter diagram with one thousand points. If you combine your results
so that you're left with one average income for each family size, you have a
line graph.
Axes
All Presentation Graphics charts except pie charts are displayed with two
perpendicular reference lines called "axes." The vertical or y axis runs
from top to bottom of the chart and is placed against the left side of the
screen. The horizontal or x axis runs from left to right across the bottom
of the screen.
The chart type determines which axes are used for category and value data.
The x axis is the category axis for column and line charts and the value
axis for bar charts. The y axis is the value axis for column and line charts
and the category axis for bar charts.
Chart Windows
The "chart window" defines that part of the screen on which the chart is
drawn. Normally the window fills the entire screen, but Presentation
Graphics allows you to resize the window for smaller graphs. By redefining
the chart window to different screen locations, you can view separate graphs
together on the same screen.
Data Windows
While the chart window defines the entire graph including axes and labels,
the "data window" defines only the actual plotting area. This is the portion
of the graph to the right of the y axis and above the x axis. You cannot
directly specify the size of the data window. Presentation Graphics
automatically determines its size based on the dimensions of the chart
window.
Chart Styles
Each of the five types of Presentation Graphics charts can appear in two
different chart styles, as described in Table 14.1.
Table 14.1 Presentation Graphics Chart Styles
╓┌───────────┌───────────────────┌───────────────────────────────────────────╖
Chart Type Chart Style #1 Chart Style #2
────────────────────────────────────────────────────────────────────────────
Pie With percentages Without percentages
Bar Side-by-side Stacked
Column Side-by-side Stacked
Line Points with lines Points only
Scatter Points with lines Points only
────────────────────────────────────────────────────────────────────────────
Bar and column charts have only one style when displaying a single series of
data. The styles "side-by-side" and "stacked" are applicable when more than
one series appear on the same chart. The first style arranges the bars or
columns for the different series side by side, showing relative heights or
lengths. The stacked style, illustrated in Figure 14.1 for a column chart,
emphasizes relative sizes between bars or columns.
(This figure may be found in the printed book.)
Legends
Legends help identify individual data series.
When displaying more than one data series on a chart, Presentation Graphics
uses different colors, line styles, or patterns to differentiate the series.
Presentation Graphics also can display a "legend" that labels the different
series of a chart. For a pie chart, the legend labels individual slices of
the pie.
The format is similar to the legends found on printed graphs and maps. A
sample of the color and pattern used to graph the series appears next to the
series label. This identifies which set of data the labels belong to. The
"Palettes" section later in this chapter explains how different data series
are identified by color and pattern.
Presentation Graphics Program Structure
QuickC programs that use Presentation Graphics typically follow seven steps:
Step Comments
────────────────────────────────────────────────────────────────────────────
Include required header files. Along with other header files your
program may need, you must include the
files GRAPH.H and PGCHART.H.
Set video mode to graphics. Refer to Chapter 13, "Graphics," for a
discussion
of video modes supported by QuickC. This
chapter explains how to change modes
within a QuickC program.
Initialize Presentation Graphics Presentation Graphics places charting
chart parameters in a data structure. These
environment. parameters determine how a graph will
appear on the screen. Collectively they
make up the "chart environment,"
described in the section "Customizing
Presentation Graphics." Presentation
Graphics sets the environment parameters
to default values. The amount of
initialization that must be done by your
program depends on how extensively it
relies on defaults.
Assemble plot data. Data can be collected in a variety of
ways: by calculating it elsewhere in
the program, reading it from files, or
entering it from the keyboard. All plot
data
must be assembled in arrays because the
Presentation Graphics functions locate
them through
pointers.
Call Presentation Graphics Display your chart.
functions.
Pause while chart is on the Your program should pause after a chart
screen. is displayed. This step allows
sufficient time to read the chart. A
common method is to wait for a keyboard
entry before resuming.
Reset video mode. When your program detects the signal to
continue,
it should normally reset the video to
its original
mode.
Once your program successfully compiles, you must link it to the library
modules PGCHART.LIB and GRAPHICS.LIB. Use the Microsoft Overlay Linker
QLINK.EXE or the QCL command-line interface to link programs outside the
QuickC environment. For descriptions of QLINK and QCL, see the Microsoft
QuickC Tool Kit, Chapter 1, "Creating Executable Programs."
Five Example Chart Programs
You'll have a better idea of Presentation Graphics capabilities once you've
seen what it can do. To that end some simple examples are presented in this
section. The sample programs that follow use only five of the 22
Presentation Graphics functions: _pg_initchart, _pg_defaultchart,
_pg_chartpie, _pg_chart, and _pg_chartscatter. Appendix B, "C Library
Guide," and online help document these functions and their arguments. But
the example code is straightforward, and you should be able to follow easily
for now. Each program is commented so that you can recognize the seven steps
given above.
A Sample Data Set
Suppose a grocer wants to graph the sales of orange juice over the course of
a single year. Sales figures are on a monthly basis, so the grocer selects
as category data the months of the year from January through December. The
sales figures are shown below.
╓┌─────────────────────────┌─────────────────────────────────────────────────╖
Month Quantity (cases)
────────────────────────────────────────────────────────────────────────────
January 33
Month Quantity (cases)
────────────────────────────────────────────────────────────────────────────
January 33
February 27
March 42
April 64
May 106
June 157
July 182
August 217
September 128
October 62
November 43
December 36
────────────────────────────────────────────────────────────────────────────
Example: Pie Chart
The following program uses Presentation Graphics to display a pie chart for
the grocer's data. The chart, which is shown in Figure 14.2, remains on the
screen until a key is pressed.
The Presentation Graphics functions return values that identify error
conditions. A return value of 0 indicates that the function has completed
its work without error. Refer to the header file PGCHART.H and online help
for descriptions of the nonzero error codes.
/* PIE.C: Create sample pie chart. */
#include <conio.h>
#include <string.h>
#include <graph.h>
#include <pgchart.h>
#define MONTHS 12
typedef enum {FALSE, TRUE} boolean;
float far value[MONTHS] =
{
33.0, 27.0, 42.0, 64.0,106.0,157.0,
182.0,217.0,128.0, 62.0, 43.0, 36.0
};
char far *category[MONTHS] =
{
"Jan", "Feb", "Mar", "Apr",
"May", "Jun", "Jly", "Aug",
"Sep", "Oct", "Nov", "Dec"
};
short far explode[MONTHS] = {0};
main()
{
chartenv env;
int mode = _VRES16COLOR;
/* Set highest video mode available */
while(!_setvideomode( mode ))
mode--;
if(mode == _TEXTMONO)
return( 0 );
/* Initialize chart library and a default pie chart */
_pg_initchart();
_pg_defaultchart( &env, _PG_PIECHART, _PG_PERCENT );
/* Add titles and some chart options */
strcpy( env.maintitle.title, "Good Neighbor Grocery" );
env.maintitle.titlecolor = 6;
env.maintitle.justify = _PG_RIGHT;
strcpy( env.subtitle.title, "Orange Juice Sales" );
env.subtitle.titlecolor = 6;
env.subtitle.justify = _PG_RIGHT;
env.chartwindow.border = FALSE;
/* Parameters for call to _pg_chartpie are:
*
* env - Environment variable
* category - Category labels
* value - Data to chart
* explode - Separated pieces
* MONTHS - Number of data values
*/
if(_pg_chartpie( &env, category, value,
explode, MONTHS ))
{
_setvideomode( _DEFAULTMODE );
_outtext( "Error: can't draw chart" );
}
else
{
getch();
_setvideomode( _DEFAULTMODE );
}
return( 0 );
}
(This figure may be found in the printed book.)
Example: Bar Chart
The code for the PIE.C program needs only minor alterations to produce bar,
column, and line charts for the same data:
■ Replace the call to _pg_chartpie with _pg_chart. This function
produces bar, column, and line charts depending on the value of the
second argument for _pg_defaultchart.
■ Give new arguments to _pg_defaultchart that specify chart type and
style.
■ Assign titles for the x axis and y axis in the structure env .
■ Remove references to array explode (applicable only to pie charts).
The following example produces the bar chart shown in Figure 14.3.
/* BAR.C: Create sample bar chart. */
#include <conio.h>
#include <string.h>
#include <graph.h>
#include <pgchart.h>
#define MONTHS 12
typedef enum {FALSE, TRUE} boolean;
float far value[MONTHS] =
{
33.0, 27.0, 42.0, 64.0,106.0,157.0,
182.0,217.0,128.0, 62.0, 43.0, 36.0
};
char far *category[MONTHS] =
{
"Jan", "Feb", "Mar", "Apr",
"May", "Jun", "Jly", "Aug",
"Sep", "Oct", "Nov", "Dec"
};
main()
{
chartenv env;
int mode = _VRES16COLOR;
/* Set highest video mode available */
while(!_setvideomode( mode ))
mode--;
if(mode == _TEXTMONO)
return(0);
/* Initialize chart library and a default bar chart */
_pg_initchart();
_pg_defaultchart( &env, _PG_BARCHART, _PG_PLAINBARS );
/* Add titles and some chart options */
strcpy( env.maintitle.title, "Good Neighbor Grocery" );
env.maintitle.titlecolor = 6;
env.maintitle.justify = _PG_RIGHT;
strcpy( env.subtitle.title, "Orange Juice Sales" );
env.subtitle.titlecolor = 6;
env.subtitle.justify = _PG_RIGHT;
strcpy( env.yaxis.axistitle.title, "Months" );
strcpy( env.xaxis.axistitle.title, "Quantity (cases)" );
env.chartwindow.border = FALSE;
/* Parameters for call to _pg_chart are:
* env - Environment variable
* category - Category labels
* value - Data to chart
* MONTHS - Number of data values */
if(_pg_chart( &env, category, value, MONTHS ))
{
_setvideomode( _DEFAULTMODE );
_outtext( "Error: can't draw chart" );
}
else
{
getch();
_setvideomode( _DEFAULTMODE );
}
return(0);
}
(This figure may be found in the printed book.)
Example: Column Chart
The grocer's bar chart becomes a column chart in two easy steps. Simply
specify the new chart type when calling _pg_defaultchart and switch the axis
titles. To produce a column chart for the data, replace the call to
_pg_defaultchart with:
_pg_defaultchart( &env, _PG_COLUMNCHART, _PG_PLAINBARS );
and replace the last two calls to strcpy with:
strcpy( env.xaxis.axistitle.title, "Months" );
strcpy( env.yaxis.axistitle.title, "Quantity (cases)" );
Notice that now the x axis is labeled "Months" and the y axis is labeled
"Quantity (cases)." Figure 14.4 shows the resulting column chart.
(This figure may be found in the printed book.)
Example: Line Chart
Creating an equivalent line chart requires only one change. Use the same
code as for the column chart and replace the call to _pg_defaultchart with:
_pg_defaultchart( &env, _PG_LINECHART, _PG_POINTANDLINE );
Figure 14.5 shows the line chart for the grocer's data.
(This figure may be found in the printed book.)
Example: Scatter Diagram
Now suppose that the store owner wants to compare the sales of orange juice
to the sales of another product, say hot chocolate. Possible monthly sales
are shown below.
╓┌───────────┌─────────────────────┌─────────────────────────────────────────╖
Months Orange Juice (cases) Hot Chocolate (cases)
────────────────────────────────────────────────────────────────────────────
January 33 37
February 27 37
Months Orange Juice (cases) Hot Chocolate (cases)
────────────────────────────────────────────────────────────────────────────
February 27 37
March 42 30
April 64 19
May 106 10
June 157 5
July 182 2
August 217 1
September 128 7
October 62 15
November 43 28
December 36 39
────────────────────────────────────────────────────────────────────────────
The program SCATTER.C displays a scatter diagram that illustrates the
relationship between the sales of orange juice and hot chocolate throughout
a 12-month period.
/* SCATTER.C: Create sample scatter diagram. */
#include <conio.h>
#include <string.h>
#include <graph.h>
#include <pgchart.h>
#define MONTHS 12
typedef enum {FALSE, TRUE} boolean;
/* Orange juice sales */
float far xvalue[MONTHS] =
{
33.0, 27.0, 42.0, 64.0,106.0,157.0,
182.0,217.0,128.0, 62.0, 43.0, 36.0
};
/* Hot chocolate sales */
float far yvalue[MONTHS] =
{
37.0, 37.0, 30.0, 19.0, 10.0, 5.0,
2.0, 1.0, 7.0, 15.0, 28.0, 39.0
};
main()
{
chartenv env;
int mode = _VRES16COLOR;
/* Set highest video mode available */
while(!_setvideomode( mode ))
mode--;
if(mode == _TEXTMONO)
return(0);
/* Initialize chart library and default
* scatter diagram
*/
_pg_initchart();
_pg_defaultchart( &env, _PG_SCATTERCHART,
_PG_POINTONLY );
/* Add titles and some chart options */
strcpy( env.maintitle.title, "Good Neighbor Grocery" );
env.maintitle.titlecolor = 6;
env.maintitle.justify = _PG_RIGHT;
strcpy( env.subtitle.title,
"Orange Juice vs Hot Chocolate" );
env.subtitle.titlecolor = 6;
env.subtitle.justify = _PG_RIGHT;
env.yaxis.grid = TRUE;
strcpy( env.xaxis.axistitle.title,
"Orange Juice Sales" );
strcpy( env.yaxis.axistitle.title,
"Hot Chocolate Sales" );
env.chartwindow.border = FALSE;
/* Parameters for call to _pg_chartscatter are:
* env - Environment variable
* xvalue - X-axis data
* yvalue - Y-axis data
* MONTHS - Number of data values
*/
if(_pg_chartscatter( &env, xvalue,
yvalue, MONTHS ))
{
_setvideomode( _DEFAULTMODE );
_outtext( "Error: can't draw chart" );
}
else
{
getch();
_setvideomode( _DEFAULTMODE );
}
return(0);
}
Figure 14.6 shows the results of SCATTER.C. Notice that the scatter points
form a slightly curved line, indicating a correlation exists between the
sales of the two products. The store owner can conclude from the scatter
diagram that the demand for orange juice is roughly inverse to the demand
for hot chocolate.
(This figure may be found in the printed book.)
Palettes
Presentation Graphics displays each data series in a way that makes it
discernible from other series. It does this by defining a separate "palette"
for every data series in a chart. Palettes consist of entries that determine
color, line style, fill pattern, and plot character used to graph the
series.
Presentation Graphics maintains its palettes as an array of structures. The
header file PGCHART.H defines the palette structures as:
/* Typedef for pattern bitmap */
typedef unsigned char fillmap[8];
/* Typedef for palette entry definition */
typedef struct
{
unsigned short color;
unsigned short style;
fillmap fill;
char plotchar;
} paletteentry;
/* Typedef for palette definition */
typedef paletteentry palettetype[_PG_PALETTELEN];
It's important not to confuse the Presentation Graphics palettes with the
adapter display palettes, which are register values kept by the video
controller. The function _selectpalette described in Chapter 13, "Graphics,"
sets the display palette. It does not define the data series palettes used
by Presentation Graphics.
Color Pool
Presentation Graphics organizes all chart colors into a "color pool." The
color pool consists of pixel values valid for the current graphics mode.
(Refer to Chapter 13, "Graphics," or the Glossary for a definition of pixel
values.) Palette structures contain color codes that refer to the color
pool. A palette's color code determines the color used to graph the data
series associated with the palette. Colors of labels, titles, legends, and
axes are also determined by the contents of the color pool.
The first element of the color pool is always 0, which is the pixel value
for the screen background color. The second element is always the highest
pixel value available for the graphics mode. The remaining elements are
repeating sequences of available pixel values, beginning with 1.
As shown above, the first member of a palette data structure is:
unsigned short color;
This variable defines the color code for the data series associated with the
palette. The color code is neither a display attribute nor a pixel value. It
is an index number of the color pool.
An example should make this clearer. A graphics mode of _MRES4COLOR (320 x
200 graphics) provides four colors for display. Pixel values from 0 to 3
determine the possible pixel colors─say, black, green, red, and brown
respectively. In this case the first 8 elements of the color pool would be
the following:
╓┌─────────────────┌────────────┌────────────────────────────────────────────╖
Color Pool Index Pixel Value Color
────────────────────────────────────────────────────────────────────────────
0 0 Black
1 3 Brown
2 1 Green
3 2 Red
4 3 Brown
5 1 Green
6 2 Red
7 3 Brown
────────────────────────────────────────────────────────────────────────────
Notice that the sequence of available foreground colors repeats from the
third element. The first data series in this case would be plotted in brown,
the second series in green, the third series in red, the fourth series again
in brown, and so forth.
Video adapters such as the EGA or the Hercules InColor(tm) Card allow 16
on-screen colors. This allows Presentation Graphics to graph more series
without duplicating colors.
Style Pool
Presentation Graphics matches the color pool with a collection of different
line styles called the "style pool." Entries in the style pool define the
appearance of lines such as axes and grids. Lines can be solid, dotted,
dashed, or of some combination.
The second member of a palette structure defines a style code as:
unsigned short style;
Each palette contains a style code that refers to an entry in the style pool
in the same way that it contains a color code that refers to an entry in the
color pool. The style code value in a palette is applicable only to line
graphs and lined scatter diagrams. The style code determines the appearance
of the lines drawn between points.
The palette's style code adds further variety to the lines of a multiseries
graph. It is most useful when the number of lines in a chart exceeds the
number of available colors. For example, a graph of nine different data
series must repeat colors if only three foreground colors are available for
display. However, the style code for each color repetition will be
different, ensuring that none of the lines looks the same.
Pattern Pool
Presentation Graphics also maintains a pool of "fill patterns." Patterns
determine the fill design for column, bar, and pie charts. The third member
of a palette structure holds the palette's fill pattern. The pattern member
is an array:
fillmap fill;
where fillmap is type-defined as:
typedef unsigned char fillmap[8];
Each fill pattern array holds an 8 x 8 bit map that defines the fill pattern
for the data series associated with the palette. Table 14.2 shows how a fill
pattern of diagonal stripes is created with the fill pattern array.
The bit map below corresponds to screen pixels. Each of the 8 layers of the
map are binary numbers, where a solid circle signifies 1 and an open circle
signifies 0. Thus the first layer of the map─that is, the first
byte─represents the binary number 10011001, which is the decimal number 153.
Table 14.2 Fill Patterns
╓┌──────────────────────────────────┌────────────────────────────────────────╖
Bit Map Value in fill
────────────────────────────────────────────────────────────────────────────
☼ ∙ ∙ ☼ ☼ ∙ ∙ ☼ fill[0] = 153
☼ ☼ ∙ ∙ ☼ ☼ ∙ ∙ fill[1] = 204
∙ ☼ ☼ ∙ ∙ ☼ ☼ ∙ fill[2] = 102
Bit Map Value in fill
────────────────────────────────────────────────────────────────────────────
∙ ☼ ☼ ∙ ∙ ☼ ☼ ∙ fill[2] = 102
∙ ∙ ☼ ☼ ∙ ∙ ☼ ☼ fill[3] = 51
☼ ∙ ∙ ☼ ☼ ∙ ∙ ☼ fill[4] = 153
☼ ☼ ∙ ∙ ☼ ☼ ∙ ∙ fill[5] = 204
∙ ☼ ☼ ∙ ∙ ☼ ☼ ∙ fill[6] = 102
∙ ∙ ☼ ☼ ∙ ∙ ☼ ☼ fill[7] = 51
────────────────────────────────────────────────────────────────────────────
If you wish to create the above pattern for your chart's first data series,
you must reset the fill array for the first palette structure. You can do
this in five steps:
1. Declare a structure of type palettetype to hold the palette
parameters.
2. Call _pg_initchart to initialize the palettes with default values.
3. Call the Presentation Graphics function _pg_getpalette to retrieve a
copy of the current palette data.
4. Assign the values given in Table 14.2 to the array fill for the
first palette.
5. Call the Presentation Graphics function _pg_setpalette to load the
modified palette values.
The following lines of code demonstrate these five steps:
/* Declare a structure array for palette data. */
palettetype palette_struct;
.
.
.
/* Initialize chart library */
_pg_initchart();
.
.
.
/* Copy current palette data into palette_struct */
_pg_getpalette( palette_struct );
/* Reinitialize fill pattern for first palette using
values in Table 14.2 */
palette_struct[1].fill[0] = 153;
palette_struct[1].fill[1] = 204;
palette_struct[1].fill[2] = 102;
palette_struct[1].fill[3] = 51;
palette_struct[1].fill[4] = 153;
palette_struct[1].fill[5] = 204;
palette_struct[1].fill[6] = 102;
palette_struct[1].fill[7] = 51;
/* Load new palette data */
_pg_setpalette( palette_struct );
Now when you display your bar or column chart the first series appears
filled with the striped pattern shown in Table 14.2.
Pie charts are a bit different. The idea of multiple series does not really
apply to them. Instead, palette structures correspond to individual slices.
If the number of slices exceeds the constant _PG_PALETTELEN, palettes are
recycled. Thus the first palette dictates not only the appearance of the
first slice, but of slice number _PG_PALETTELEN as well. The second palette
determines the appearance of both the second slice and of slice number
_PG_PALETTELEN + 1, and so forth.
Character Pool
The last member of a palette structure is an index number in a pool of ASCII
characters:
char plotchar;
The member plotchar represents plot points on line graphs and scatter
diagrams. Each palette uses a different character to distinguish plot points
between data series.
Customizing Presentation Graphics
Presentation Graphics is built for flexibility. You can use its system of
default values to produce professional-looking charts with a minimum of
programming effort. Or you can fine-tune the appearance of your charts by
overriding default values and initializing variables explicitly in your
program. The following section shows you how.
Chart Environment
The header file PGCHART.H defines a structure type chartenv. This structure
type organizes the set of variables known as the "chart environment." The
chart environment describes everything about a chart except the plots
themselves. It's the blank page, in other words, ready for plotting data.
The environment determines the appearance of text, axes, grid lines, and
legends.
Calling the _pg_defaultchart function fills the chart environment with
default values. Presentation Graphics allows you to reset any variable in
the environment before displaying a chart. Except for adjusting the palette
values, all initialization of data is done through a chartenv type
structure.
The sample chart programs provided earlier illustrate how to adjust
variables in the chart environment. These programs create a structure env
of the type chartenv. The structure env contains the chart environment
variables, initialized by the call to _pg_defaultchart. Environment
variables such as the chart title are then given specific values, as in:
strcpy( env.maintitle.title, "Good Neighbor Grocery" );
Environment variables that determine colors and line styles deserve special
mention. The chart environment holds several such variables, which can be
recognized by their names. For example, the variable titlecolor specifies
the color of title text. Similarly, the variable gridstyle specifies the
line style used to draw the chart grid.
Colors and line styles in the chart environment are taken from palettes.
These variables are index numbers, but do not refer directly to the color
pool or line pool. They correspond instead to palette numbers. If you set
titlecolor to 2, Presentation Graphics uses the color code in the second
palette to determine the title's color. Thus the title in this case would be
the same color as the chart's second data series. If you change the color
code in the palette, you'll also change the title's color.
A structure of type chartenv consists of four secondary structures. The file
PGCHART.H type-defines the secondary structures as:
titletype
axistype
windowtype
legendtype
The remainder of this section describes the chart environment of
Presentation Graphics. It first examines structures of the four secondary
types that make up the chart environment structure. The section concludes
with a description of the chartenv structure type. Each discussion begins
with a brief explanation of the structure's purpose, followed by a listing
of the structure type definition as it appears in the PGCHART.H file. All
symbolic constants are defined in the file PGCHART.H.
titletype
Structures of type titletype determine text, color, and placement of titles
appearing in the graph. The PGCHART.H file defines the structure type as:
typedef struct
{
char title[_PG_TITLELEN]; /* Title text */
short titlecolor; /* Palette color
for title text */
short justify; /* _PG_LEFT, _PG_CENTER,
_PG_RIGHT */
} titletype;
The following list describes titletype members:
Member Variable Description
────────────────────────────────────────────────────────────────────────────
justify An integer specifying how the title is
justified
within the chart window. The symbolic
constants
defined in the PGCHART.H file for this
variable are _PG_LEFT, _PG_CENTER, and
_PG_RIGHT.
titlecolor An integer between 1 and _PG_PALETTELEN
that specifies a title's color. The
default value for titlecolor is 1.
title[_PG_TITLELEN] A character array containing title text.
For example, if env is a structure of
type chartenv, then env.maintitle.title
holds the character string used for the
main title of the chart. Similarly,
env.xaxis.axistitle.title contains the
axis title. The number of characters in
a title must be one less than
_PG_TITLELEN to allow room for a null
terminator.
axistype
Structures of type axistype contain variables for the axes such as color,
scale, grid style, and tick marks. The PGCHART.H file defines the structure
type as:
typedef struct
{
short grid; /* TRUE=grid lines drawn;
FALSE=no lines */
short gridstyle; /* Style bytes for grid */
titletype axistitle; /* Title definition
for axis */
short axiscolor; /* Color for axis */
short labeled; /* TRUE=ticks marks and titles
drawn */
short rangetype; /* _PG_LINEARAXIS,
_PG_LOGAXIS */
float logbase; /* Base used if log axis */
short autoscale; /* TRUE=next 7 values
calculated by system */
float scalemin; /* Minimum value of scale */
float scalemax; /* Maximum value of scale */
float scalefactor; /* Scale factor for data on
this axis */
titletype scaletitle; /* Title definition for
scaling factor */
float ticinterval; /* Distance between tick marks
(world coord.) */
short ticformat; /* _PG_EXPFORMAT or
_PG_DECFORMAT */
short ticdecimals; /* Number of decimals for tick
labels (max=9) */
} axistype;
The following list describes axistype member variables:
Member Variable Description
────────────────────────────────────────────────────────────────────────────
autoscale A boolean variable. If autoscale is TRUE,
Presentation Graphics automatically
determines values
for scalefactor, scalemax, scalemin,
scaletitle, ticdecimals, ticformat, and
ticinterval (see below).
If autoscale equals FALSE, these seven
variables must be specified in your
program.
axiscolor An integer between 1 and _PG_PALETTELEN
that specifies the color used for the
axis and parallel grid lines. (See
description for gridstyle above.) Note
that this member does not determine the
color of the axis title. That selection
is made through the structure
axistitle.
axistitle A titletype structure that defines the
title of the associated axis. The title
of the y axis displays vertically to the
left of the y axis, and the title of the
x axis displays horizontally below the x
axis.
grid A boolean true/false value that
determines whether grid lines are drawn
for the associated axis. Grid lines span
the data window perpendicular to the
axis.
gridstyle An integer between 1 and _PG_PALETTELEN
that specifies the grid's line style.
Lines can be solid, dashed, dotted, or
some combination. The default value for
gridstyle is 1. Note that the color of
the parallel axis determines the color
of the grid lines. Thus the x axis grid
is the same color as the y axis, and the
y axis grid is the same color as the x
axis.
labeled A boolean value that determines whether
tick marks and labels are drawn on the
axis. Axis labels should not be confused
with axis titles. Axis labels are
numbers or descriptions such as "23.2"
or "January" attached to each tick mark.
logbase If rangetype is logarithmic, the logbase
variable determines the log base used to
scale the axis. Default value is 10.
rangetype An integer that determines whether the
scale of the axis is linear or
logarithmic. The rangetype variable
applies only to value data.
Specify a linear scale with the
_PG_LINEARAXIS constant. A linear scale
is best when the difference between axis
minimum and maximum is relatively small.
For example, a linear axis range 0-10
results in 10 tick marks evenly spaced
along the axis.
Use _PG_LOGAXIS to specify a logarithmic
rangetype. Logarithmic scales are useful
when the range is very large or when the
data varies exponentially. Line graphs
of exponentially varying data can be
made straight with a logarithmic
rangetype.
scalefactor All numeric data are scaled by dividing
each value by scalefactor. For
relatively small values, the variable
scalefactor should be 1, which is the
default.
But data with large values should be
scaled by an
appropriate factor. For example, data in
the range
2 million-20 million should be plotted
with scale-min set to 2, scalemax set to
20, and scalefactor set to 1 million.
If autoscale is set to TRUE,
Presentation Graphics automatically
determines a suitable value for
scalefactor based on the range of data
to be plotted. Presentation Graphics
selects only values that are
a factor of 1 thousand─that is, values
such as 1
thousand, 1 million, or 1 billion. It
then labels the scaletitle appropriately
(see below). If you desire some other
value for scaling, you must set
autoscale to FALSE and set scalefactor
to the desired scaling value.
scalemax Highest value represented by the axis.
scalemin Lowest value represented by the axis.
scaletitle A titletype structure defining a string
of text that
describes the value of scalefactor. If
autoscale is TRUE, Presentation Graphics
automatically writes
a scale description to scaletitle. If
autoscale equals FALSE and scalefactor
is 1, scaletitle.title should be blank.
Otherwise your program should copy an
appropriate scale description to
scaletitle.title, such as "( x 1000),"
"(in millions of units)," "times 10
thousand dollars," etc.
For the y axis, the scaletitle text
displays vertically between the axis
title and the y axis. For the x axis,
the scale title appears below the x axis
title.
ticdecimals Number of digits to display after the
decimal point in tick labels. Maximum
value is 9. Note that this variable
applies only to axes with value data. It
is
ignored for the category axis.
ticformat An integer that determines the format of
the labels assigned to each tick mark.
Set ticformat to _PG_EXPFORMAT for
exponential format or set it to
_PG_DECFORMAT for decimal. The default
is _PG_DECFORMAT. Note that this
variable applies only to axes with value
data. It is ignored for the category
axis.
ticinterval Sets interval between tick marks on the
axis. The tick interval is measured in
the same units as the numeric data
associated with the axis. For example,
if 2 sequential tick marks correspond to
the values 20 and 25, the tick interval
between them is 5. Note that this
variable applies only to axes with value
data. It is ignored for the category
axis.
windowtype
Structures of type windowtype contain sizes, locations, and color codes for
the three windows produced by Presentation Graphics: the chart window, the
data window, and the legend. Refer to the "Terminology" section at the
beginning of this chapter for definitions of these terms. Windows are
located on the screen relative to the screen's logical origin. By changing
the logical origin, you can display charts that are partly or completely off
the screen. The PGCHART.H file defines windowtype as:
typedef struct
{
short x1; /* Left edge of window in
pixels */
short y1; /* Top edge of window in
pixels */
short x2; /* Right edge of window in
pixels */
short y2; /* Bottom edge of window in
pixels */
short border; /* TRUE for border, FALSE
otherwise */
short background; /* Internal palette color for
window background */
short borderstyle; /* Style bytes for window
border */
short bordercolor; /* Internal palette color for
window border */
} windowtype;
The following list describes windowtype member variables:
Member Variable Description
────────────────────────────────────────────────────────────────────────────
x1, y1, x2, y2 Window coordinates in pixels. The
ordered pair (x1, y1) specifies the
coordinate of the upper left corner of
the window. The ordered pair (x2, y2)
specifies the coordinate of the lower
right corner.
The reference point for the coordinates
depends on the type of window. The chart
window is located relative to the
logical origin, usually the upper left
corner of the screen. The data and
legend windows are located relative to
the upper left corner of the chart
window. This allows you to change the
position of the chart window without
having to redefine coordinates for the
other two windows.
background An integer between 1 and _PG_PALETTELEN
that specifies the window's background
color. The default value for background
is 1.
border A boolean variable that determines
whether a border frame is drawn around a
window.
bordercolor An integer between 1 and _PG_PALETTELEN
that specifies the color of the window's
border frame. The default value is 1.
borderstyle An integer between 1 and _PG_PALETTELEN
that specifies the line style of the
window's border frame. The default value
is 1.
legendtype
Structures of type legendtype contain size, location, and colors of the
chart legend. The PGCHART.H file defines the structure type as:
typedef struct
{
short legend; /* TRUE=draw legend;
FALSE=no legend */
short place; /* _PG_RIGHT, _PG_BOTTOM,
_PG_OVERLAY */
short textcolor; /* Palette color for text*/
short autosize; /* TRUE=system calculates
legend size */
windowtypelegendwindow; /* Window definition for
legend */
} legendtype;
The following list describes legendtype member variables:
Member Variable Description
────────────────────────────────────────────────────────────────────────────
autosize A boolean true/false variable that
determines whether Presentation Graphics
is to automatically calculate the size
of the legend. If autosize equals FALSE,
the legend window must be specified in
the legendwindow structure (see below).
legend A boolean true/false variable that
determines whether a legend is to appear
on the chart. The legend variable is
ignored by functions that graph
single-series charts.
legendwindow A windowtype structure that defines
coordinates, background color, and
border frame for the legend. Coordinates
given in legendwindow are ignored if
autosize is TRUE.
place An integer that specifies the location
of the legend relative to the data
window. Setting the variable place equal
to the constant _PG_RIGHT positions the
legend to the right of the data window.
Setting place to _PG_BOTTOM positions
the legend below the data window.
Setting place to _PG_OVERLAY positions
the legend within the data window.
These settings influence the size of the
data
window. If place is equal to _PG_BOTTOM
or _PG_RIGHT, Presentation Graphics
automatically sizes the data window to
accommodate the legend.
If place equals _PG_OVERLAY the data
window is sized without regard to the
legend.
textcolor An integer between 1 and _PG_PALETTELEN
that specifies the color of text within
the legend window.
chartenv
A structure of type chartenv defines the chart environment. The following
code shows that a chartenv type structure consists almost entirely of
structures of the four types discussed above.
The PGCHART.H file defines the chartenv structure type as:
typedef struct
{
short charttype; /* Chart type */
short chartstyle; /* Chart style */
windowtype chartwindow; /* Window definition for
overall chart */
windowtype datawindow; /* Window definition for data
part of chart */
titletype maintitle; /* Main chart title */
titletype subtitle; /* Chart subtitle */
axistype xaxis; /* Definition for x axis */
axistype yaxis; /* Definition for y axis */
legendtype legend; /* Definition for legend */
} chartenv;
Initialize the chart environment with the _pg_defaultchart function.
Note that all the data in a chartenv type structure is initialized by
calling the _pg_defaultchart function. If your program does not call
_pg_defaultchart, it must explicitly define every variable in the chart
environment─a tedious and unnecessary procedure. The recommended method for
adjusting the appearance of your chart is to initialize variables for the
proper chart type by calling the _pg_defaultchart function, and then
reassign selected environment variables such as titles.
The following list describes chartenv member variables:
Member Variable Description
────────────────────────────────────────────────────────────────────────────
chartstyle An integer that determines the style of
the chart
(see Table 14.1). Legal values for
chartstyle
are _PG_PERCENT and _PG_NOPERCENT
for pie charts; _PG_STACKEDBARS and
_PG_PLAINBARS for bar and column charts;
and _PG_POINTANDLINE and _PG_POINTONLY
for line graphs and scatter diagrams.
This variable corresponds to the third
argument for the _pg_defaultchart
function.
charttype An integer that determines the type of
chart displayed. The value of the
variable charttype is _PG_BARCHART,
_PG_COLUMNCHART, _PG_LINECHART,
_PG_SCATTERCHART, or _PG_PIECHART. This
variable corresponds to the second
argument for the _pg_defaultchart
function.
chartwindow A windowtype structure that defines the
appearance of the chart window.
datawindow A windowtype structure that defines the
appearance of the data window.
legend A legendtype structure that defines the
appearance of the legend window.
maintitle A titletype structure that defines the
appearance of the main title of the
chart.
subtitle A titletype structure that defines the
appearance of the chart's subtitle.
xaxis An axistype structure that defines the
appearance of the x axis. (This variable
is not applicable for pie charts.)
yaxis An axistype structure that defines the
appearance of the y axis. (This variable
is not applicable for pie charts.)
An Overview of the Presentation Graphics Functions
The chapter concludes with a few words about the 22 functions that make up
the Presentation Graphics library. They are listed in Table 14.3 for
convenient reference. Refer to Appendix B, "C Library Guide," or online help
for a description of the functions and their arguments.
Table 14.3 Presentation Graphics Functions
╓┌────────────────────┌─────────────────────┌────────────────────────────────╖
Primary Functions Secondary Functions
────────────────────────────────────────────────────────────────────────────
_pg_initchart _pg_hlabelchart _pg_setpalette
_pg_defaultchart _pg_vlabelchart _pg_resetpalette
_pg_chart _pg_analyzechart _pg_getstyleset
_pg_chartms _pg_analyzechartms _pg_setstyleset
_pg_chartscatter _pg_analyzescatter _pg_resetstyleset
_pg_chartscatterms _pg_analyzescatterms _pg_getchardef
Primary Functions Secondary Functions
────────────────────────────────────────────────────────────────────────────
_pg_chartscatterms _pg_analyzescatterms _pg_getchardef
_pg_chartpie _pg_analyzepie _pg_setchardef
_pg_getpalette
────────────────────────────────────────────────────────────────────────────
In most cases you need only be concerned with seven of the routines, called
the "primary functions." These functions initialize variables and display
the selected chart types. As demonstrated in example programs earlier in
this chapter, you can create very acceptable charts with programs that call
only three of the Presentation Graphics primary functions.
The 15 secondary functions of Presentation Graphics do not directly display
charts. Most of them retrieve or set data in the Presentation Graphics chart
environment.
Of special interest among the secondary functions are the "analysis
functions," identified by the prefix _pg_analyze in their function names.
These five functions calculate default values that pertain to a given chart
type and data set. Calling an analysis function has the same effect as
calling a corresponding primary function, except that the chart is not
displayed. This allows you to pass on to the library the burden of
calculating values. You can then make modifications to the resulting values
and call a primary routine to display the chart.
Use the _pg_hlabelchart and _pg_vlabelchart functions to display text on
your chart that is not part of a title or axis label. These functions enable
you to attach notes or other messages to your chart. You may also find them
useful for labeling separate lines of a multiseries line graph.
Chapter 15 Fonts
────────────────────────────────────────────────────────────────────────────
Preceding chapters have discussed how to write QuickC programs that generate
graphics and display charts. QuickC has yet another capability when it comes
to graphics: fonted text. Your programs can display various styles and sizes
of text in any graphics image or chart.
This chapter tells how. It assumes you have already read Chapter 13,
"Graphics." You should understand such terms as "graphics mode" and "text
mode," and be familiar with the functions _setvideomode and _moveto. Other
than that, there's very little to it. Fonts are simple to learn and even
simpler to use, yet they can add to your graphics a final touch of polish.
QuickC Fonts
A "font" is a collection of stylized text characters. Each font consists of
several type sizes and a typeface.
"Typeface" is a printer's term that refers to the style of the displayed
text─ Courier, for example, or Roman. The list on the following page shows
six of the typefaces available with QuickC's font library.
"Type size" measures the screen area occupied by individual characters. This
term is also borrowed from the printer's lexicon, but for our purposes is
specified in units of screen pixels. For example, "Courier 16 x 9" denotes
text of Courier typeface, with each character occupying a screen area of 16
vertical pixels by 9 horizontal pixels.
QuickC's font functions use two methods to create fonts. The first technique
generates Courier, Helv, and Tms Rmn fonts through a "bit-mapping" (or
"raster-mapping") technique. Bit-mapping defines character images with
binary data. Each bit in the map corresponds to a screen pixel. If a bit is
1, its associated pixel is set to the current screen color. A bit value of 0
clears the pixel. Video adapters use this same technique to display text in
graphics mode.
The second method creates the remaining three type styles─Modern, Script,
and Roman─as "vector-mapped" fonts. Vector-mapping represents each character
in terms of lines and arcs. In a literal sense vector-mapped characters are
drawn on the screen. You might think of bit-mapped characters as being
stenciled.
Each method has advantages and disadvantages. Bit-mapped characters are more
completely formed since the pixel mapping is predetermined. However, they
cannot be scaled. Vector-mapped text can be scaled to any size, but the
characters tend to lack the solid appearance of the bit-mapped characters.
(This figure may be found in the printed book.)
Table 15.1 lists available sizes for each font. Notice that the bit-mapped
fonts come in preset sizes as measured in pixels. The exact size of any
fonted character depends on screen resolution and display type.
Table 15.1 Typefaces and Type Sizes in the QuickC Library
╓┌─────────┌────────┌─────────────────────────┌──────────────────────────────╖
Typeface Mapping Size (in pixels) Spacing
────────────────────────────────────────────────────────────────────────────
Courier Bit 13 x 8, 16 x 9, Fixed
20 x 12
Helv Bit 13 x 5, 16 x 7, 20 x 8, Fixed
13 x 15, 16 x 6, 19 x 8
Tms Rmn Bit 10 x 5, 12 x 6, 15 x 8, Fixed
16 x 9, 20 x 12, 26 x 16
Typeface Mapping Size (in pixels) Spacing
────────────────────────────────────────────────────────────────────────────
16 x 9, 20 x 12, 26 x 16
Modern Vector Scaled Proportional
Script Vector Scaled Proportional
Roman Vector Scaled Proportional
────────────────────────────────────────────────────────────────────────────
QuickC's font routines can display characters 32-255, including most
extended characters (ASCII 128-255). A few extended characters cannot be
displayed; these are represented as either an underscore (_) or period (.)
character.
Using QuickC's Font Library
Data for both bit-mapped and vector-mapped fonts reside in files on disk. A
.FON extension identifies the files. The names of the .FON files indicate
their content. For example, the files MODERN.FON, ROMAN.FON, and SCRIPT.FON
hold data for the three vector-mapped fonts.
You can use Microsoft Windows .FON files.
QuickC .FON files are identical to the .FON files used in the Microsoft
Windows operating environment. If you have access to Windows you can use any
of its .FON files with QuickC's font functions. Windows .FON files are also
available for purchase separately. In addition, several vendors offer
software that can create or modify .FON files, allowing you to design your
own fonts.
Your programs should follow these three steps to display fonted text:
1. Register fonts
2. Set the current font from the register
3. Display text using the current font
The following sections describe each of the three steps in detail. An
example program later in the chapter demonstrates the steps.
Register Fonts
The fonts you plan to use must first be organized into a list in memory, a
process called "registering." The register list contains information about
the available .FON files. Register fonts by calling the function
_registerfonts. This function reads header information from specified .FON
files. It builds a list of file information but does not read mapping data
from the files.
The GRAPH.H file prototypes the _registerfonts function as:
short far _registerfonts( unsigned char far * );
The argument points to a string containing a file name. The file name is the
name of the .FON file for the desired font. The file name can include wild
cards, allowing you to register several fonts with one call to
_registerfonts.
If it successfully reads one or more .FON files, _registerfonts returns the
number of fonts registered. If the function fails, it returns a negative
error code. Refer to Appendix B, "C Library Guide," or to online help for a
description of error codes.
Set Current Font
Call the function _setfont to select a current font. This function checks to
see if the requested font is registered, then reads the mapping data from
the appropriate .FON file. A font must be registered and marked current
before your program can display text of that font.
The GRAPH.H file prototypes _setfonts as
short far _setfont( unsigned char far * );
The function's argument is a pointer to a character string. The string
consists of letter codes that describe the desired font, as outlined below:
Option Code Meaning
────────────────────────────────────────────────────────────────────────────
b Select the best fit from the registered
fonts. This option instructs _setfont to
accept the closest-fitting font if a
font of the specified size is not
registered.
If at least one font is registered, the
b option always sets a current font. If
you do not specify the b option and an
exact matching font is not registered,
_setfont will fail. In this case, any
existing current font remains current.
Refer to online help for a description
of error codes returned by _setfont.
The _setfont function uses four criteria
for selecting the best fit. In
descending order of precedence the four
criteria are pixel height, typeface,
pixel width, and spacing (fixed or
proportional). If you request a
vector-mapped font,_setfont sizes the
font to correspond with the specified
pixel height and width. If you request a
raster-mapped (bit-mapped) font,
_setfont chooses the closest available
size. If the requested type size for a
raster-mapped font fits exactly between
two registered fonts, the smaller size
takes precedence.
f Select only a fixed-spaced font.
hy Character height, where y is the height
in pixels.
nx Select font number x, where x is less
than or equal to the value returned by
_registerfonts. For example, the option
n3 makes the third registered font
current, assuming that three or more
fonts are registered.
p Select only a proportional-spaced font.
r Select only a raster-mapped (bit-mapped)
font.
t`fontname' Typeface of the font in single quotes.
The fontname string is one of the
following:
courier modern
helv script
tms rmn roman
Notice the space in "tms rmn."
Additional font files use other names
for fontname. Refer to the vendor's
documentation for these names.
v Select only a vector-mapped font.
wx Character width, where x is the width in
pixels.
Option codes are not case-sensitive and can be listed in any order. You can
separate codes with spaces or any other character that is not a valid option
code. The _setfont function ignores all invalid codes.
The _setfont function updates a data area with parameters of the current
font. The data area is in the form of a structure, defined in the GRAPH.H
file as
struct _fontinfo
{
int type; /* set = vector,clear = bit map */
int ascent; /* pix dist from top to base */
int pixwidth; /* character width in pixels */
int pixheight; /* character height in pixels */
int avgwidth; /* average character width */
char filename[81]; /* file name including path */
char faceName[32]; /* font name */
};
If you wish to retrieve the parameters of the current font, call the
function _getfontinfo. Refer to Appendix B, "C Library Guide," or online
help for a description of this function.
Display Text
The last step consists of two parts. First, select a screen position for the
text with the graphics function _moveto. Then display fonted text at that
position with the function _outgtext. The _moveto function takes pixel
coordinates as arguments. The coordinates locate the top left of the first
character in the text string.
An Example Program
QuickC's font functions shine when used in conjunction with your other
graphics functions. They allow you to dress up any image on the screen. Yet
they can make a visual impression when used by themselves, as an example
will show.
The program SAMPLER.C displays sample text in all the available fonts, then
exits when a key is pressed. Make sure the .FON files are in the current
directory before running the program.
/* SAMPLER.C: Display sample text in various fonts. */
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <graph.h>
#include <string.h>
#define NFONTS 6
main()
{
static unsigned char *text[2*NFONTS] =
{
"COURIER", "courier",
"HELV", "helv",
"TMS RMN", "tms rmn",
"MODERN", "modern",
"SCRIPT", "script",
"ROMAN", "roman"
};
static unsigned char *face[NFONTS] =
{
"t'courier'",
"t'helv'",
"t'tms rmn'",
"t'modern'",
"t'script'",
"t'roman'"
};
static unsigned char list[20];
struct videoconfig vc;
int mode = _VRES16COLOR;
register i;
/* Read header info from all .FON files in
* current directory */
if(_registerfonts( "*.FON" )<0 )
{
_outtext("Error: can't register fonts");
exit( 0 );
}
/* Set highest available video mode */
while( !_setvideomode( mode ) )
mode--;
if( mode == _TEXTMONO )
exit ( 0 );
/* Copy video configuration into structure vc */
_getvideoconfig( &vc );
/* Display six lines of sample text */
for( i = 0; i<NFONTS; i++ )
{
strcpy( list, face[i] );
strcat( list, "h30w24b" );
if( !_setfont( list ) )
{
_setcolor( i + 1 );
_moveto( 0, (i * vc.numypixels) / NFONTS );
_outgtext( text[i * 2] );
_moveto( vc.numxpixels / 2,
(i * vc.numypixels) / NFONTS );
_outgtext( text[(i * 2) + 1] );
}
else
{
_setvideomode( _DEFAULTMODE );
_outtext( "Error: can't set font" );
exit( 0 );
}
}
getch();
_setvideomode( _DEFAULTMODE );
/* Return memory when finished with fonts */
_unregisterfonts();
exit ( 0 );
}
Notice that SAMPLER.C calls the graphics function _moveto to establish the
starting position for each text string. Chapter 13, "Graphics," describes
the _moveto function in the section "Graphics Coordinates." The function
_setfont takes a character string as an argument. The string is a list of
options that specifies typeface and the best fit for a character height of
30 pixels, and a width of 24 pixels. See Appendix B, "C Library Guide," and
online help for complete descriptions of the QuickC font functions.
A Few Hints
Fonted text is simply another form of graphics, and using fonts effectively
requires little programming effort. Still, there are a few things to watch:
■ Remember the video should be set only once to establish a graphics
mode. If you generate an image─say, with Presentation Graphics─and
wish to incorporate fonted text into it, don't reset the video mode
prior to calling the font routines. Doing so will blank the screen,
destroying the original image.
■ The _setfont function reads specified .FON files to obtain mapping
data for the current font. Each call to _setfont causes a disk access
and overwrites the old font data in memory. If you wish to show text
of different styles on the same screen, display all text of one font
before moving on to the others. By minimizing the number of calls to
_setfont you'll save time spent in disk I/O and memory reloads.
■ When your program finishes with the fonts library, you might wish to
free the memory occupied by the register list. Call the function
_unregisterfonts to do this. As its name implies, this function frees
the memory allocated by _registerfonts. The register information for
each type size of each font takes up approximately 140 bytes of
memory. Thus the amount of memory returned by _unregisterfonts is
significant only if you have many fonts registered.
■ As for aesthetics, the same suggestions for the printed page apply to
fonted screen text. Typefaces are more effective when they are not
competing with each other for attention. Restricting the number of
styles per screen to one or two generally results in a more pleasing,
less cluttered image.
Chapter 16 In-Line Assembly
────────────────────────────────────────────────────────────────────────────
QuickC has the ability to handle assembly-language instructions right in
your C programs. This powerful feature is called "in-line assembly."
Assembly language serves many purposes, such as improving program speed,
reducing memory needs, and controlling hardware. The in-line assembler lets
you embed assembly-language instructions directly in your C source programs
without extra assembly and link steps. And the assembler is built into the
compiler─you don't need a separate assembler such as the Microsoft Macro
Assembler (MASM).
This chapter assumes that you are familiar with assembly-language terms and
concepts. If you have never programmed in assembly language, refer to the
section "References and Books on Assembly Language," at the end of this
chapter.
Advantages of In-Line Assembly
Because QuickC's in-line assembler doesn't require separate assembly and
link steps, it is more convenient than a separate assembler. In-line
assembly code can use any C variable or function name that is visible (in
scope), so it is easy to integrate it with your program's C code. And
because the assembly code can be mixed in-line with C statements, it can do
tasks that are cumbersome or impossible in C alone.
The uses of in-line assembly include
■ Writing the body of a function in assembly language
■ Spot-optimizing speed-critical sections of code
■ Calling DOS and BIOS routines with the INT instruction
■ Creating TSR (terminate-and-stay-resident) code or handler routines
that require knowledge of processor states
In-line assembly is a special-purpose tool. If you plan to transport an
application, you'll probably want to place machine-specific code in a
separate module. And because the in-line assembler doesn't support all MASM
directives, you may find it more convenient to use MASM for such modules.
The _asm Keyword
The _asm keyword invokes the in-line assembler and can appear wherever a C
statement is legal. It cannot appear by itself. It must be followed by an
assembly instruction, a group of instructions enclosed in braces, or, at the
very least, an empty pair of braces. The term "_asm block" here refers to
any instruction or group of instructions, whether or not in braces.
Below is a simple _asm block enclosed in braces. (The code prints the "beep"
character, ASCII 7.)
_asm
{
mov ah, 2
mov dl, 7
int 21h
}
Alternatively, you can put _asm in front of each assembly instruction:
_asm mov ah, 2
_asm mov dl, 7
_asm int 21h
Since the _asm keyword is a statement separator, you can also put assembly
instructions on the same line:
_asm mov ah, 2 _asm mov dl, 7 _asm int 21h
Braces can prevent ambiguity and needless repetition.
All three examples generate the same code, but the first style─enclosing the
_asm block in braces─has some advantages. The braces clearly separate
assembly code from C code and avoid needless repetition of the _asm keyword.
Braces can also prevent ambiguities. If you want to put a C statement on the
same line as an _asm block, you must enclose the block in braces. Without
the braces, the compiler cannot tell where assembly code stops and C
statements begin. Finally, since the text in braces has the same format as
ordinary MASM text, you can easily cut and paste text from existing MASM
source files.
The braces enclosing an _asm block don't affect variable visibility, as do
braces in C. You can also nest _asm blocks, but the nesting doesn't affect
variable visibility.
Using Assembly Language in _asm Blocks
The in-line assembler has much in common with other assemblers. For example,
it accepts any expression that is legal in MASM, and it supports almost all
80286 and 80287 instructions. This section describes the use of
assembly-language features in _asm blocks.
Instruction Set
The in-line assembler supports the full instruction set of the Intel(R)
80286 and 80287 processors, except for privileged instructions that control
the processor's protected mode (protected mode is available in the OS/2 and
XENIX(R) operating systems, but not in DOS). It does not recognize 80386-
and 80387-specific instructions. To use assembly instructions specific to
the 80286 and 80287 processors, you must compile your QuickC program with
the /G2 switch included in the command line. For a description of the
compiler /G command-line switch, refer to Chapter 4, "QCL Command
Reference," in the Microsoft QuickC Tool Kit.
Expressions
In-line assembly code can use any MASM expression, that is, any combination
of operands and operators that evaluates to a single value or address.
Data Directives and Operators
Although an _asm block can reference C data types and objects, it cannot
define data objects with MASM directives or operators. Specifically, you
cannot use the definition directives DB, DW, DD, DQ, DT, and DF, or the
operators DUP or THIS. Nor are MASM structures and records available. The
in-line assembler doesn't accept the directives STRUC, RECORD, WIDTH, or
MASK.
EVEN and ALIGN Directives
While the in-line assembler doesn't support most MASM directives, it does
support EVEN and ALIGN. These directives put NOP (no operation) instructions
in the assembly code as needed to align labels to specific boundaries. This
makes instruction-fetch operations more efficient for some processors (not
including eight-bit processors such as the Intel 8088).
Macros
The in-line assembler is not a macro assembler. You cannot use MASM macro
directives (MACRO, REPT, IRC, IRP, and ENDM) or macro operators ( <>, !, &,
%, and .TYPE). An _asm block can use C preprocessor directives, however. See
the section "Using C in _asm Blocks" for more information.
Segment References
You must refer to segments by register rather than by name (the segment name
_TEXT is invalid, for instance). Segment overrides must use the register
explicitly, as in ES:[BX].
Type and Variable Sizes
The LENGTH, SIZE, and TYPE operators have a limited meaning in in-line
assembly. They cannot be used at all with the DUP operator (because you
cannot define data with MASM directives or operators). But you can use them
to find the size of C variables or types:
■ The LENGTH operator can return the number of elements in an array. It
returns the value 1 for nonarray variables.
■ The SIZE operator can return the size of a C variable. A variable's
size is the product of its LENGTH and TYPE.
■ The TYPE operator can return the size of a C type or variable. If the
variable is an array, TYPE returns the size of a single element of the
array.
For instance, if your program has an eight-element int array,
int arr[8];
the following C and assembly expressions yield the size of arr and its
elements:
╓┌───────────┌───────────────────────────┌───────────────────────────────────╖
_asm C Size
────────────────────────────────────────────────────────────────────────────
LENGTH arr sizeof(arr)/sizeof(arr[0]) 8
SIZE arr sizeof(arr) 16
_asm C Size
────────────────────────────────────────────────────────────────────────────
SIZE arr sizeof(arr) 16
TYPE arr sizeof(arr[0]) 2
────────────────────────────────────────────────────────────────────────────
Comments
Instructions in an _asm block can use assembly-language comments:
_asm mov ax, offset buff ; Load address of buff
Because C macros expand into a single logical line, avoid using
assemblylanguage comments in macros (see the section "Defining _asm Blocks
as C Macros," below). An _asm block can also contain C-style comments, as
noted below.
Debugging with the CodeView(R) Debugger
In-line assembly code can be debugged with CodeView.
Programs containing in-line assembly code can be debugged with the CodeView
debugger, assuming you compile with the /Zi option.
Note that putting multiple assembly instructions or C statements on one line
can hamper debugging with CodeView. In source mode, the CodeView debugger
lets you set breakpoints on a single line but not on individual statements
on the same line. The same principle applies to an _asm block defined as a C
macro, which expands to a single logical line.
Using C in _asm Blocks
Because in-line assembly instructions can be mixed with C statements, they
can refer to C variables by name and use many other elements of C. An _asm
block can use the following C language elements:
■ Symbols, including labels and variable and function names
■ Constants, including symbolic constants and enum members
■ Macros and preprocessor directives
■ Comments ( /* */ )
■ Type names (wherever a MASM type would be legal)
■ typedef names, generally used with operators such as PTR and TYPE or
to specify structure or union members
Within an _asm block, you can specify integer constants with either C
notation or assembler radix notation (0x100 and 100h are equivalent, for
instance). This allows you to define (using #define) a constant in C, and
use it in both C and assembly portions of the program. You can also specify
constants in octal by preceding them with a 0. For example, 0777 specifies
an octal constant.
Using Operators
An _asm block cannot use C-specific operators, such as the operator.
However, operators shared by QuickC and MASM, such as the * operator, are
interpreted as assembly-language operators. For instance, outside an _asm
block, square brackets ( [] ) are interpreted as enclosing array subscripts,
which C automatically scales to the size of an element in the array. Inside
an _asm block, they are seen as the MASM index operator, which yields an
unscaled byte offset from any data object or label (not just an array). The
following code illustrates the difference:
int array[10];
_asm mov array[6], bx ; Store BX at array+6 (not scaled)
array[6] = 0; /* Store 0 at array+12 (scaled) */
The first reference to array is not scaled, but the second is. Note that
you can use the TYPE operator to achieve scaling based on a constant. For
instance, the following statements are equivalent:
_asm mov array[6 * TYPE int], 0 ; Store 0 at array + 12
array[6] = 0; /* Store 0 at array + 12 */
Using C Symbols
An _asm block can refer to any C symbol that is visible (in scope) where the
block appears. (C symbols are variable names, function names, and labels─in
other words, names that aren't symbolic constants or enum members.)
A few restrictions apply to the use of C symbols:
■ Each assembly-language statement can contain only one C symbol.
Multiple symbols can appear in the same assembly instruction only with
OFFSET, LENGTH, TYPE, and SIZE expressions.
■ Functions referenced in an _asm block must be declared (prototyped)
earlier in the program. Otherwise, the compiler cannot distinguish
between function names and labels in the _asm block.
■ An _asm block cannot use any C symbols with the same spelling as MASM
reserved words (regardless of case). MASM reserved words include
instruction names such as PUSH and register names such as SI.
■ Structure and union tags are not recognized in _asm blocks.
Accessing C Data
A great convenience of in-line assembly is the ability to refer to C
variables by name. An _asm block can refer to any symbols─including variable
names─that are visible where the block appears. For instance, if the C
variable var is visible, the instruction
_asm mov ax, var
stores the value of var in AX.
If a structure or union member has a unique name, an _asm block can refer to
it using only the member name, without specifying the C variable or typedef
name before the period (.) operator. If the member name is not unique,
however, you must place a variable or typedef name immediately before the
period (.) operator. For instance, the following structure types share
same_name as their member name:
struct first_type
{
char *weasel;
int same_name;
};
struct second_type
{
int wonton;
long same_name;
};
If you declare variables with the types
struct first_type hal;
struct second_type oat;
all references to the member same_name must use the variable name, because
same_name is not unique. But the member weasel has a unique name, so you
can refer to it using only its member name:
_asm
{
mov bx, OFFSET hal
mov cx, [bx]hal.same_name ; Must use 'hal'
mov si, [bx].weasel ; Can omit 'hal'
}
Note that omitting the variable name is merely a coding convenience. The
same assembly instructions are generated whether or not it is present.
Writing Functions
If you write a function with in-line assembly code, it's a simple matter to
pass arguments to the function and return a value from it. The following
examples compare a function first written for a separate assembler and then
rewritten for the in-line assembler. The function, called power2, receives
two parameters, multiplying the first parameter by 2 to the power of the
second parameter. Written for a separate assembler, the function might look
like this:
; POWER.ASM
; Compute the power of an integer
;
PUBLIC _power2
_TEXT SEGMENT WORD PUBLIC 'CODE'
_power2 PROC
push bp ; Save BP
mov bp, sp ; Move SP into BP so we can refer
; to arguments on the stack
mov ax, [bp+4] ; Get first argument
mov cx, [bp+6] ; Get second argument
shl ax, cl ; AX = AX * ( 2 ^ CL )
pop bp ; Restore BP
ret ; Return with sum in AX
_power2 ENDP
_TEXT ENDS
END
Function arguments are usually passed on the stack.
Since it's written for a separate assembler, the function requires a
separate source file and assembly and link steps. C function arguments
usually are passed on the stack, so this version of the power2 function
accesses its arguments by their positions on the stack. (Note that the MODEL
directive, available in MASM and some other assemblers, also allows you to
access stack arguments and local stack variables by name.)
The POWER2.C program below writes the power2 function with in-line
assembly code:
/* POWER2.C */
#include <stdio.h>
int power2( int num, int power );
void main( void )
{
printf( "3 times 2 to the power of 5 is %d\n", \
power2( 3, 5) );
}
int power2( int num, int power )
{
_asm
{
mov ax, num ; Get first argument
mov cx, power ; Get second argument
shl ax, cl ; AX = AX * ( 2 to the power of CL )
}
/* Return with result in AX */
}
The in-line version of the power2 function refers to its arguments by name
and appears in the same source file as the rest of the program. This version
also requires fewer assembly instructions. Since C automatically preserves
BP, the _asm block doesn't need to do so. It can also dispense with the RET
instruction, since the C part of the function performs the return.
Because the in-line version of power2 doesn't execute a C return
statement, it causes a harmless warning if you compile at warning levels 2
or higher:
warning C4035: 'power2' : no return value
The function does return a value, but QuickC cannot tell that in the absence
of a return statement. Simply ignore the warning in this context.
Using and Preserving Registers
In general, you should not assume that a register will have a given value
when an _asm block begins. An _asm block inherits whatever register values
happen to result from the normal flow of control.
As you may have noticed in the POWER2.C example in the previous section, the
power2 function doesn't preserve the value in the AX register. When you
write a function in assembly language, you don't need to preserve the AX,
BX, CX, DX, ES, and flags registers. However, you should preserve any other
registers you use (DI, SI, DS, SS, SP, and BP).
────────────────────────────────────────────────────────────────────────────
WARNING
If your in-line assembly code changes the direction flag using the STD or
CLD instructions, you must restore the flag to its original value.
────────────────────────────────────────────────────────────────────────────
The POWER2.C example in the previous section also shows that functions
return values in registers. This is true whether the function is written in
assembly language or in C.
Functions return values in the AX and DX registers.
If the return value is short (a char, int, or near pointer), it is stored in
AX. The POWER2.C example returned a value by terminating with the desired
value in AX.
If the return value is long, store the high word in DX and the low word in
AX. To return a longer value (such as a floating-point value), store the
value in memory and return a pointer to the value (in AX if near or in DX:AX
if far).
Assembly instructions that appear in-line with C statements are free to
alter the AX, BX, CX, and DX registers. C doesn't expect these registers to
be maintained between statements, so you don't need to preserve them. The
same is true of the SI and DI registers, with some exceptions (see the
section "Optimizing," below). You should preserve the SP and BP registers
unless you have some reason to change them─to switch stacks, for instance.
Jumping to Labels
Like an ordinary C label, a label in an _asm block is visible (has scope)
throughout the function in which it is defined (not only in the block). Both
assembly instructions and C goto statements can jump to labels inside or
outside the _asm block.
Labels in _asm blocks have function scope and are not case sensitive.
Unlike C labels, labels defined in _asm blocks are not case sensitive, even
when used in C statements. C labels are not case sensitive in an _asm block,
either. (Outside an _asm block, a C label is case sensitive as usual.) The
following do-nothing code shows all the permutations.
void func( void )
{
goto C_Dest; /* legal */
goto c_dest; /* error */
goto A_Dest; /* legal */
goto a_dest; /* legal */
_asm
{
jmp C_Dest ; legal
jmp c_dest ; legal
jmp A_Dest ; legal
jmp a_dest ; legal
a_dest: ; _asm label
}
C_Dest: /* C label */
return;
}
Don't use C library function names as labels in _asm blocks. For instance,
you might be tempted to use exit as a label,
jne exit
.
.
.
exit:
; More _asm code follows
forgetting that exit is the name of a C library function. The code doesn't
cause a compiler error, but it might cause a jump to the exit function
instead of the desired location.
As in MASM programs, the dollar symbol ($) serves as the current location
counter─a label for the instruction currently being assembled. In _asm
blocks, its main use is to make long conditional jumps:
jne $+5 ; next instruction is 5 bytes long
jmp farlabel
; $+5
.
.
.
farlabel:
Calling C Functions
An _asm block can call C functions, including C library routines. The
following example calls the printf library routine:
#include <stdio.h>
char format[] = "%s %s\n";
char hello[] = "Hello";
char world[] = "world";
void main( void )
{
_asm
{
mov ax, offset world
push ax
mov ax, offset hello
push ax
mov ax, offset format
push ax
call printf
add sp, 6
}
}
Since function arguments are passed on the stack, you simply push the needed
arguments─string pointers, in the example above─before calling the function.
The arguments are pushed in reverse order, so they come off the stack in the
desired order. To emulate the C statement
printf( format, hello, world );
the example pushes pointers to world, hello, and format, in that order,
then calls printf. The last instruction in the _asm block adjusts the stack
to account for the arguments previously pushed onto it.
Defining _asm Blocks as C Macros
C macros offer a convenient way to insert assembly code into C code, but
they demand extra care because a macro expands into a single logical line.
To create trouble-free macros, follow these rules:
■ Enclose the _asm block in braces
■ Put the _asm keyword in front of each assembly instruction
■ Use old-style C comments ( /* comment */ ) instead of assembly-style
comments ( ; comment )
To illustrate, the following example defines a simple macro:
#define BEEP _asm \
/* Beep sound */ \
{ \
_asm mov ah, 2 \
_asm mov dl, 7 \
_asm int 21h \
}
At first glance, the last three _asm keywords seem superfluous. They are
needed, however, because the macro expands into a single line:
_asm /* Beep sound */ { _asm mov ah, 2 _asm mov dl, 7 _asm int 21h }
The third and fourth _asm keywords are needed as statement separators. The
only statement separators recognized in _asm blocks are the newline
character and _asm keyword. And since a block defined as a macro is one
logical line, you must separate each instruction with _asm.
The braces are essential as well. If you omit them, the compiler can be
confused by C statements on the same line to the right of the macro
invocation. Without the closing brace, QuickC cannot tell where assembly
code stops, and it sees C statements after the _asm block as assembly
instructions.
Use C comments in _asm blocks written as macros.
Assembly-style comments that start with a semicolon (;) continue to the end
of the line. This causes problems in macros because QuickC ignores
everything after the comment, all the way to the end of the logical line. To
prevent errors, use C comments ( /* comment */ ) in _asm blocks defined as
macros.
An _asm block written as a C macro can take arguments. Unlike an ordinary C
macro, however, an _asm macro cannot return a value. So you cannot use such
macros in C expressions.
You can convert MASM macros to C macros.
Note that some MASM-style macros can be written as C macros. Below is a MASM
macro that sets the video page to the value specified in the page
argument:
setpage MACRO page
mov ah, 5
mov al, page
int 10h
ENDM
The following code defines setpage as a C macro:
#define setpage( page ) _asm \
{ \
_asm mov ah, 5 \
_asm mov al, page \
_asm int 10h \
}
Both macros do the same job.
Optimizing
The presence of an _asm block in a function affects optimization in a few
different ways. First, as you might expect, QuickC doesn't try to optimize
the _asm block itself. What you write in assembly language is exactly what
you get.
Second, the presence of an _asm block affects register variable storage.
(See the section "Register Variables" in Chapter 5, "Advanced Data Types,"
for a discussion of register variables.) Under normal circumstances, QuickC
automatically stores variables in registers. This is not done, however, in
any function that contains an _asm block. To get register variable storage
in such a function, you must request it with the register keyword.
Since the compiler stores register variables in the SI and DI registers,
these registers represent variables in functions that request register
storage. The first eligible variable is stored in SI and the second in DI.
Preserve SI and DI in such functions unless you want to change the register
variables.
Keep in mind that the name of a variable declared with register translates
directly into a register reference (assuming a register is available for
such use). For instance, if you declare
register int sample;
and the variable sample happens to be stored in SI, then the _asm
instruction
_asm mov ax, sample
is equivalent to
_asm mov ax, si
If you declare a variable with register and the compiler cannot store the
variable in a register, QuickC issues a compiler error if you reference the
variable in an _asm block. The solution is to remove the register
declaration from that variable.
Register variables form a slight exception to the general rule that an
assembly-language statement can contain no more than one C symbol. If one of
the symbols is a register variable, for example,
register int v1;
int v2;
then an instruction can use two C symbols, as in
mov v1, v2
Finally, the presence of in-line assembly code inhibits loop optimization
for the entire function in which the code appears. (Loop optimization can be
selected with the /Ol command-line switch; see Chapter 4, "QCL Command
Reference," in Microsoft QuickC Tool Kit.) This optimization is suppressed
no matter which compiler options you use.
References and Books on Assembly Language
Assembly language varies widely for different computer processors. In
selecting a reference on assembly language, make sure it describes assembly
for the Intel 8086 family of processors or compatibles. These are the
microprocessors used in the IBM and IBM-compatible computers able to run
QuickC.
The following books and articles may be useful in learning to program in
assembly language:
Chesley, Harry R. and Mitchell Waite. Supercharging C with Assembly
Language.
Reading, Massachusetts: Addison-Wesley Publishing Company, Inc., 1987.
Duncan, Ray. Advanced MS-DOS Programming, 2nd ed. Redmond, Washington:
Microsoft Press, 1988.
Lafore, Robert. Assembly Language Primer for the IBM PC & XT.
New York, New York: Plume/Waite, 1984.
Metcalf, Christopher D. and Marc B. Sugiyama. COMPUTE!'s Beginner's Guide to
Machine Language on the IBM PC & PCjr.
Greensboro, North Carolina: COMPUTE! Publications, Inc., 1985.
Microsoft. Microsoft Macro Assembler 5.1 Programmer's Guide.
Redmond, Washington, 1987. (Included with Microsoft Macro Assembler.)
Microsoft. Microsoft Macro Assembler 5.1 Reference.
Redmond, Washington, 1987. (Included with Microsoft Macro Assembler.)
Sargent, Murray and Richard L. Shoemaker. The IBM Personal Computer from the
Inside Out.
Reading, Massachusetts: Addison-Wesley Publishing Company, Inc., 1986.
The above references are listed for your convenience only. With the
exception of those published by Microsoft, Microsoft Corporation does not
endorse these books or recommend them over others on the same subject.
Appendix A C Language Guide
────────────────────────────────────────────────────────────────────────────
This appendix provides a quick summary of C language fundamentals. It does
not attempt to teach you the C language (Part 1 of this book does that) or
document all the details of C. Use it as a refresher or ready reference
after you have read all the material in Chapters 1 through 10.
To simplify reference, this appendix has the same general organization as
the chapters in Part 1. Each major section lists the chapter(s) where you
may find more detailed information on a given topic.
You can also use QuickC's online help to get instant information on any
topic. The online help index and table of contents provide alternate ways to
access information.
General Syntax
Basic C-language syntax is explained in Chapter 1, "Anatomy of a C Program."
A C statement consists of keywords, expressions, and function calls. A
statement always ends with a semicolon. A statement block is a collection of
statements enclosed by braces ({ }). A statement block can appear anywhere a
simple C statement appears. No semicolon occurs after the closing brace.
C is a free-format programming language. You can insert "whitespace"
characters (spaces, tabs, carriage returns, and form feeds) almost anywhere,
to indent statement blocks and otherwise make your code more readable.
Comments begin with the slash-asterisk sequence (/*) and end with the
asterisk-slash sequence (*/). Comments are legal anywhere a space is legal,
but they cannot be nested.
User-Defined Names
The rules governing user-defined names are explained in Chapter 1, "Anatomy
of a C Program," and Chapter 4, "Basic Data Types."
You can define your own names ("identifiers") for variables, functions, and
user-defined types. Identifiers are case sensitive. For instance, the
identifier myVariable is not the same as the identifier Myvariable. You
cannot use a C keyword (see the list below) as an identifier.
An identifier can contain only the following characters:
■ abcdefghijklmnopqrstuvwxyz
■ ABCDEFGHIJKLMNOPQRSTUVWXYZ
■ 0123456789
■ _ (underscore)
The first character of an identifier must be a letter or the underscore
character. The first 31 characters of local identifiers are significant. The
name can contain more than 31 characters, but QuickC ignores everything
beyond the thirty-first character. Global identifiers are normally
significant to 30 characters.
Keywords
A keyword has a special meaning in the C language. You must spell keywords
as shown in the following list, and you cannot use them as user-defined
names (see above).
_asm _emit _interrupt signed
auto enum _loadds sizeof
_based _export long static
break extern _near struct
case _far _pascal switch
_cdecl _fastcall register typedef
char float return union
const for _saveregs unsigned
continue _fortran _segname void
default goto _segment volatile
do _huge _self while
double if short
else int
A few other words, such as main, have a special meaning but are not keywords
in the strict sense. Use online help to get details on all such words.
Functions
The rules governing C functions are explained in Chapter 2, "Functions."
Every C program must have at least one function, named main, which marks the
beginning and end of the program's execution. Every executable statement in
a C program must occur within a function.
Variables can be declared inside or outside functions. Variables declared
inside a function are "local" and can only be accessed in that function.
Variables declared outside all functions are "global" and can be accessed
from any function in your program.
You call a C function by stating its name. If the function requires
"arguments" (data), you list the arguments in the parentheses that follow
the function name. Arguments that you pass to a function become local
variables in the function.
A function can return a value (using the return keyword) or return nothing.
If the function contains no return statement, it ends automatically when
execution reaches the closing brace of the function definition.
A function "prototype" (declaration) tells QuickC the function's name, the
type of value it returns, and the number and type of arguments it requires.
Function prototypes normally appear near the beginning of the program. They
allow QuickC to check the accuracy of every reference to the function.
Flow Control
Flow-control statements are explained in Chapter 3, "Flow Control."
The C language provides several kinds of flow-control statements. The for,
while, and do statements create loops. The if and switch statements perform
a branch. The break, continue, return, and goto statements perform an
unconditional "jump" to another location in your program.
The following sections describe the C flow-control statements in
alphabetical order.
The break Statement
The break statement terminates the smallest enclosing do, for, switch, or
while statement in which it appears. It passes control to the statement
following the terminated statement.
This statement is often used to exit from a loop or switch statement (see
below). The following example illustrates break:
while( c != 'Q' )
{
/* Some C statements here */
if( number_of_characters > 80 )
break; /* Break out of while loop */
/* More C statements here */
}
/* Execution continues here after break statement */
The continue Statement
The continue statement is the opposite of the break statement. It passes
control to the next iteration of the smallest enclosing do, for, or while
statement in which it appears.
This statement is often used to return to the start of a loop from within a
deeply nested loop.
The following example illustrates continue:
while( c != 'Q' )
{
/* Some C statements here*/
if( c == 0x20 )
continue; /* Skip rest of loop */
/* More C statements here */
}
In the example, the continue statement skips to the next iteration of the
loop whenever c equals 0x20, the ASCII value for a space character.
The do Statement
The do statement repeats a statement until a specified expression becomes
false. The test expression in the loop is evaluated after the body of the
loop executes. Thus, the body of a do loop always executes at least once.
Use a break, goto, or return statement when you need to exit a do loop
early. Use the continue statement to terminate an iteration without exiting
the loop. The continue statement passes control to the next iteration of the
loop.
The following example illustrates do:
sample = 1;
do
printf( "%d\t%d\n", sample, sample * sample );
while( ++x <= 7 );
The printf statement in the example always executes at least once, no matter
what value x has when the loop begins.
The for Statement
The for statement lets you repeat a statement a specified number of times.
It consists of three expressions:
■ An initializing expression, which is evaluated when the loop begins
■ A test expression, which is evaluated before each iteration of the
loop
■ A modifying expression, which is evaluated at the end of each
iteration of the loop
These expressions are enclosed in parentheses and followed by the loop
body─the statement the loop is to execute. Each expression in the
parentheses can be any legal C statement.
The for statement works as follows:
1. The initializing expression is evaluated.
2. As long as the test expression evaluates to a nonzero value, the loop
body is executed. When the test expression becomes 0, control passes
to the statement following the loop body.
3. At the end of each iteration of the loop, the modifying expression is
evaluated.
You can use a break, goto, or return statement to exit a for loop early. Use
the continue statement to terminate an iteration without exiting the for
loop. The continue statement passes control to the next iteration of the
loop.
The following example illustrates for:
for( counter = 0; counter < 100; counter++ )
{
x[counter] = 0; /* Set every array element to zero */
}
The goto Statement
The goto statement performs a jump to the statement following the specified
label. A goto statement can jump anywhere within the current function.
A common use of goto is to exit immediately from a deeply nested loop. For
instance:
for( ... )
{
for( ... )
{
/* Do something here */
if(c == CTRL_C)
goto myplace;
}
/* Do something else here */
}
/* The goto label is named myplace */
myplace:
/* The goto statement transfers control here */
The if Statement
The if statement performs a branch based on the outcome of a conditional
test. If the test expression is true, the body of the if statement executes.
If it is false, the statement body is skipped.
The else keyword is used with if to form an either-or construct that
executes one statement when the test expression is true and another when
it's false. C does not offer an "else-if" keyword. You can combine if and
else statements to achieve the same effect. C pairs each else with the most
recent if that lacks an else.
Below is a simple if statement:
if( score < 70 )
grade = 'F';
else
grade = 'P';
If the value of the variable score is less than 70, the variable grade
is set to the constant F. Otherwise, score is set to P.
The return Statement
The return statement ends the execution of the function in which it appears.
It can also return a value to the calling function. For example:
return; /* End function and return no value */
return myvariable; /* End function and return value of myvariable */
The switch Statement
The switch statement allows you to branch to various sections of code based
on the value of a single variable. This variable must evaluate to a char,
int, or long constant.
Each section of code in the switch statement is marked with a case label─the
keyword case followed by a constant or constant expression. The value of the
switch test expression is compared to the constant in each case label. If a
match is found, control transfers to the statement after the matching label
and continues until you reach a break statement or the end of the switch
statement.
For example:
switch( answer )
{
case 'y': /* First case */
printf( "lowercase y\n" );
break;
case 'n': /* Another case */
printf( "lowercase n\n" );
break;
default: /* Default case */
printf( "not a lowercase y or n\n" );
break;
}
The example tests the value of the variable answer. If answer evaluates
to the constant 'y', control transfers to the first case in the switch
statement. If it equals 'n', control transfers to the second case.
A case labelled with the default keyword executes when none of the other
case constants matches the value of the switch test expression. In the
example, the default case executes when answer equals any value other than
'y' or 'n'.
If you omit the break statement at the end of a case, execution falls
through to the next case.
If you omit the default case and no matching case is found, nothing in the
switch statement executes.
No two case constants in the same switch statement can have the same value.
The while Statement
The while statement repeats a statement until its test expression becomes
false. A while loop evaluates its test expression before executing its loop
body. If the test expression is false when the loop begins, the loop body
never executes. (Contrast this behavior with the do loop, which always
executes its loop body at least once.)
For example:
while( !sample ) /* Repeat until sample equals 1 */
{
printf( "%d\t%d\n", x, x*x );
x += 6;
if( x > 20 )
sample = 1;
}
You can exit a while loop early with a break or goto statement. The continue
statement skips to the next iteration of the loop.
Data Types
Data types are explained in Chapter 4, "Data Types," and Chapter 5,
"Advanced Data Types." A brief description is given here.
Basic Data Types
The basic data types in C are character (char), integer (int), and floating
point (float and double). All other data types are derived from these basic
types. For example, a string is an array of char values.
Table A.1 lists the range of values for each data type.
Table A.1 Basic Data Types
╓┌────────────────┌────────────────────────────┌─────────────────────────────╖
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
char signed char -128 to 127
unsigned char none 0 to 255
int signed, signed int -32,768 to 32,767
unsigned unsigned int 0 to 65,535
unsigned short unsigned short int 0 to 65,535
short short int, signed short -32,768 to
signed short int 32,767
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
signed short int 32,767
long long int, signed long -2,147,483,647 to
signed long int 2,147,483,648
unsigned long unsigned long int 0 to 4,294,967,295
_segment none 0 to 65,535
enum none -32,768 to 32,767
float none Approximately 1.2E-38 to
3.4E+38 (7-digit precision)
double none Approximately 2.2E-308 to
1.8E+308 (15-digit precision)
long double none Approximately 3.4E-4932 to
Type Name Other Names Range of Values
────────────────────────────────────────────────────────────────────────────
long double none Approximately 3.4E-4932 to
1.2E+4932 (19-digit
precision)
────────────────────────────────────────────────────────────────────────────
Character Type
The character type (char) occupies one byte of storage and can express a
whole number in the range of -128 to 127. Unsigned characters have a range
of 0 to 255. You can represent any ASCII character as an unsigned char
value.
Typical declarations of character types are shown below:
char answer; /* Declare a character variable answer */
char alpha = 'a'; /* Declare character variable alpha
and initialize it */
A character constant represents a single ASCII character. Typical character
constants are shown below:
char alpha = 'a'; /* Declare and initialize */
char c2 = 0x61; /* Declare and initialize with
hexadecimal value for 'a' */
Escape Sequences - Escape sequences represent special characters, such as
the carriage return. An escape sequence consists of a backslash character
plus a letter or punctuation mark. Table A.2 lists the C escape sequences;
they are also listed in online help.
Table A.2 C Escape Sequences
╓┌──────────┌─────────────────────┌──────────────────────────────────────────╖
Character Meaning Hexadecimal Value
Character Meaning Hexadecimal Value
────────────────────────────────────────────────────────────────────────────
\a Alert (bell) 0x07
\n New line (linefeed) 0x0A
\b Backspace 0x08
\r Carriage return 0x0D
\f Formfeed 0x0C
\t Tab 0x09
\v Vertical tab 0x0B
\\ Backslash 0x5C
\' Single quote 0x27
\" Double quote 0x22
\0 Null 0x00
────────────────────────────────────────────────────────────────────────────
Integer Type
The integer (int) type occupies two bytes of storage and can express a whole
number in the range -32,768 to 32,767. Unsigned integers (unsigned or
unsigned int) have a range of 0 to 65,535.
In QuickC, short integers (short or short int) are the same as integers
(int). Note that the short and int types are not the same in some operating
systems other than DOS.
Signed long integers (long) occupy four bytes and have a range of
-2,147,483,648 to 2,147,483,647. Unsigned long integers have a range of 0 to
4,294,967,295.
Integer variables are declared with the keywords int, short, unsigned, or
long. Typical declarations of integer types are shown below:
int z; /* Declare an int variable z */
int ten = 10; /* Declare int variable and
assign it the value 10 */
unsigned int a; /* Declare unsigned int variable */
unsigned long BigInt = 2000000001UL; /* Declare and
initialize */
Integer constants are used to represent decimal, octal, and hexadecimal
numbers. There are three types of integer constants:
1. Decimal constants can only contain the digits 0-9. The first digit
must not be 0.
2. Octal constants can only contain the digits 0-7. The first digit must
be 0.
3. Hexadecimal constants can only contain the digits 0-9, plus the
letters a-f or A-F. The constant must begin with either 0x or 0X.
You can specify that an integer constant is long by adding the suffix l or
L. The suffix can be used with decimal, hexadecimal, or octal notation.
To specify that an integer constant is short, add the suffix u or U. This
suffix can also be used with decimal, hexadecimal, or octal notation.
Typical integer constants are shown below:
42 /* Decimal constant */
0x34 /* Hexadecimal constant */
0x3cL /* Long hexadecimal constant */
052 /* Octal constant */
Floating-Point Types
You can declare floating-point variables using the keywords float or double.
The float type occupies four bytes of storage and can express a
floating-point value in the range 1.2E-38 to 3.4E+38. This type has
seven-digit precision.
The double type occupies eight bytes of storage and can express a
floating-point value in the range 2.2E-308 to 1.8E+308. This type has
fifteen-digit precision.
The long double type occupies ten bytes of storage and can express a
floating-point value in the range 3.4E-4932 to 1.2E+4932. This type has
nineteen-digit precision.
Typical declarations of floating-point types are shown below:
float SmallPi = 3.14; /* Declare floating-point variable */
double AccuratePi = 3.141592653596 /* Declare
double-precision */
Floating-point constants can represent decimal numbers in either single or
double precision. A floating-point constant must either contain a decimal
point or end with the suffix e or E. Typical floating-point constants are
shown below:
2.78 /* Floating-point constant */
3E /* Floating-point constant */
Aggregate Data Types
Aggregate data types are built from one or more of the basic data types.
These include the following:
■ Arrays (including strings)
■ Structures
■ Unions
Arrays and Strings
An "array" is a collection of data elements of a single type. An array can
contain any data type. You can access an element of an array by using the
array name and a numeric subscript.
A "string" is an array of characters that terminates with the null character
(\0). Arrays that contain strings must allow space for the final null
character.
Typical arrays and strings are shown below:
int id_number[10]; /* One-dimensional;
10 elements; integer */
char name[30]; /* String */
float matrix[5][3]; /* Two-dimensional array,
5 rows, 3 columns */
char baby[30] = "Peter Roddy"; /* String initialization */
Structures
A "structure" is a collection of data items of different types. Once you
have defined a structure type, you can declare a structure variable using
that type.
The following example illustrates a simple structure:
struct date
{
int month;
int day;
int year;
}
struct date today;
The example defines a structure type named date and declares a structure
variable today to be of type date.
Use the structure-member operator ( . ) to access the "elements" (members)
of a structure. The name
today.month
refers to the month member of the today structure in the example.
Unions
A "union" is a set of data items of different types sharing the same storage
space in memory. One use of unions is accessing the computer's DOS
registers. For instance, QuickC defines the union REGS as the following:
union REGS
{
struct WORDREGS x;
struct BYTEREGS h;
};
Advanced Data Types
Advanced data topics are explained in Chapter 5, "Advanced Data Types." A
brief description of each topic is given here.
Visibility
Variables declared outside all functions are global and can be accessed
anywhere in the current source file. Variables declared inside a function
are local and can be accessed only in that function. Use the extern keyword
to make a variable declared in another source file visible in the current
source file.
Lifetime
Global variables, and local variables declared with the static keyword,
exist for the lifetime of the program. Other local variables are
"automatic;" they come into being when the function starts and evaporate
when it ends.
Type Conversions
A type conversion occurs automatically when an expression mixes two
different data types. QuickC converts the lower-ranking type to the
higher-ranking type before it performs the specified operation.
You can also "cast" (manually convert) a value to any type by placing the
desired type name in parentheses in front of the value. The example below
casts the value of sample to type float and assigns the value to x:
int sample;
float x;
x = (float)sample;
User-Defined Types
The typedef keyword allows you to create user-defined types, which are
synonyms for existing data types. User-defined types can make your program
more readable. For example, a type called string may be easier to
understand than a type called char *.
A simple typedef declaration is shown below. The name of an existing type
(long int) is followed by the synonym income.
typedef long int income;
Once you have created a new type name, you can use it wherever the original
type name could be used:
income net_income, gross_income;
In the example above, the variables net_income and gross_income are of
type income, which is the same as long int.
Enumerated Types
An enumerated type (declared with enum) has values limited to a specified
set. If the enum declaration does not specify any values, QuickC assigns
sequential integers to the enumeration identifiers beginning at zero.
The example below assigns the values of 0, 1, and 2 to the enumeration
identifiers zero, one, and two, respectively. It also creates an
enumerated type small_numbers that can be used to declare other variables.
/* Enumerated data type */
enum small_numbers {zero, one, two};
/* Variable my_numbers is of type small_numbers */
enum small_numbers my_numbers;
The following example explicitly assigns values to the enumeration
identifiers:
/* Enumerated data type */
enum even_numbers { two = 2, four = 4, six = 6 };
Operators
C-language operators are explained in Chapter 6, "Operators."
An "operand" is a constant or variable manipulated by an operator in an
expression. An "operator" specifies how the operands in an expression are to
be evaluated. Operators also produce a result that can be nested within a
larger expression.
C provides a rich set of operators covering everything from basic arithmetic
operations to logical and bitwise operations. You can also combine the
assignment operator (=) with any arithmetic or bitwise operator to form a
combined assignment operator.
C operators have two properties, precedence and associativity. You can
change the normal order of evaluation by enclosing expressions in
parentheses.
Table A.3 lists the C operators and their precedence and associativity
values. The lines in the table separate precedence levels. The highest
precedence level is at the top of the table.
Table A.3 C Operators
╓┌───────────────────────┌────────────────────────────────────┌──────────────╖
Symbol Name or Meaning Associativity
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
( ) Function call Left to right
[ ] Array element
. Structure or union
member
-> Pointer to structure
member
────────────────────────────────────────────────────────────────────────────
- - Decrement Right to left
++ Increment
────────────────────────────────────────────────────────────────────────────
:> Base operator Left to right
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
:> Base operator Left to right
────────────────────────────────────────────────────────────────────────────
! Logical NOT Right to left
~ One's complement
- Unary minus
+ Unary plus
& Address
* Indirection
sizeof Size in bytes
(type) Type cast [for example, (float) i]
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
(type) Type cast [for example, (float) i]
────────────────────────────────────────────────────────────────────────────
* Multiply Left to right
/ Divide
% Modulus (remainder)
────────────────────────────────────────────────────────────────────────────
+ Add Left to right
- Subtract
────────────────────────────────────────────────────────────────────────────
<< Left shift Left to right
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
<< Left shift Left to right
>> Right shift
────────────────────────────────────────────────────────────────────────────
< Less than Left to right
<= Less than or equal to
> Greater than
>= Greater than or equal to
────────────────────────────────────────────────────────────────────────────
== Equal Left to right
!= Not equal
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
!= Not equal
────────────────────────────────────────────────────────────────────────────
& Bitwise AND Left to right
────────────────────────────────────────────────────────────────────────────
^ Bitwise exclusive OR Left to right
────────────────────────────────────────────────────────────────────────────
| Bitwise OR Left to right
────────────────────────────────────────────────────────────────────────────
&& Logical AND Left to right
────────────────────────────────────────────────────────────────────────────
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
|| Logical OR Left to right
────────────────────────────────────────────────────────────────────────────
? : Conditional Right to left
────────────────────────────────────────────────────────────────────────────
= Assignment Right to left
*=, /=, %=, +=, -= Compound assignment
<<=, >>=, &=, ^=, |=
────────────────────────────────────────────────────────────────────────────
, Comma Left to right
Symbol Name or Meaning Associativity
────────────────────────────────────────────────────────────────────────────
, Comma Left to right
────────────────────────────────────────────────────────────────────────────
Preprocessor Directives
Preprocessor directives are explained in Chapter 7, "Preprocessor
Directives."
A "preprocessor directive" is a command to the QuickC compiler, which
processes all such commands before it compiles your source program. A
preprocessor directive begins with the # symbol, followed by the directive
and any arguments the directive needs. Since a preprocessor directive is not
a C language statement, it doesn't end in a semicolon.
The two most commonly used directives are #define and #include. Use the
#define directive to give a meaningful name to some constant in your
program. The following directive tells QuickC to replace PI with 3.14159
everywhere in the source program:
#define PI 3.14159
The #include directive below tells QuickC to insert the contents of a
specified file at the current location in your source program.
#include <stdio.h> /* Standard header file */
Such files are called "include files" or "header files." Standard header
files, such as STDIO.H, end with the .H extension and contain function
prototypes and other definitions needed for QuickC library routines.
Table A.4 lists and describes the QuickC standard header files. Consult
online help for information on the header files needed by individual library
functions.
Table A.4 QuickC Header Files
╓┌─────────────────────────────────┌─────────────────────────────────────────╖
File Name Major Contents
────────────────────────────────────────────────────────────────────────────
ASSERT.H assert debugging macro
BIOS.H BIOS service functions
CONIO.H Console and port I/O routines
CTYPE.H Character classification
DIRECT.H Directory control
DOS.H MS-DOS interface functions
ERRNO.H System-wide error numbers
FCNTL.H Flags used in open and sopen functions
FLOAT.H Constants needed by math functions
File Name Major Contents
────────────────────────────────────────────────────────────────────────────
GRAPH.H Low-level graphics and font routines
IO.H File-handling and low-level I/O
LIMITS.H Ranges of integers and character types
LOCALE.H Internationalization functions
MALLOC.H Memory-allocation functions
MATH.H Floating-point math routines
MEMORY.H Buffer-manipulation routines
PGCHART.H Presentation graphics
PROCESS.H Process-control routines
File Name Major Contents
────────────────────────────────────────────────────────────────────────────
SEARCH.H Searching and sorting functions
SETJMP.H setjmp and longjmp functions
SHARE.H Flags used in sopen
SIGNAL.H Constants used by signal function
STDARG.H Macros used to access variable-length
argument-list functions
STDDEF.H Commonly used data types and values
STDIO.H Standard I/O header file
STDLIB.H Commonly used library functions
STRING.H String-manipulation functions
File Name Major Contents
────────────────────────────────────────────────────────────────────────────
STRING.H String-manipulation functions
TIME.H General time functions
VARARGS.H Variable-length argument-list functions
SYS\LOCKING.H Flags used by locking function
SYS\STAT.H File-status structures and functions
SYS\TIMEB.H time function
SYS\TYPES.H File-status and time types
SYS\UTIME.H utime function
────────────────────────────────────────────────────────────────────────────
Pointers
Pointers are described in Chapter 8, "Pointers," and Chapter 9, "Advanced
Pointers."
A "pointer" is a variable that contains the memory address of an item rather
than its value. A pointer can point to any type of data item or to a
function. The following code illustrates pointer declarations:
int *intptr; /* Pointer to an integer */
char *name; /* Pointer to char */
The following operators are used with pointers:
■ The indirection operator (*) has two uses. In a declaration, it means
that the declared item is a pointer. In an expression, it denotes the
data being pointed to.
■ The address-of operator (&) yields the memory address at which an item
is stored.
You can perform four arithmetic operations on pointers:
1. Adding a pointer and an integer
2. Subtracting an integer from a pointer
3. Subtracting two pointers
4. Comparing two pointers
Pointer arithmetic operations are automatically scaled by the size of the
object pointed to. For instance, adding 1 to a pointer to a float item
causes the address stored in the pointer to be incremented four bytes, the
size of one float item.
QuickC 2.5 also supports based pointers, a highly advanced feature, that are
compatible with Microsoft C version 6.0. Please refer to your C 6.0
documentation for more information about based pointers and objects.
Appendix B C Library Guide
────────────────────────────────────────────────────────────────────────────
This appendix outlines the features of the C run-time library provided with
QuickC. It does not intend to be a complete presentation of the complete C
run-time library. Instead, this appendix presents the most fundamental
routines, grouped by category so you can begin experimenting with C and with
QuickC.
Remember, use online help to get instant help on any topic of interest. The
online help system provided with QuickC provides complete reference
information for all C library functions, keywords, and preprocessor
directives.
Overview of the C Run-Time Library
At last count, the C run-time library contained over 400 functions to use in
C programs. This appendix describes the major categories of functions
included in the library and, within those categories, the fundamental
routines every C programmer should know.
The discussions of these categories give only a brief overview of the
capabilities of the run-time library. You can find a complete description of
the syntax and use of each routine in online help.
The routines in the C run-time library are divided into the following
categories:
Table B.1 C Run-Time Library Routines
╓┌───────────────────────────────┌──────────────────────────────────┌────────╖
Category Function Routines Page
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
Buffer Manipulation memchr, memcmp, memcpy, memmove, 345
memset
────────────────────────────────────────────────────────────────────────────
Character Classification isalnum, isalpha, isascii, 346
and Conversion iscntrl, isdigit, isgraph,
islower, isprint, ispunct,
isspace, isupper, isxdigit,
tolower, toupper
────────────────────────────────────────────────────────────────────────────
Data Conversion atof, atoi, atol, itoa, ltoa, 348
ultoa, strtod, strtol, strtoul
────────────────────────────────────────────────────────────────────────────
Error Message assert, perror, strerror, 349
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
Error Message assert, perror, strerror, 349
_strerror
────────────────────────────────────────────────────────────────────────────
Graphics 1: Low-Level 350
Graphics
Configure Mode and _displaycursor, _getvideoconfig, 351
Environment _setvideomode
Set Coordinates _getcurrentposition, 352
_getphyscoord,_getviewcoord,
_getwindowcoord,
_setcliprgn,_setvieworg,
_setviewport, _setwindow
Set Palette _remapallpalette, remappallette, 354
_selectpalette
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
_selectpalette
Set Attributes _getbkcolor, _getcolor, 355
_setbkcolor, _setcolor
Output Images _arc, _clearscreen, _ellipse, 356
_floodfill, _getpixel,
_lineto, _moveto, _pie,
_rectangle, _setpixel
Output Text _displaycursor, _gettextcolor, 359
_gettextcursor,
_gettextposition, _outtext,
_settextposition,
_settextcolor, _settextwindow
Transfer Images _getimage, _imagesize, _putimage 361
────────────────────────────────────────────────────────────────────────────
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Graphics 2: Presentation _pg_chart, _pg_chartms, 362
Graphics _pg_chartpie, _pg_chartscatter,
_pg_chartscatterms,
_pg_defaultchart, _pg_initchart
────────────────────────────────────────────────────────────────────────────
Graphics 3: Font Display _getfontinfo, _getgtextextent, 365
_outgtext, _registerfonts,
_setfont, _unregisterfonts
────────────────────────────────────────────────────────────────────────────
Input and Output 367
Stream Routines clearerr, fclose, feof, ferror, 367
fflush, fgetc, fgetpos, fgets,
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
fflush, fgetc, fgetpos, fgets,
fopen, fprintf, fputc, fputs,
fread, freopen, fscanf, fseek,
fsetpos, ftell, fwrite, getc,
getchar, gets, printf, putc,
putchar, puts, rewind, scanf,
sprintf, sscanf, tmpfile, tmpnam,
ungetc
Low-Level Routines close, creat, eof, lseek, open, 373
read, tell, write
Console and Port cgets, cprintf, cputs, cscanf, 375
I/O Routines getch, getche, kbhit,
putch, ungetch
────────────────────────────────────────────────────────────────────────────
Math abs, fabs, labs, acos, asin, atan, 377
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
Math abs, fabs, labs, acos, asin, atan, 377
atan2, ceil, cos, cosh,
exp, floor, fmod, frexp, ldexp,
log, log10, modf, pow, rand,
srand, sin, sinh, sqrt, tan, tanh
────────────────────────────────────────────────────────────────────────────
Memory Allocation calloc, free, _ffree, hfree, 381
_nfree, malloc, _fmalloc,
_nmalloc, realloc
────────────────────────────────────────────────────────────────────────────
Process Control abort, atexit, exit, _exit, 383
system
────────────────────────────────────────────────────────────────────────────
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
Searching and Sorting bsearch, lfind, lsearch, qsort 384
────────────────────────────────────────────────────────────────────────────
String Manipulation strcat, strcpy, strdup, strncat, 385
strncpy, strchr,
strcspn, strpbrk, strrchr, strspn,
strstr, strcmp,
strcmpi, stricmp, strncmp,
strnicmp, strlen, strlwr,
strupr, strnset, strset, strtok
────────────────────────────────────────────────────────────────────────────
Time asctime, clock, ctime, difftime, 389
ftime, gmtime,
mktime, time
Category Function Routines Page
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Buffer-Manipulation Routines
Buffer manipulation routines are used with areas of memory on a
character-by-character basis. Buffers are arrays of characters (bytes).
Unlike strings, however, they are not usually terminated with a null
character (\0).
memchr - Returns a pointer to the first occurrence, within a specified
number of characters, of a given character in the buffer.
Include STRING.H
Prototype void *memchr( const void *buf, int c,
size_t count );
Arguments buf Pointer to buffer
c Character to copy
count Number of characters
Returns A pointer to the location of c in buf
if successful;
NULL if c is not within first count
bytes of buf
────────────────────────────────────────────────────────────────────────────
memcmp - Compares a specified number of characters from two buffers.
Include STRING.H
Prototype int memcmp( const void *buf1, const
void *buf2, size_t count );
Arguments buf1 First buffer
buf2 Second buffer
count Number of characters
Returns A negative value if buf1 < buf2, 0 if
buf1 = buf2,
a positive value if buf1 > buf2
────────────────────────────────────────────────────────────────────────────
memcpy - Copies a specified number of characters from one buffer to another.
Include STRING.H
Prototype void *memcpy( void *dest, const void *
src, size_t count );
Arguments dest New buffer
src Buffer to copy from
count Number of characters to copy
Returns A pointer to dest
────────────────────────────────────────────────────────────────────────────
memmove - Copies a specified number of characters from one buffer to
another. When the source and target areas overlap, the memmove function is
guaranteed to properly copy the full source.
Include STRING.H
Prototype void *memmove( void *dest, const void *
src, size_t count );
Arguments dest Target object
src Source object
count Number of characters to copy
Returns The value of dest
────────────────────────────────────────────────────────────────────────────
memset - Uses a given character to initialize a specified number of bytes in
the buffer.
Include STRING.H
Prototype void *memset( void *dest, int c, size_t
count );
Arguments dest Pointer to destination
c Character to set
count Number of characters
Returns A pointer to dest
Character Classification and Conversion Routines
The classification routines (is...) test a character and return a one (1) if
the character is in the set that the routine is testing for. The conversion
routines (to...) convert characters between uppercase and lowercase. These
routines are generally faster than writing a test expression such as the
following:
if ((c >= 0) || c <= 0x7f))
────────────────────────────────────────────────────────────────────────────
isalnum, isalpha, isascii, iscntrl, isdigit, isgraph, islower, isprint,
ispunct, isspace, isupper, isxdigit These routines test a character for a
specified condition and return a nonzero value if the condition is true. -
Include CTYPE.H
Prototypes int isalnum( int c ); (alphanumeric
character: A-Z, a-z, 0-9)
int isalpha( int c ); (alphabetic
character: A-Z, a-z)
int isascii( int c ); (ASCII
character: 0x00-0x7F)
int iscntrl( int c ); (control
character: 0x00-0x1F, 0x7F)
int isdigit( int c ); (decimal
digit: 0-9)
int isgraph( int c ); (printable
character, not space: 0x21-0x7E)
int islower( int c ); (lowercase
letter: a-z)
int isprint( int c ); (printable
character: 0x20-0x7E)
int ispunct( int c ); (punctuation
character)
int isspace( int c ); (white-space
character: 0x09-0x0D, 0x20)
int isupper( int c ); (uppercase
letter: A-Z)
int isxdigit( int c ); (hexadecimal
digit: A-F, a-f, 0-9)
Argument c Character to be tested
Returns A nonzero value if the condition is true
────────────────────────────────────────────────────────────────────────────
tolower, toupper - These routines accept a character argument and return a
converted character. The tolower and toupper routines are also implemented
as functions. To use the function versions, you must do the following:
■ Include CTYPE.H if necessary for other macro definitions
■ If CTYPE.H is included, give #undef directives for tolower and toupper
■ Include STDLIB.H (which contains the function prototypes)
Include CTYPE.H
Prototypes int tolower( int c );
int toupper( int c );
Argument c Character to be converted
Returns tolower: the lowercase equivalent of c
only if c is an uppercase letter
toupper: the uppercase equivalent of c
only if c is a lowercase letter
Data Conversion Routines
The data conversion routines convert numbers to strings of ASCII characters
and vice versa.
────────────────────────────────────────────────────────────────────────────
atof, atoi, atol - These ASCII-to-number routines convert an ASCII string to
a float, an integer, and a long, respectively.
Include STDLIB.H or MATH.H (atof)
STDLIB.H (atoi, atol)
Prototypes double atof( const char *string );
int atoi( const char *string );
long atol( const char *string );
Argument string String to be converted
Returns The converted string, or 0 if string
cannot be converted
────────────────────────────────────────────────────────────────────────────
itoa, ltoa, ultoa - These number-to-ASCII routines convert an integer, a
long value, or an unsigned long value to an ASCII string.
Include STDLIB.H
Prototypes char * itoa( int value, char *string,
int radix );
char * ltoa( long value, char *string,
int radix );
char * ultoa( unsigned long value, char
*string, int radix );
Arguments value Number to be converted
string String result
radix Number base of value
Returns A pointer to string; there is no error
return
────────────────────────────────────────────────────────────────────────────
strtod, strtol, strtoul - These routines convert a string to a double, a
long, and an unsigned long, respectively.
Include STDLIB.H
Prototypes double strtod( const char *nptr, char
**endptr );
long strtol( const char *nptr, char **
endptr, int base );
unsigned long strtoul( const char *nptr,
char **endptr, int base );
Arguments nptr String to convert
endptr End of scan
base Number base to use
Returns strtod: the converted value; overflow
returns HUGE_VAL,
underflow returns 0
strtol: the converted value; overflow
returns LONG_MAX
or LONG_MIN, depending on sign of
converted value
strtoul: the converted value if
successful, 0 if not, and
ULONG_MAX on overflow
Error Message Routines
The routines in this category handle the display of error messages.
The assert macro is typically used to test for program logic errors; it
prints a message when a given "assertion" fails to hold true. Defining the
identifier NDEBUG to any value causes occurrences of assert to be disabled
in the source file, thus allowing you to turn off assertion checking without
modifying the source file.
The perror routine prints the system-error message, along with a
user-supplied message, for the last system-level call that produced an
error. The perror routine is declared in the include files STDLIB.H and
STDIO.H. The error number is obtained from the errno variable. The system
message is taken from the sys_errlist array.
The strerror and _strerror routines store error messages in a string.
────────────────────────────────────────────────────────────────────────────
assert - Tests for logic error.
Include ASSERT.H, STDIO.H
Prototype void assert( expression );
Argument expression Expression to test
Returns Void
────────────────────────────────────────────────────────────────────────────
perror - Prints error message.
Include STDIO.H
Prototypes void perror( const char *string );
int errno;
int sys_nerr;
char *sys_errlist [sys_nerr];
Arguments string User-supplied message
errno Error number
sys_nerr Number of system-error
messages
sys_errlis Array of error messages
t
Returns Void
────────────────────────────────────────────────────────────────────────────
strerror, _strerror - Saves system-error message and optional user-error
message in string. The routine strerror is the ANSI-compatible version.
Include STRING.H
Prototypes char *strerror( int errnum );
char *_strerror( char *string );
int errno;
int sys_nerr;
char *sys_errlist [sys_nerr];
Arguments errnum Error number
string User-supplied message
errno Error number
sys_nerr Number of system-error
messages
sys_errlis Array of error messages
t
Returns A pointer to the error-message string
Graphics 1: Low-Level Graphics Routines
The low-level graphics routines provide line, figure, and pixel manipulation
capabilities. The routines for presentation graphics are described in the
next section. The routines for displaying fonts follow the presentation
graphics section.
The graphics package supports the IBM(R) (and compatible) Enhanced Graphics
Adapter (EGA), Color Graphics Adapter (CGA), certain operating modes of the
Video Graphics Array (VGA) hardware configurations, and the MCGA (Multicolor
Graphics Array). The graphics package also supports the Hercules Graphics
Card, Graphics Card Plus, InColor Card, and 100 percent compatible cards, as
well as the special Olivetti(R) modes available on AT&T(R) computers.
The low-level graphics routines can be divided into the seven categories
listed below, corresponding to the different tasks involved with creating
and manipulating graphic objects:
Category Task
────────────────────────────────────────────────────────────────────────────
Configure mode and environment Selects the proper display mode for the
hardware and establishes memory areas
for writing and displaying images
Set coordinates Specifies the logical origin and the
active display area within the screen
Set palette Specifies a palette mapping
Set attributes Specifies background and foreground
colors and mask and line styles
Output images Draws and fills figures on the screen
Output text Writes text to the screen
Transfer images Stores images in memory and retrieves
them
Configure Mode and Environment
The configure category of functions sets the status of the cursor, sets
active and visual pages, and determines and sets video display modes.
The _setvideomode and _getvideoconfig functions are generally used at the
very beginning of a graphics program.
────────────────────────────────────────────────────────────────────────────
_displaycursor - Determines whether the cursor will be left on or turned off
on exit from graphics routines.
Include GRAPH.H
Prototype short _far _displaycursor( short toggle
);
Argument toggle Cursor state (_GCURSORON,
_GCURSOROFF )
Returns The previous value of toggle
────────────────────────────────────────────────────────────────────────────
_getvideoconfig - Obtains status of current graphics environment.
Include GRAPH.H
Prototype struct videoconfig _far * _far
_getvideoconfig
( struct videoconfig _far *config );
Argument config Configuration information
Returns The configuration information as a
videoconfig structure
────────────────────────────────────────────────────────────────────────────
_setvideomode - Selects screen display mode.
Include GRAPH.H
Prototype short _far _setvideomode( short mode );
Argument mode Desired mode (_DEFAULTMODE,
_TEXTBW40,
_TEXTC40, _TEXTBW80, _TEXTC80,
_MRES4COLOR,
_MRESNOCOLOR, _HRESBW, _TEXTMONO,
_HERCMONO, _MRES16COLOR, _HRES16COLOR,
_ERESNOCOLOR, _ERESCOLOR, _VRES2COLOR,
_VRES16COLOR, _MRES256COLOR, _ORESCOLOR,
_MAXRESMODE, _MAXCOLORMODE)
Returns A nonzero value if successful, 0 if not
Set Coordinates
The Microsoft C graphics routines recognize three sets of coordinates:
1. Window coordinates defined with real-number values that are mapped to
a specified viewport
2. Viewport coordinates defined by the application (viewport coordinates)
3. Fixed physical coordinates determined by the hardware and display
configuration of the user's environment (physical coordinates)
The functions in this category alter the coordinate systems and provide a
means to translate coordinates between the various systems.
Most of these routines have two or three forms. The functions are listed by
the "base" name, without a suffix. Note, though, that function names that
end with a _w, such as _getcurrentposition_w, use the window-coordinate
system. Those that end with a _wxy, such as _getviewcoord_wxy, use the
window-coordinate system and a _wxycoord structure to define the
coordinates.
The default viewport-coordinate system is identical to the physical one. The
physical origin (0, 0) is always in the upper left corner of the display.
The x axis extends in the positive direction left to right, and the y axis
extends in the positive direction top to bottom.
The dimensions of the x and y axes depend upon the hardware display
configuration and the selected mode. These values are accessible at run time
by examining the numxpixels and numypixels fields of the videoconfig
structure returned by _getvideoconfig.
────────────────────────────────────────────────────────────────────────────
_getcurrentposition - Obtains the coordinates of the current graphic-output
position. The _getcurrentposition function returns the position as an
xycoord structure and the _getcurrentposition_w function returns the
position as a _wxycoord structure.
Include GRAPH.H
Prototypes struct xycoord _far _getcurrentposition(
void );
struct _wxycoord _far
_getcurrentposition_w( void );
Arguments None
Returns _getcurrentposition: the coordinates of
the current position as
an xycoord structure
_getcurrentposition_w: the coordinates
of the current position
as a _wxycoord structure
────────────────────────────────────────────────────────────────────────────
_getphyscoord - Converts viewport coordinates to physical coordinates.
Include GRAPH.H
Prototype struct xycoord _far _getphyscoord( short
x, short y );
Argument x, y View point to translate
Returns A pair of physical coordinates as an
xycoord structure
────────────────────────────────────────────────────────────────────────────
_getviewcoord - Converts specified coordinates to viewport coordinates.
Include GRAPH.H
Prototypes struct xycoord _far _getviewcoord( short
x, short );
struct xycoord _far _getviewcoord_w(
double wx, double wy );
struct xycoord _far _getviewcoord_wxy(
struct _wxycoord _far
*pwxy1 );
Arguments x, y Physical point to translate
wx, wy Window-coordinate point to
translate
pwxy1 Window-coordinate point to
translate
Returns A pair of logical coordinates as an
xycoord structure
────────────────────────────────────────────────────────────────────────────
_getwindowcoord - Converts physical coordinates to window coordinates.
Include GRAPH.H
Prototype struct _wxycoord _far _getwindowcoord(
short x, short y );
Argument x, y Physical point to translate
Returns A pair of window coordinates as a
_wxycoord structure
────────────────────────────────────────────────────────────────────────────
_setcliprgn - Limits graphic output to part of the screen.
Include GRAPH.H
Prototype void _far _setcliprgn( short x1, short
y1, short x2, short y2 );
Arguments x1, y1 Upper left corner of clip
region
x2, y2 Lower right corner of clip
region
Returns Void
────────────────────────────────────────────────────────────────────────────
_setvieworg - Positions the logical origin.
Include GRAPH.H
Prototype struct xycoord _far _setvieworg( short x
, short y );
Argument x, y New origin point
Returns The physical coordinates of the previous
viewport origin in an
xycoord structure
────────────────────────────────────────────────────────────────────────────
_setviewport - Limits graphic output and positions the logical origin within
a limited area.
Include GRAPH.H
Prototype void _far _setviewport( short x1, short
y1, short x2, short y2 );
Arguments x1, y1 Upper left corner of window
x2, y2 Lower right corner of window
Returns Void
────────────────────────────────────────────────────────────────────────────
_setwindow - Defines a window-coordinate system.
Include GRAPH.H
Prototype void _far _setwindow( short finvert,
double wx1, double wy1,
double wx2, double wy2 );
Arguments wx1, wy1 Upper left corner of window
wx2, wy2 Lower right corner of window
finvert Invert flag (TRUE, FALSE)
Returns Void
Set Palette
A screen pixel can be represented as a one-, two-, or four-bit value,
depending on the particular mode. The byte representation is called the
"color value."
Each color that can be displayed is represented by a unique ordinal value
called a "color index." A palette is simply a mapping of the actual display
colors to the legal values.
────────────────────────────────────────────────────────────────────────────
_remapallpalette, _remappalette - The _remapallpalette routine assigns
colors to all color values. The _remappalette routine assigns color indexes
to selected color values.
Include GRAPH.H
Prototypes short _far _remapallpalette( long _far *
colors );
long _far _remappalette( short index,
long color );
Arguments colors Color value array: (_BLACK,
_BLUE, _GREEN,
_CYAN, _RED, _MAGENTA, _BROWN, _WHITE,
_GRAY, _LIGHTBLUE, ,_LIGHTGREEN,
_LIGHTCYAN, _LIGHTRED, _LIGHTMAGENTA,
_LIGHTYELL _BRIGHTWHITE)
OW,
index Color index to reassign
color Color value to assign color
index
Returns _remapallpalette: 0 if successful, -1
if not
_remappalette: the previous color value
of the index argument
if successful, -1 if not
────────────────────────────────────────────────────────────────────────────
_selectpalette - Selects a predefined palette.
Include GRAPH.H
Prototype short _far _selectpalette( short number
);
Argument number Palette number
Returns The value of the previous palette
Set Attributes
Attributes are characteristics (color, fill pattern, or line style) that can
be specified for low-level graphics routines.
A fill mask is an 8-by-8-bit template array, with each bit representing a
pixel. If a bit is 0, the pixel in memory is left untouched: the mask is
transparent to that pixel. If a bit is 1, the pixel is assigned the current
color value. The template is repeated over the entire fill area.
A line style is a 16-bit template buffer, with each bit corresponding to a
pixel. If a bit is 0, the pixel is set to the current background color. If a
bit is 1, the pixel is set to the current color. The template is repeated
for the length of the line.
────────────────────────────────────────────────────────────────────────────
_getbkcolor - Reports the current background color.
Include GRAPH.H
Prototype long _far _getbkcolor( void );
Arguments None
Returns The current background color
────────────────────────────────────────────────────────────────────────────
_getcolor - Obtains the current color.
Include GRAPH.H
Prototype short _far _getcolor( void );
Arguments None
Returns The current color
────────────────────────────────────────────────────────────────────────────
_setbkcolor - Sets the current background color.
Include GRAPH.H
Prototype long _far _setbkcolor( long color );
Argument color Desired color value
Returns The color value of the previous
background color
────────────────────────────────────────────────────────────────────────────
_setcolor - Sets the current color.
Include GRAPH.H
Prototype short _far _setcolor( short color );
Argument color Desired color index
Returns The previous color
Output Images
These routines display graphic elements (arcs, lines, pixels, etc.) on the
screen.
Circular figures such as arcs and ellipses are centered within a "bounding
rectangle," specified by two points that define the diagonally opposed
corners of the rectangle. The center of the rectangle becomes the center of
the figure, and the rectangle's borders determine the size of the figure.
Most of these routines have two or three forms. The functions are listed by
the "base" name, without a suffix. Note, though, that function names that
end with a _w, such as _arc_w, use the window coordinate system. Those that
end with a _wxy, such as _ellipse_wxy, use the window coordinate system and
a _wxycoord structure to define the coordinates.
────────────────────────────────────────────────────────────────────────────
_arc - Draws an arc.
Include GRAPH.H
Prototypes short _far _arc( short x1, short y1,
short x2, short y2,
short x3, short y3, short x4, short y4
);
short _far _arc_wxy( struct _wxycoord
pwxy1,
struct _wxycoord*pwxy2, struct
_wxycoord*pwxy3,
struct _wxycoord*pwxy4 );
Arguments x1, y1 Upper left corner of
bounding rectangle
x2, y2 Lower right corner of
bounding rectangle
x3, y3 Start vector
x4, y4 End vector
pwxy1 Upper left corner of
bounding rectangle
pwxy2 Lower right corner of
bounding rectangle
pwxy3 Start vector
pwxy4 End vector
Returns A nonzero value if the arc is drawn
successfully, 0 if not
────────────────────────────────────────────────────────────────────────────
_clearscreen - Erases the screen and fills it with the current background
color.
Include GRAPH.H
Prototype void _far _clearscreen( short area );
Argument area Target area (_GCLEARSCREEN,
_GVIEWPORT,
_GWINDOW)
Returns Void
────────────────────────────────────────────────────────────────────────────
_ellipse - Draws an ellipse.
Include GRAPH.H
Prototypes short _far _ellipse( short control,
short x1, short y1,
short x2, short y2 );
short _far _ellipse_w( short control,
double wx1, double wy1,
double wx2, double wy2 );
short _far _ellipse_wxy( short control,
struct _wxycoord* pwxy1,
struct _wxycoord*pwxy2 );
Arguments control Fill flag ( _GFILLINTERIOR,
_GBORDER)
x1, y1 Upper left corner of
bounding rectangle (view
coordinates)
x2, y2 Lower right corner of
bounding rectangle (view
coordinates)
wx1, wy1 Upper left corner of
bounding rectangle (window
coordinates)
wx2, wy2 Lower right corner of
bounding rectangle (window
coordinates)
pwxy1 Upper left corner of
bounding rectangle (window
coordinates)
pwxy2 Lower right corner of
bounding rectangle (window
coordinates)
Returns A nonzero value if the ellipse is drawn
successfully, 0 if not
────────────────────────────────────────────────────────────────────────────
_floodfill - Fills an area of the screen with the current color.
Include GRAPH.H
Prototypes short _far _floodfill( short x, short y
, short boundary );
short _far _floodfill_w( double wx,
double wy, short boundary );
Arguments x, y Start point (view
coordinates)
wx, wy Start point (window
coordinates)
boundary Boundary color
Returns A nonzero value if successful, 0 if not
────────────────────────────────────────────────────────────────────────────
_getpixel - Obtains a pixel's color index. The coordinates can be specified
in either view coordinates (_getpixel) or in window coordinates
(_getpixel_w).
Include GRAPH.H
Prototypes short _far _getpixel( short x, short y
);
short _far _getpixel_w( double wx,
double wy );
Arguments x, y Pixel position (view
coordinates)
wx, wy Pixel position (window
coordinates)
Returns The color index if successful, -1 if not
────────────────────────────────────────────────────────────────────────────
_lineto - Draws a line from the current graphic output position to a
specified point. The coordinate of the end point can be specified in either
view coordinates (_lineto) or in window coordinates (_lineto_w).
Include GRAPH.H
Prototypes short _far _lineto( short x, short y );
short _far _lineto_w( double wx, double
wy );
Arguments x, y End point (view coordinates)
wx, wy End point (window
coordinates)
Returns A nonzero value if the line is drawn
successfully, 0 if not
────────────────────────────────────────────────────────────────────────────
_moveto - Moves the current graphic-output position to a specified point.
The coordinates can be specified in either view coordinates (_moveto) or in
window coordinates (_moveto_w).
Include GRAPH.H
Prototypes struct xycoord _far _moveto( short x,
short );
struct _wxycoord _far _moveto_w( double
wx, double wy );
Arguments x, y Target position (view
coordinates)
wx, wy Target position (window
coordinates)
Returns The logical coordinates of the previous
position as an xycoord
structure (_moveto) or as a _wxycoord
structure (_moveto_w)
────────────────────────────────────────────────────────────────────────────
_pie - Draws a figure shaped like a pie wedge.
Include GRAPH.H
Prototypes short _far _pie( short control, short
x1, short y1,
short x2, short y2, short x3, short y3,
short x4, short y4 );
short _far _pie_wxy( short control,
struct _wxycoord* pwxy1,
struct _wxycoord*pwxy2, struct
_wxycoord* pwxy3,
struct _wxycoord*pwxy4 );
Arguments control Fill flag (_GFILLINTERIOR,
_GBORDER)
x1, y1 Upper left corner of
bounding rectangle (view
coordinates)
x2, y2 Lower right corner of
bounding rectangle (view
coordinates)
x3, y3 Start vector(view
coordinates)
x4, y4 End vector (view coordinates)
pwxy1 Upper left corner of
bounding rectangle (window
coordinates)
pwxy2 Lower right corner of
bounding rectangle (window
coordinates)
pwxy3 Start vector (window
coordinates)
pwxy4 End vector (window
coordinates)
Returns A nonzero value if the pie is drawn
successfully, 0 if not
────────────────────────────────────────────────────────────────────────────
_rectangle - Draws a rectangle.
Include GRAPH.H
Prototypes short _far _rectangle( short control,
short x1, short y1,
short x2, short y2 );
short _far _rectangle_w( short control,
double wx1, double wy1,
double wx2, double wy2 );
short _far _rectangle_wxy( short control
, struct _wxycoord*pwxy1,
struct _wxycoord*pwxy2 );
Arguments control Fill flag (_GFILLINTERIOR,
_GBORDER)
x1, y1 Upper left corner (view
coordinates)
x2, y2 Lower right corner (view
coordinates)
wx1, wy1 Upper left corner (window
coordinates)
wx2, wy2 Lower right corner (window
coordinates)
pwxy1 Upper left corner (window
coordinates)
pwxy2 Lower right corner (window
coordinates)
Returns A nonzero value if the rectangle is
drawn successfully, 0 if not
────────────────────────────────────────────────────────────────────────────
_setpixel - Sets a pixel's color index.
Include GRAPH.H
Prototypes short _far _setpixel( short x, short y
);
short _far _setpixel_w( double wx,
double wy );
Arguments x, y Target pixel (view
coordinates)
wx, wy Target pixel (window
coordinates)
Returns The pixel's previous value if successful,
-1 if not
Output Text
These routines provide text output in both graphics and text modes.
These functions recognize text window boundaries and should be used in
applications using text windows.
No formatting capability is provided. If you want to output integer or
floating-point values, you must convert the values into a string variable
before calling these routines. All screen positions are specified as
character-row and character-column coordinates.
────────────────────────────────────────────────────────────────────────────
_displaycursor - Sets the cursor "on" or "off" on exit from a graphics
routine.
Include GRAPH.H
Prototype short _far _displaycursor( short toggle
);
Argument toggle Cursor state (_GCURSORON,
_GCURSOROFF)
Returns The previous value of toggle
────────────────────────────────────────────────────────────────────────────
_gettextcolor - Obtains the current text color.
Include GRAPH.H
Prototype short _far _gettextcolor( void );
Arguments None
Returns The color index of the current text
color
────────────────────────────────────────────────────────────────────────────
_gettextcursor - Obtains the current cursor attribute.
Include GRAPH.H
Prototype short _far _gettextcursor( void );
Arguments None
Returns The current cursor attribute
────────────────────────────────────────────────────────────────────────────
_gettextposition - Obtains the current text-output position.
Include GRAPH.H
Prototype struct rccoord _far _gettextposition(
void );
Arguments None
Returns The current text position as an rccoord
structure
────────────────────────────────────────────────────────────────────────────
_outtext - Outputs text to the screen at the current position.
Include GRAPH.H
Prototype void _far _outtext( unsigned char _far *
text );
Argument text Text to be output
Returns Void
────────────────────────────────────────────────────────────────────────────
_settextposition - Relocates the current text position.
Include GRAPH.H
Prototype struct rccoord _far _settextposition(
short row, short col );
Arguments row Row coordinate of new output
position
col Column coordinate of new
output position
Returns The previous text position in an rccoord
structure
────────────────────────────────────────────────────────────────────────────
_settextcolor - Sets the current text color.
Include GRAPH.H
Prototype short _far _settextcolor( short index
);
Argument index Desired color index
Returns The value of the previous color
────────────────────────────────────────────────────────────────────────────
_settextwindow - Sets the current text-display window.
Include GRAPH.H
Prototype void _far _settextwindow( short r1,
short c1, short r2, short c2 );
Arguments r1, c1 Upper left corner of window
r2, c2 Lower right corner of window
Returns Void
Transfer Images
These functions transfer screen images between memory and the display, using
a buffer allocated by the application. You can use these functions to
animate graphics elements on the screen.
Most of these routines have two or three forms. The functions are listed by
the "base" name, without a suffix. Note, though, that function names that
end in a _w, such as _getimage_w, use the window-coordinate system. Those
that end with a _wxy, such as _imagesize_wxy, use the window-coordinate
system and a _wxycoord structure to define the coordinates.
The _imagesize function is used to find the size in bytes of the buffer
needed to store a given image.
────────────────────────────────────────────────────────────────────────────
_getimage - Stores a screen image in memory.
Include GRAPH.H
Prototypes void _far _getimage( short x1, short y1
,
short x2, short y2, char _huge *image
);
void _far _getimage_w( double wx1,
double wy1,
double wx2, double wy2, char _huge *
image );
void _far _getimage_wxy( struct
_wxycoord*pwxy1,
struct _wxycoord*pwxy2, char _huge *
image );
Arguments x1, y1 Upper left corner of
bounding rectangle (view
coordinates)
x2, y2 Lower right corner of
bounding rectangle (view
coordinates)
wx1, wy1 Upper left corner of
bounding rectangle (window
coordinates)
wx2, wy2 Lower right corner of
bounding rectangle (window
coordinates)
pwxy1 Upper left corner of
bounding rectangle (window
coordinates)
pwxy2 Lower right corner of
bounding rectangle (window
coordinates)
image Storage buffer for screen
image
Returns Void
────────────────────────────────────────────────────────────────────────────
_imagesize - Returns image size in bytes.
Include GRAPH.H
Prototypes long _far _imagesize( short x1, short
y1, short x2, short y2 );
long _far _imagesize_w( double wx1,
double wy1, double wx2,
double wy2 );
long _far _imagesize_wxy( struct
_wxycoord* pwxy1,
struct _wxycoord* pwxy2 );
Arguments x1, y1 Upper left corner of
bounding rectangle (view
coordinates)
x2, y2 Lower right corner of
bounding rectangle (view
coordinates)
wx1, wy1 Upper left corner of
bounding rectangle (window
coordinates)
wx2, wy2 Lower right corner of
bounding rectangle (window
coordinates)
pwxy1 Upper left corner of
bounding rectangle (window
coordinates)
pwxy2 Lower right corner of
bounding rectangle (window
coordinates)
Returns The storage size of the image in bytes
────────────────────────────────────────────────────────────────────────────
_putimage - Retrieves an image from memory and displays it.
Include GRAPH.H
Prototypes void _far _putimage( short x, short y,
char _huge *image, short action );
void _far _putimage_w( double wx, double
wy,
char _huge *image, short action );
Arguments x, y Position of upper left
corner of image (view
coordinates)
wx, wy Position of upper left
corner of image (window
coordinates)
image Stored image buffer
action Interaction with existing
screen image (_GAND, _GOR,
_GXOR, _GPSET, _GPRESET)
Returns Void
Graphics 2: Presentation Graphics Routines
The presentation graphics routines provide complete charting capabilities
for line, bar, column, scatter, and pie charts.
Some charts plot both "categories," or non-numeric data such as time
periods, and "values," or specific numeric data, such as sales. Presentation
graphics routines support the following kinds of charts:
Chart Name Description
────────────────────────────────────────────────────────────────────────────
Line Category/value chart, with styles for
lines between points and for no lines
Bar Category/value chart, horizontal bars,
styles for stacked and unstacked
multiple series charts
Column Category/value chart, vertical bars,
with styles for stacked and unstacked
multiple series charts
Scatter Value/value chart, with styles for lines
connecting points or for no lines
Pie Pie chart, with optional percentages and
exploded sections
The graphics package supports the IBM (and compatible) Enhanced Graphics
Adapter (EGA), Color Graphics Adapter (CGA), certain operating modes of the
Video Graphics Array (VGA) hardware configurations, and the Multicolor
Graphics Array (MCGA). The graphics package also supports the Hercules
Graphics Card, Graphics Card Plus, InColor Card, and 100-percent compatible
cards, as well as the special Olivetti modes available on AT&T computers.
The _pg_initchart and _pg_defaultchart functions are generally used at the
very beginning of a presentation graphics program.
The _pg_chart functions produce column charts, line charts, and bar charts.
The _pg_chartscatter functions produce a scatter plot of data. The
_pg_chartpie function generates a pie chart.
────────────────────────────────────────────────────────────────────────────
_pg_chart - Generates a chart of the type specified in the env environment
variable. It produces a column, bar, or line chart for a single series of
data.
Include PGCHART.H
Prototype short _far _pg_chart( chartenv _far*env,
char _far*_far *categories, float _far*
values, short n );
Arguments env Chart environment variable
categories Array of category variables
values Array of data values
n Number of data values to
chart
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_chartms - Generates a multiple series of charts of the type specified in
the env environment variable. It produces a column, bar, or line chart for a
multiple series of data. All series must be the same length.
Include PGCHART.H
Prototype short _far _pg_chartms( chartenv _far *
env,
char _far * _far*categories, float _far*
values, short n, short nseries,
short arraydim, char _far* _far*
serieslabels );
Arguments env Chart environment variable
categories Array of category variables
values Two-dimensional array of
data values (series, data)
n Number of data values to
chart in a series
nseries Number of series to chart
arraydim Second (row) dimension of
data array
serieslabe Array of labels for series
ls
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_chartpie - Generates a pie chart for a single series of data.
Include PGCHART.H
Prototype short _far _pg_chartpie( chartenv _far*
env,
char _far* _far*categories, float _far*
values,
short _far*explode, short n );
Arguments env Chart environment variable
categories Array of category names
values Array of data values
explode Array of explode flags;
1=explode, 0=do not explode
n Number of data values to
chart
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_chartscatter - Generates a scatter chart for a single series of data.
Include PGCHART.H
Prototype short _far _pg_chartscatter( chartenv
_far *env, float _far *xvalues,
float _far *yvalues, short n );
Arguments env Chart environment variable
xvalues Array of x-axis data values
yvalues Array of y-axis data values
n Number of data values to
chart
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_chartscatterms - Generates a scatter chart for a multiple series of
data.
Include PGCHART.H
Prototype short _far _pg_chartscatterms( chartenv
_far *env,
float _far*xvalues, float _far*yvalues,
short nseries, short n,
short rowdim, char _far* _far *
serieslabels );
Arguments env Chart environment variable
xvalues Two-dimensional array of x
-axis values
yvalues Two-dimensional array of y
-axis values
n Number of data values to
chart in a series
nseries Number of series to chart
rowdim Second (row) dimension of
data array
serieslabe Array of labels for series
ls
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_defaultchart - Initializes all necessary variables in the chart
environment for the specified default chart and chart style.
Include PGCHART.H
Prototype short _far _pg_defaultchart( chartenv
_far *env, short charttype,
short chartstyle );
Arguments env Chart environment variable
charttype Chart type (_PG_BARCHART,
_PG_COLUMNCHART, _PG_LINECHART,
_PG_SCATTERCHART, _PG_PIECHART)
chartstyle Chart style
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
_pg_initchart - Initializes chart line-style set, default palettes, screen
modes, and character fonts. You must call this routine before any other
charting routine.
Include PGCHART.H
Prototype short _far _pg_initchart( void );
Arguments None
Returns 0 if successful, nonzero if not
Graphics 3: Font Display Routines
The font graphics routines display font-based characters on the screen.
The _registerfonts function initializes the fonts package with a set of
disk-based type fonts. This must be done at the very beginning of any fonts
program. The _unregisterfonts function frees fonts from memory when they are
no longer needed.
The _setfont function makes a specified font the current active font for
output. The _outgtext function displays text on the screen using the current
font.
────────────────────────────────────────────────────────────────────────────
_getfontinfo - Obtains the current font characteristics.
Include GRAPH.H
Prototype short _far _getfontinfo( struct
_fontinfo _far *fontbuffer );
Argument fontbuffer Font information
Returns Font information as a _fontinfo
structure
────────────────────────────────────────────────────────────────────────────
_getgtextextent - Determines the width of the specified text in the current
font.
Include GRAPH.H
Prototype short _far _getgtextextent( unsigned
char _far * text );
Argument text Text to be analyzed
Returns The width of the text in pixels
────────────────────────────────────────────────────────────────────────────
_outgtext - Outputs text in the current font to the screen at the current
position.
Include GRAPH.H
Prototype void _far _outgtext( unsigned char _far
*text );
Argument text Text to be output
Returns Void
────────────────────────────────────────────────────────────────────────────
_registerfonts - Initializes the font library.
Include GRAPH.H
Prototype short _far _registerfonts( unsigned char
_far *filename );
Argument file name File name of .FON files to
register
Returns Void
────────────────────────────────────────────────────────────────────────────
_setfont - Finds a single font that matches a specified set of
characteristics and makes this font the current font.
Include GRAPH.H
Prototype short _far _setfont( unsigned char _far
*options );
Argument options Font options string
Returns Void
────────────────────────────────────────────────────────────────────────────
_unregisterfonts - Frees memory associated with fonts.
Include GRAPH.H
Prototype void _far _unregisterfonts( void );
Arguments None
Returns Void
Input and Output Routines
The input and output (I/O) routines of the standard C library allow you to
read and write data to and from files and devices. In C, there are no
predefined file structures; all data is treated as sequences of bytes.
Three types of I/O functions are available:
■ Stream I/O, in which the data file is a stream of individual
characters
■ Low-level I/O, which uses the system's I/O capabilities directly
■ Console and port I/O, which are stream routines for console or port
Stream I/O uses the FILE structure. The stream routines provide for
buffered, formatted, or unformatted input and output.
Low-level I/O uses a file "handle" to access files. This handle is an
integer value that is used to refer to the file in subsequent operations.
Do not mix stream and low-level routines on the same file or device.
Stream Routines
In the stream routines listed below, the following manifest constants are
used:
■ EOF is defined to be the value returned at end-of-file
■ NULL is the null pointer
■ FILE is the structure that maintains information about a stream
■ BUFSIZ defines the default size of stream buffers, in bytes
────────────────────────────────────────────────────────────────────────────
clearerr - Clears the error indicator for a stream.
Include STDIO.H
Prototype void clearerr( FILE *stream );
Argument stream Pointer to FILE structure
Returns Void
────────────────────────────────────────────────────────────────────────────
fclose - Closes a stream.
Include STDIO.H
Prototype int fclose( FILE *stream );
Argument stream Target FILE structure
Returns 0 if successful, EOF if not
────────────────────────────────────────────────────────────────────────────
feof - Tests for end-of-file on a stream.
Include STDIO.H
Prototype int feof( FILE *stream );
Argument stream Pointer to FILE structure
Returns A nonzero value when the current
position is the end-of-file,
0 if not
────────────────────────────────────────────────────────────────────────────
ferror - Tests for error on a stream.
Include STDIO.H
Prototype int ferror( FILE *stream );
Argument stream Pointer to FILE structure
Returns A nonzero value to indicate an error in
stream, 0 to indicate
no error
────────────────────────────────────────────────────────────────────────────
fflush - Flushes a stream.
Include STDIO.H
Prototype int fflush( FILE *stream );
Argument stream Pointer to FILE structure
Returns 0 if successful, if stream has no buffer,
or if stream is open
only for reading; returns EOF otherwise
────────────────────────────────────────────────────────────────────────────
fgetc - Reads a character from a stream (function version).
Include STDIO.H
Prototype int fgetc( FILE *stream );
Argument stream Pointer to FILE structure
Returns The character read; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
fgetpos - Gets the position indicator of a stream.
Include STDIO.H
Prototype int fgetpos( FILE *stream, fpos_t *pos
);
Arguments stream Target stream
pos Position indicator storage
Returns 0 if successful, a nonzero value if not
errno: EINVAL, EBADF
────────────────────────────────────────────────────────────────────────────
fgets - Reads a string from a stream.
Include STDIO.H
Prototype char *fgets( char *string, int n, FILE
*stream );
Arguments string Storage location for data
n Number of characters stored
stream Pointer to FILE structure
Returns A pointer to string if successful, NULL
if unsuccessful or at
end-of-file
────────────────────────────────────────────────────────────────────────────
fopen - Opens a stream.
Include STDIO.H
Prototype FILE *fopen( const char *filename,
const char *mode );
Arguments filename Path name of file
mode Type of access permitted
such as r, w, a, r+, w+, a+
, t, b
(appended to type to indicate mode)
Returns A pointer to the open file if successful,
NULL if not
────────────────────────────────────────────────────────────────────────────
fprintf - Writes formatted data to a stream.
Include STDIO.H
Prototype int fprintf( FILE *stream, const char *
format [[, argument]]... );
Arguments stream Pointer to FILE structure
format Format-control string
Returns The number of characters printed
────────────────────────────────────────────────────────────────────────────
fputc - Writes a character to a stream (function version).
Include STDIO.H
Prototype int fputc( int c, FILE *stream );
Arguments c Character to be written
stream Pointer to FILE structure
Returns The character written; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
fputs - Writes a string to a stream.
Include STDIO.H
Prototype int fputs( const char *string, FILE *
stream );
Arguments string String to be output
stream Pointer to FILE structure
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
fread - Reads unformatted data from a stream.
Include STDIO.H
Prototype size_t fread( void *buffer, size_t size
, size_t count, FILE *stream );
Arguments buffer Storage location for data
size Item size in bytes
count Maximum number of items to
be read
stream Pointer to FILE structure
Returns The number of items actually read
────────────────────────────────────────────────────────────────────────────
freopen - Reassigns a FILE pointer.
Include STDIO.H
Prototype FILE *freopen( const char *filename,
const char *mode,
FILE *stream );
Arguments filename Path name of new file
mode Type of access permitted
such as r, w, a, r+, w+,
a+, t, b (appended to type to indicate
mode)
stream Pointer to FILE structure
Returns A pointer to the newly opened file if
successful, a NULL
pointer if not
────────────────────────────────────────────────────────────────────────────
fscanf - Reads formatted data from a stream.
Include STDIO.H
Prototype int fscanf( FILE *stream, const char*
format [[, argument]] ... );
Arguments stream Pointer to FILE structure
format Format-control string
Returns The number of fields successfully
converted and assigned;
EOF indicates an attempt to read the
end-of-file
────────────────────────────────────────────────────────────────────────────
fseek - Repositions FILE pointer to given location.
Include STDIO.H
Prototype int fseek( FILE *stream, long offset,
int origin );
Arguments stream Pointer to FILE structure
offset Number of bytes from origin
origin Initial position (SEEK_SET,
SEEK_CUR, SEEK_END)
Returns 0 if successful, a nonzero value if not
────────────────────────────────────────────────────────────────────────────
fsetpos - Sets the position indicator of a stream.
Include STDIO.H
Prototype int fsetpos( FILE *stream, const fpos_t
*pos );
Arguments stream Target stream
pos Position-indicator storage
Returns 0 if successful, a nonzero value if not
errno: EINVAL, EBADF
────────────────────────────────────────────────────────────────────────────
ftell - Gets current FILE pointer position.
Include STDIO.H
Prototype long ftell( FILE *stream );
Argument stream Target FILE structure
Returns The current position if successful, -1L
if not
errno: EINVAL, EBADF
────────────────────────────────────────────────────────────────────────────
fwrite - Writes unformatted data items to a stream.
Include STDIO.H
Prototype size_t fwrite( const void *buffer,
size_t size, size_t count,
FILE *stream );
Arguments buffer Pointer to data to be
written
size Item size in bytes
count Maximum number of items to
be written
stream Pointer to FILE structure
Returns The number of full items actually
written
────────────────────────────────────────────────────────────────────────────
getc - Reads a character from a stream (macro version).
Include STDIO.H
Prototype int getc( FILE *stream );
Argument stream Pointer to FILE structure
Returns The character read; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
getchar - Reads a character from stdin (macro version).
Include STDIO.H
Prototype int getchar( void );
Arguments None
Returns The character read; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
gets - Reads a line from stdin.
Include STDIO.H
Prototype char *gets( char *buffer );
Argument buffer Storage location for input
string
Returns A pointer to its argument if successful,
a NULL pointer if at
end-of-file or unsuccessful
────────────────────────────────────────────────────────────────────────────
printf - Writes formatted data to stdout.
Include STDIO.H
Prototype int printf( const char *format [[,
argument]]... );
Argument format Format-control string
Returns The number of characters printed
────────────────────────────────────────────────────────────────────────────
putc - Writes a character to a stream (macro version).
Include STDIO.H
Prototype int putc( int c, FILE *stream );
Arguments c Character to be written
stream Pointer to FILE structure
Returns The character written; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
putchar - Writes a character to stdout (macro version).
Include STDIO.H
Prototype int putchar( int c );
Argument c Character to be written
Returns The character written; EOF may indicate
error
────────────────────────────────────────────────────────────────────────────
puts - Writes a line to a stream.
Include STDIO.H
Prototype int puts( const char *string );
Argument string String to be output
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
rewind - Repositions FILE pointer to beginning of a stream.
Include STDIO.H
Prototype void rewind( FILE *stream );
Argument stream Pointer to FILE structure
Returns Void
────────────────────────────────────────────────────────────────────────────
scanf - Reads formatted data from stdin.
Include STDIO.H
Prototype int scanf( const char *format [[,
argument]]... );
Argument format Format control string
Returns The number of fields converted and
assigned if successful, 0 if
no fields were assigned, EOF for an
attempt to read end-of-file
────────────────────────────────────────────────────────────────────────────
sprintf - Writes formatted data to string.
Include STDIO.H
Prototype int sprintf( char *buffer, const char *
format [[, argument]] ... );
Arguments buffer Storage location for output
format Format-control string
Returns The number of characters stored in
buffer
────────────────────────────────────────────────────────────────────────────
sscanf - Reads formatted data from string.
Include STDIO.H
Prototype int sscanf( const char *buffer, const
char *format [[, argument]] ... );
Arguments buffer Stored data
format Format-control string
Returns The number of fields converted and
assigned if successful, 0 if
no fields were assigned, EOF for an
attempt to read at end-of-string
────────────────────────────────────────────────────────────────────────────
tmpfile - Creates a temporary file.
Include STDIO.H
Prototype FILE *tmpfile( void );
Arguments None
Returns A stream pointer if successful, NULL if
not
────────────────────────────────────────────────────────────────────────────
tmpnam - Generates a temporary file name.
Include STDIO.H
Prototype char *tmpnam( char *string );
Argument string Pointer to temporary name
Returns A pointer to the new name if successful,
NULL if not
────────────────────────────────────────────────────────────────────────────
ungetc - Places a character in the input stream buffer.
Include STDIO.H
Prototype int ungetc( int c, FILE *stream );
Arguments c Character to be pushed
stream Pointer to FILE structure
Returns The character argument c if successful,
EOF if not
Low-Level Routines
The low-level input and output calls do not buffer or format data.
Files opened by low-level calls are referenced by a "file handle," an
integer value used by the operating system to refer to the file.
────────────────────────────────────────────────────────────────────────────
close - Closes a file.
Include IO.H
Prototype int close( int handle );
Argument handle Handle referring to open
file
Returns 0 if successful, -1 if not
errno: EBADF
────────────────────────────────────────────────────────────────────────────
creat - Creates a file.
Include IO.H, SYS\TYPES.H, SYS\STAT.H
Prototype int creat( char *filename, int pmode );
Arguments filename Path name of new file
pmode Permission setting (
S_IWRITE , S_IREAD,
S_IREAD | S_IWRITE)
Returns A handle if successful, -1 if not
errno: EACCES, EMFILE, ENOENT
────────────────────────────────────────────────────────────────────────────
eof - Tests for end-of-file.
Include IO.H
Prototype int eof( int handle );
Argument handle Handle referring to open
file
Returns 1 if the current position is the
end-of-file and 0 if it is not,
-1 to indicate an error
errno: EBADF
────────────────────────────────────────────────────────────────────────────
lseek - Repositions file pointer to a given location.
Include IO.H, STDIO.H
Prototype long lseek( int handle, long offset,
int origin );
Arguments handle Handle referring to open
file
offset Number of bytes from origin
origin Initial position (SEEK_SET,
SEEK_CUR, SEEK_END)
Returns The new position offset (in bytes) from
the beginning of
the file if successful, -1L if not
errno: EBADF, EINVAL
────────────────────────────────────────────────────────────────────────────
open - Opens a file.
Include FCNTL.H, IO.H, SYS\TYPES.H, SYS\STAT.H
Prototype int open( char *path, int oflag [[, int
pmode]] );
Arguments path File path name
oflag Type of operations allowed
such as O_APPEND,
O_BINARY, O_CREAT, O_EXCL, O_RDONLY,
O_RDWR, O_TEXT, O_TRUNC, O_WRONLY
(may be joined by | )
pmode Permission setting (S_IWRITE
, S_IREAD,
S_IREAD | S_IWRITE)
Returns A handle if successful, -1 if not
errno: EACCES, EEXIST, EMFILE, ENOENT
────────────────────────────────────────────────────────────────────────────
read - Reads data from a file.
Include IO.H
Prototype int read( int handle, char *buffer,
unsigned int count );
Arguments handle Handle referring to open
file
buffer Storage location of data
count Maximum number of bytes
Returns The number of bytes actually read or 0
at end-of-file if
successful; -1 if not
errno: EBADF
────────────────────────────────────────────────────────────────────────────
tell - Gets current file-pointer position.
Include IO.H
Prototype long tell( int handle );
Argument handle Handle referring to open
file
Returns The current position if successful, -1L
if not
errno: EBADF
────────────────────────────────────────────────────────────────────────────
write - Writes data to a file.
Include IO.H
Prototype int write( int handle, void *buffer,
unsigned int count );
Arguments handle Handle referring to open
file
buffer Data to be written
count Number of bytes
Returns The number of bytes actually written if
successful, -1 if not
errno: EBADF, ENOSPC
Console and Port I/O Routines
The console and port I/O routines perform reading and writing operations on
your console or on the specified port.
The cgets, cscanf, getch, getche, and kbhit routines take input from the
console.
The cprintf, cputs, putch, and ungetch routines write to the console.
The console or port does not have to be opened or closed before I/O is
performed.
The console I/O routines use the corresponding MS-DOS system calls to read
and write characters. Since these routines are not compatible with stream or
low-level library routines, console routines should not be used with them.
────────────────────────────────────────────────────────────────────────────
cgets - Reads a string from the console.
Include CONIO.H
Prototype char *cgets( char *buffer );
Argument buffer Storage location for data
Returns A pointer to the start of the string,
which is at str[2]
────────────────────────────────────────────────────────────────────────────
cprintf - Writes formatted data to the console.
Include CONIO.H
Prototype int cprintf( char *format [[, argument]]
... );
Argument format Format-control string
Returns The number of characters printed
────────────────────────────────────────────────────────────────────────────
cputs - Writes a string to the console.
Include CONIO.H
Prototype int cputs( char *string );
Argument string Output string
Returns 0 if successful, nonzero if not
────────────────────────────────────────────────────────────────────────────
cscanf - Reads formatted data from the console.
Include CONIO.H
Prototype int cscanf( char *format [[, argument
]]... );
Argument format Format-control string
Returns The number of fields converted and
assigned if successful
(0 means no fields were assigned), EOF
for an attempt to read
end-of-file
────────────────────────────────────────────────────────────────────────────
getch - Reads a character from the console.
Include CONIO.H
Prototype int getch( void );
Arguments None
Returns The character read
────────────────────────────────────────────────────────────────────────────
getche - Reads a character from the console and echoes it.
Include CONIO.H
Prototype int getche( void );
Arguments None
Returns The character read
────────────────────────────────────────────────────────────────────────────
kbhit - Checks for a keystroke at the console.
Include CONIO.H
Prototype int kbhit( void );
Arguments None
Returns A nonzero value if a key has been
pressed, 0 if not
────────────────────────────────────────────────────────────────────────────
putch - Writes a character to the console.
Include CONIO.H
Prototype int putch( int c );
Argument c Character to be output
Returns The argument c if successful, EOF if
not
────────────────────────────────────────────────────────────────────────────
ungetch - "Ungets" the last character read from the console so that it
becomes the next character read.
Include CONIO.H
Prototype int ungetch( int c );
Argument c Character to be pushed
Returns The argument c if successful, EOF if
not
Math Routines
The math routines allow you to perform common mathematical calculations.
All math routines work with floating-point values and therefore require
floating-point support.
────────────────────────────────────────────────────────────────────────────
abs, fabs, labs - The abs, fabs, and labs routines return the absolute value
of an integer, a double, and a long argument, respectively.
Includes STDLIB.H (abs, labs), MATH.H (fabs)
Prototypes int abs( int n );
double fabs( double x );
long labs( long x );
Arguments n Integer (abs) or long (labs)
value
x Floating-point value
Returns Absolute value of its argument
────────────────────────────────────────────────────────────────────────────
acos - Calculates the arccosine.
Includes FLOAT.H, MATH.H
Prototype double acos( double x );
Argument x Value whose arccosine is to
be calculated
Returns The arccosine result if successful, or 0
if x > 1
errno: EDOM
────────────────────────────────────────────────────────────────────────────
asin - Calculates the arcsine.
Includes FLOAT.H, MATH.H
Prototype double asin( double x );
Argument x Value whose arcsine is to be
calculated
Returns The arcsine result if successful, or 0
if x >1
errno: EDOM
────────────────────────────────────────────────────────────────────────────
atan, atan2 - Calculates the arctangent of x (atan) or the arctangent of y/x
(atan2).
Includes FLOAT.H, MATH.H
Prototypes double atan( double x );
double atan2( double y, double x );
Argument x, y Floating-point values
Returns atan: the arctangent result
atan2: the arctangent of y/x, or 0 if
both arguments are 0
errno: EDOM
────────────────────────────────────────────────────────────────────────────
ceil - Rounds the argument up to an integer.
Includes FLOAT.H, MATH.H
Prototype double ceil( double x );
Argument x Floating-point value
Returns The double result
────────────────────────────────────────────────────────────────────────────
cos, cosh - Calculates the cosine (cos) or the hyperbolic cosine (cosh).
Includes FLOAT.H, MATH.H
Prototypes double cos( double x );
double cosh( double x );
Argument x Angle (in radians)
Returns cos: the cosine result if successful, 0
if not
cosh: the hyperbolic result if
successful, or HUGE_VAL if the
result is too large
errno: ERANGE
────────────────────────────────────────────────────────────────────────────
exp - Calculates the exponential function.
Includes FLOAT.H, MATH.H
Prototype double exp( double x );
Argument x Floating-point value
Returns The exponential value if successful,
HUGE_VAL on overflow,
0 on underflow
errno: ERANGE
────────────────────────────────────────────────────────────────────────────
floor - Rounds the argument down to an integer.
Includes FLOAT.H, MATH.H
Prototype double floor( double x );
Argument x Floating-point value
Returns The floating-point result
────────────────────────────────────────────────────────────────────────────
fmod - Finds the floating-point remainder.
Includes FLOAT.H, MATH.H
Prototype double fmod( double x, double y );
Argument x, y Floating-point values
Returns The floating-point remainder, or 0 if y
is 0
────────────────────────────────────────────────────────────────────────────
frexp - Calculates an exponential value.
Includes FLOAT.H, MATH.H
Prototype double frexp( double x, int *expptr );
Argument x Floating-point value
expptr Pointer to stored integer
exponent
Returns The mantissa, or 0 if x is 0
────────────────────────────────────────────────────────────────────────────
ldexp - Calculates the argument times 2exp.
Includes FLOAT.H, MATH.H
Prototype double ldexp( double x, int exp );
Arguments x Floating-point value
exp Integer exponent
Returns An exponential value if successful,
HUGE_VAL on overflow
errno: ERANGE
────────────────────────────────────────────────────────────────────────────
log, log10 - Calculates the natural logarithm (log) or the base-10 log
(log10).
Includes FLOAT.H, MATH.H
Prototypes double log( double x );
double log10( double x );
Argument x Floating-point value
Returns A logarithm result if successful,
-HUGE_VAL if not
errno: EDOM (if x < 0), ERANGE (if x =
0)
────────────────────────────────────────────────────────────────────────────
modf - Breaks argument into integer and fractional parts.
Includes FLOAT.H, MATH.H
Prototype double modf( double x, double *intptr
);
Arguments x Floating-point value
intptr Pointer to stored integer
position
Returns The signed fractional portion of x
────────────────────────────────────────────────────────────────────────────
pow - Calculates a value raised to a power.
Includes FLOAT.H, MATH.H
Prototype double pow( double x, double y );
Arguments x Number to be raised
y Power of x
Returns The argument x raised to the y power if
successful,
HUGE_VAL if not
────────────────────────────────────────────────────────────────────────────
rand, srand - The rand function returns a pseudorandom integer in the range
0-32,767. The srand function initializes the random number generator.
Include STDLIB.H
Prototypes int rand( void );
void srand( unsigned seed );
Argument seed Seed for random-number
generation (srand)
Returns rand: a pseudorandom number
srand: void
────────────────────────────────────────────────────────────────────────────
sin, sinh - Calculates the sine (sin) or hyperbolic sine (sinh).
Includes FLOAT.H, MATH.H
Prototypes double sin( double x );
double sinh( double x );
Argument x Angle (in radians)
Returns sin: the sine of x if successful, 0 if
not
sinh: the hyperbolic sine of x if
successful, HUGE_VAL if not
errno: ERANGE
────────────────────────────────────────────────────────────────────────────
sqrt - Finds the square root.
Includes FLOAT.H, MATH.H
Prototype double sqrt( double x );
Argument x Nonnegative floating-point
value
Returns A square root if successful, 0 if not
errno: EDOM
────────────────────────────────────────────────────────────────────────────
tan, tanh - Calculates the tangent (tan) or hyperbolic tangent (tanh).
Includes FLOAT.H, MATH.H
Prototypes double tan( double x );
double tanh( double x );
Argument x Angle (in radians)
Returns tan: the tangent of x if successful, 0
if not
tanh: the hyperbolic tangent of x
errno: ERANGE (tan only)
Memory-Allocation Routines
The memory-allocation routines allocate, free, analyze, and reallocate
blocks of memory.
Many of the memory-allocation functions are prefixed by an _f or an _n. This
notation means use the far (_f) heap or the near (_n) heap.
The malloc family of routines (malloc, _fmalloc, and _nmalloc) allocates
memory blocks of a specified size. The calloc function allocates storage for
an array. The halloc function allocates storage for a huge array.
The realloc routine changes the size of an allocated block.
────────────────────────────────────────────────────────────────────────────
calloc - Allocates storage for an array.
Includes MALLOC.H, STDLIB.H
Prototype void *calloc( size_t num, size_t size
);
Arguments num Number of elements
size Length in bytes of each
element
Returns A void pointer to the allocated space if
successful,
NULL if not
────────────────────────────────────────────────────────────────────────────
free, _ffree, hfree, _nfree - Frees a block of memory previously allocated
by the corresponding malloc routine. The corresponding routines are listed
below:
Free Function Allocation Function
────────────────────────────────────────────────────────────────────────────
_ffree _fmalloc
free calloc, malloc, realloc
hfree halloc
_nfree _nmalloc
Includes MALLOC.H, STDLIB.H (ANSI-compatible free
only)
Prototypes void _ffree( void _far *memblock );
void free( void *memblock );
void _nfree( void near *memblock );
void hfree( void huge *
memblock );
Argument memblock Allocated memory block
Returns Void
────────────────────────────────────────────────────────────────────────────
malloc , _fmalloc, _nmalloc - Allocates a block of memory. The _fmalloc
function allocates the block in the far heap. The _nmalloc function
allocates the block in the near heap.
Includes MALLOC.H, STDLIB.H (ANSI-compatible
malloc only)
Prototypes void *malloc( size_t size );
void _far *_fmalloc( size_t size );
void near *_nmalloc( size_t size );
Argument size Bytes to allocate
Returns A void pointer to the allocated space if
successful, NULL if not
────────────────────────────────────────────────────────────────────────────
realloc - Reallocates a block.
Include MALLOC.H, STDLIB.H
Prototype void *realloc( void *memblock, size_t
size );
Arguments memblock Pointer to previously
allocated memory block
size New size in bytes
Returns A pointer to the reallocated memory if
successful, NULL
if not
Process-Control Routines
The term "process" refers to a program being executed by the operating
system.
Use the process-control routines to
■ Terminate a process (abort, exit, and _exit)
■ Call a new function when a process terminates (atexit)
■ Start a new process (system)
Use the abort and _exit functions to exit without flushing stream buffers.
Use the exit function to exit after flushing stream buffers.
Use the atexit function to create a list of functions to be executed when
the calling program exits.
Use the system call to execute a given MS-DOS command.
────────────────────────────────────────────────────────────────────────────
abort - Aborts a process.
Include PROCESS.H or STDLIB.H
Prototype void abort( void );
Arguments None
Returns Void
────────────────────────────────────────────────────────────────────────────
atexit - Executes functions at program termination.
Include STDLIB.H
Prototype int atexit( void (*func)( void ) );
Argument func Function to be called
Returns A pointer to func if successful, NULL
if not
────────────────────────────────────────────────────────────────────────────
exit, _exit - Terminates the process after flushing buffers (exit);
terminates the process without flushing buffers (_exit).
Include PROCESS.H or STDLIB.H
Prototypes void exit( int status );
void _exit( int status );
Argument status Exit status
Returns Void
────────────────────────────────────────────────────────────────────────────
system - Executes an MS-DOS command.
Include PROCESS.H, STDLIB.H
Prototype int system( const char *command );
Argument command Command to be executed
Returns 0 if successful, -1 if not
errno: E2BIG, ENOENT, ENOEXEC, ENOMEM
Searching and Sorting Routines
The bsearch, lfind, lsearch, and qsort routines provide helpful
binary-search, linear-search, and quick-sort utilities.
────────────────────────────────────────────────────────────────────────────
bsearch - Performs a binary search.
Includes STDLIB.H, SEARCH.H
Prototype void *bsearch( const void *key, const
void *base,
size_t num, size_t width, int( *compare
)( const void *elem1,
const void *elem2 ) );
Arguments key Object to search for
base Pointer to base of search
data
num Number of elements
width Width of elements
compare Compare function
elem1, Array elements to compare
elem2
Returns A pointer if successful, NULL if not
────────────────────────────────────────────────────────────────────────────
lfind, lsearch - Performs a linear search for given value. If the value is
not found, lsearch adds it to the end of the list.
Includes STDLIB.H, SEARCH.H
Prototypes char *lfind( char *key, char *base,
unsigned *num,
unsigned width, int( *compare )( const
void *elem1,
const void *elem2 ) );
char *lsearch( const char *key, const
char *base,
unsigned *num, unsigned width, int( *
compare )
( const void *elem1, const void *elem2 )
);
Arguments key Object to search for
base Pointer to base of search
data
num Number of elements
width Width of elements
compare Compare function
elem1, Array elements to compare
elem2
Returns A pointer if successful, NULL if not
────────────────────────────────────────────────────────────────────────────
qsort - Performs a quick sort.
Includes STDLIB.H, SEARCH.H
Prototype void qsort( void *base, size_t num,
size_t width,
int( *compare )( const void *elem1,
const void *elem2 ) );
Arguments base Start of target array
num Array size in elements
width Element size in bytes
compare Compare function
elem1, Array elements to compare
elem2
Returns Void
String-Manipulation Routines
A wide variety of string routines is available in the run-time library. With
these functions, you can do the following:
■ Copy strings (strcat, strcpy, strdup, strncat, strncpy)
■ Search for strings, individual characters, or characters from a given
set (strchr, strcspn, strpbrk, strrchr, strspn, strstr)
■ Perform string comparisons (strcmp, strcmpi, stricmp, strncmp,
strnicmp)
■ Find the length of a string (strlen)
■ Convert strings to a different case (strlwr, strupr)
■ Set characters of the string to a given character (strnset, strset)
■ Break strings into tokens (strtok)
All string functions work on null-terminated character strings.
Use the buffer-manipulation routines described earlier in this appendix for
manipulating character arrays that do not end with a null character.
────────────────────────────────────────────────────────────────────────────
strcat, strcpy, strdup, strncat, strncpy - Use these routines to copy and
concatenate strings. The list below describes each function.
Function Action
────────────────────────────────────────────────────────────────────────────
strcat Append (concatenate) a string
strcpy Copy one string to another
strdup Duplicate a string
strncat Append a specified number of characters
to a string
strncpy Copy a specified number of characters
from one string to another
Include STRING.H
Prototypes char *strcat( char *dest, const char *
src );
char *strcpy( char *dest, const char *
src );
char *strdup( const char *string );
char *strncat( char *dest, const char *
src, size_t n );
char *strncpy( char *dest, const char *
src, size_t n );
Arguments dest Destination string
src Source string
string Null-terminated string
n Number of characters
Returns strcat: a pointer to the concatenated
string
strcpy: dest string
strdup: a pointer if successful, NULL if
not
strncat, strncpy: a pointer to dest
string
────────────────────────────────────────────────────────────────────────────
strchr, strcspn, strpbrk, strrchr, strspn, strstr - Use these routines to
search strings. The list below describes each function.
Function Action
────────────────────────────────────────────────────────────────────────────
strchr Finds first occurrence of a given
character in a string
strcspn Finds first occurrence of a character
from a given character set in a string
strpbrk Finds first occurrence of a character
from one string in another
strrchr Finds last occurrence of a given
character in a string
strspn Finds first substring from a given
character set in a string
strstr Finds first occurrence of a given string
in another string
Include STRING.H
Prototypes char *strchr( const char *string, int c
);
size_t strcspn( const char *string1,
const char *string2 );
char *strpbrk( const char *string1,
const char *string2 );
char *strrchr( const char *string, int c
);
size_t strspn( const char *string1,
const char *string2 );
char *strstr( const char *string1, const
char *string2 );
Arguments string, Null-terminated strings
string1,
string2
c Character
Returns strchr: a pointer if successful, NULL
if not
strcspn: an offset into string1
strpbrk: a pointer to the first matching
character in string1,
NULL if no match is found
strrchr: a pointer to the last
occurrence of c in string, NULL
if c is not found
strspn: the position of the first
nonmatching character in string1
strstr: a pointer to the first
occurrence of string2 in string1,
or NULL if string2 is not found
────────────────────────────────────────────────────────────────────────────
strcmp, strcmpi, stricmp, strncmp, strnicmp - Use these routines to compare
strings. The list below describes the operation of each function. An "n" in
the function name means to use up to n characters; "i" in the name means to
operate without regard to the case of the string.
Function Action
────────────────────────────────────────────────────────────────────────────
strcmp Compares two strings
strcmpi Compares two strings without regard to
case ("i"
indicates that this function is case
insensitive)
stricmp Compares two strings without regard to
case (identical to strcmpi)
strncmp Compares characters of two strings
strnicmp Compares characters of two strings
without regard to case
Include STRING.H
Prototypes int strcmp( const char *string1, const
char *string2 );
int strcmpi( const char *string1, const
char *string2 );
int stricmp( const char *string1, const
char *string2 );
int strncmp( const char *string1, const
char *string2, size_t n );
int strnicmp( const char *string1, const
char *string2, size_t n );
Arguments string1 Destination string
string2 Source string
n Number of characters
Returns A negative value if string1, 0 if
string1 = string2,
a positive value if string1>string2
────────────────────────────────────────────────────────────────────────────
strlen - The strlen function returns the length in bytes of the string, not
including the terminating null character (\0).
Include STRING.H
Prototype size_t strlen( const char *string );
Argument string Null-terminated string
Returns The string length
────────────────────────────────────────────────────────────────────────────
strlwr, strupr - The strlwr and strupr routines convert the characters of a
string to lowercase and uppercase, respectively.
Include STRING.H
Prototypes char *strlwr( char *string );
char *strupr( char *string );
Argument string String to be converted
Returns A pointer to a copy of the converted
input string
────────────────────────────────────────────────────────────────────────────
strnset, strset - The routines strnset and strset set the characters of a
string to a specified character. The strnset function sets the first n
characters in the string to the specified character. The strset function
sets the entire string to the specified character.
Include STRING.H
Prototypes char *strnset( char *string, int c,
size_t n );
char*strset( char *string, int c );
Arguments string String to be set
c Character setting
n Number of characters set
Returns A pointer to string
────────────────────────────────────────────────────────────────────────────
strtok - The strtok function finds a token in a string. A "token" is a
series of characters delimited by a character from a specified set. For
example, use the strtok function to break an input line into the component
words.
Include STRING.H
Prototype char *strtok( char *string1, const char
*string2 );
Arguments string1 String containing tokens
string2 Set of delimiter characters
Returns A pointer to a token in string1
Time Routines
Use the time routines to get the current time, convert it to a convenient
format, and store it according to your particular needs.
The current time is always taken from the system time.
The time function returns the current time as the number of seconds elapsed
since Greenwich mean time, January 1, 1970.
Use the asctime, ctime, gmtime, and mktime functions to manipulate the time
value.
────────────────────────────────────────────────────────────────────────────
asctime - Converts a time from a structure to a character string.
Include TIME.H
Prototype char *asctime( const struct tm *timeptr
);
Argument timeptr Time structure
Returns A pointer to string result
────────────────────────────────────────────────────────────────────────────
clock - Returns the elapsed CPU time for a process.
Include TIME.H
Prototype clock_t clock( void );
Arguments None
Returns The elapsed processor time if successful,
-1 if not
────────────────────────────────────────────────────────────────────────────
ctime - Converts time from a long integer to a character string.
Include TIME.H
Prototype char *ctime( const time_t *timer );
Argument timer Pointer to stored time
Returns A pointer to string result; NULL if
time represents a date
before 1980
────────────────────────────────────────────────────────────────────────────
difftime - Computes the difference between two times.
Include TIME.H
Prototype double difftime( time_t timer1, time_t
timer0 );
Arguments timer0, Beginning and ending times
timer1
Returns The difference in elapsed time between
timer1 and timer0
────────────────────────────────────────────────────────────────────────────
ftime - Gets current system time as structure.
Includes SYS\TYPES.H, SYS\TIMEB.H
Prototype void ftime( struct timeb *timeptr );
Argument timeptr Pointer to time structure
Returns Void
────────────────────────────────────────────────────────────────────────────
gmtime - Converts time from integer to structure.
Include TIME.H
Prototype struct tm *gmtime( const time_t *timer
);
Argument timer Pointer to stored time
Returns A pointer to a structure
────────────────────────────────────────────────────────────────────────────
mktime - Converts time to a calendar value.
Include TIME.H
Prototype time_t mktime( struct tm *timeptr );
Argument timeptr Local time structure
Returns The encoded calendar time if successful,
-1 if not
────────────────────────────────────────────────────────────────────────────
time - Gets current system time as a long integer.
Include TIME.H
Prototype time_t time( time_t *timer );
Argument timer Storage location for time
Returns The elapsed time
Glossary
────────────────────────────────────────────────────────────────────────────
8087 or 80287 coprocessor:
Intel hardware products that provide very fast and precise floating-point
number processing.
aggregate types:
Arrays, structures, and unions.
ANSI (American National Standards Institute):
The national institute responsible for defining programming-language
standards to promote portability of these languages between different
computer systems. The ANSI standard for C will become official in 1990.
argc:
The traditional name for the first argument to the main function in a C
source program. It is an integer that specifies how many arguments are
passed to the program from the command line.
argument:
A value passed to a function.
argv:
The traditional name for the second argument to the main function in a C
source program. It is a pointer to an array of strings. Traditionally, the
first string is the program name, and each following string is an argument
passed to the program from the command line.
array:
A set of elements with the same type.
array pointer:
A pointer that holds the address of any element of an array.
ASCII (American Standard Code for Information Interchange):
A set of 256 codes that many computers use to represent letters, digits,
special characters, and other symbols. Only the first 128 of these codes are
standardized; the remaining 128 are special characters that are defined by
the computer manufacturer.
automatic variable:
A variable, declared in a block, whose value is discarded when the program
exits from the block. See "static variable" and "lifetime."
background color:
A long integer representing the background color of the display screen. In
graphics modes, the background color applies to the entire screen. In text
modes, the background color specifies the text background for each
character. See "foreground color."
basic data types:
The integral, enumerated, floating-point, and pointer types in the C
language.
binary file:
A file that is not used for text processing. It may be an executable file, a
data file, or some other nontext file.
binary format:
A method of data representation in which data are stored directly from
memory to disk with no translations. In binary format, numeric values are
stored as binary numbers and are not translated to ASCII characters.
binary mode:
A method of accessing files in which no translations are performed. There is
no specific end-of-file character.
binary operator:
An operator that takes two operands. Binary operators in the C language are
the multiplicative operators (*/), additive operators (+ -), shift operators
(<< >>), relational operators (< >> <= >= == !=), bitwise operators (&
| ^), logical operators (&& ||), and the sequential-evaluation operator (,).
bit:
A binary digit (either 0 or 1), the smallest unit of information used with
computers. Eight bits make up one byte.
bit field:
A type of structure that allows manipulation of individual bits or groups of
bits.
bit-mapped font:
A font in which each character is defined by (mapped to) the bits of an
array.
bitwise operator:
An operator used to manipulate bits in an integer expression. Bitwise
operators in the C language are & (AND), | (inclusive OR), ^ (exclusive OR),
(left shift), >> (right shift), and ~ (one's complement).
block:
A sequence of declarations, definitions, and statements enclosed within
curly braces ({}).
bounding rectangle:
An imaginary rectangle that defines the outer limits of a rounded shape such
as an ellipse, arc, or pie.
byte:
The unit of measure used for computer memory and data storage. One byte
contains eight bits and can store one ASCII character.
case label:
The case keyword and the constant, or constant expression, that follows it.
CGA:
IBM's Color Graphics Adapter.
character code:
A numeric code that represents a character. The default ASCII character set
used in all PCs and PS/2s comprises 256 eight-bit character codes.
character constant:
A character enclosed in single quotes, for example, 'p'. A character
constant has a type of char. See "string constant."
character set:
A set of alphabetic and numeric characters and symbols.
clipping:
The process of determining which parts of a graphics image lie within the
clipping region. Parts of the image that lie outside this region are
"clipped"; that is, they are not displayed.
clipping region:
The rectangular area of the screen where graphics display occurs.
color index:
A short integer that represents a displayable color. See "remapping" and
"color value."
color value:
A long integer representing an absolute color. See "remapping" and "color
index."
command-line argument:
A value passed to a program when the program begins execution.
conditional expression:
An expression consisting of three operands joined by the ternary (? :)
operator. Similar to an if-else construct, a conditional expression is used
to evaluate either of two expressions depending on the value of a third
expression.
constant expression:
An expression that evaluates to a constant. A constant expression may
involve integer constants, character constants, floating-point constants,
enumeration constants, type casts to integral and floating-point types, and
other constant expressions.
current color:
The color index for the color in which graphics pixels are displayed. The
current color can be examined with _getcolor or changed with _setcolor.
declaration:
A construct that associates the name and the attributes of a variable,
function, or type.
default:
A condition that is assumed by a program if not specified.
definition:
A construct that initializes and allocates storage for a variable or that
specifies the name, formal parameters, body, and return type of a function.
dimension:
The number of subscripts required to specify a single array element.
directive:
An instruction to the C preprocessor to perform an action on source-program
text before compilation.
double precision:
A real (floating-point) value that occupies eight bytes of memory. Double
precision values are accurate to 15 or 16 digits.
EGA:
Enhanced Graphics Adapter.
enumeration type:
A user-defined data type with values that range over a set of named integral
constants.
escape sequence:
A specific combination of a backslash (\) followed by a letter or
combination of digits, which represents white space and nonprinting
characters within strings and character constants.
expression:
A combination of operands and operators that yields a single value.
external variable:
A variable that is defined outside any function in a C source file and is
used in other source files in the same program.
file handle:
An integer value that is returned when a library function that performs
low-level input/output opens or creates a file. The file handle is used to
refer to that file in later operations.
file pointer:
A value that keeps track of the current position in an input or output
stream. It is updated to reflect the new position each time a read or write
operation takes place.
FILE pointer:
A pointer to a structure of type FILE that contains information about a
file. It is returned by library functions that create or open files and use
stream input/output.
fill flag:
A parameter that determines whether a shape will be drawn as a solid.
fill mask:
A group of pixels that defines the pattern used to fill a graphics shape.
fill pattern:
The design defined by the fill mask and used to fill a shape.
font:
A description of the style and shapes of the characters in a character set.
foreground color:
The color index for the color in which text is displayed. See "background
color."
format specification:
A string that specifies how the printf and scanf families of functions
interpret input and output data.
function:
A collection of declarations and statements that has a unique name and can
return a value.
function body:
A statement block containing the local variable declarations and statements
of a function.
function call:
An expression that passes control and arguments (if any) to a function.
function declaration:
A declaration that states the name, return type, and storage class of a
function that is defined explicitly elsewhere in the program.
function definition:
A definition that specifies a function's name, its formal parameters, the
declarations and statements that define what it does, and (optionally) its
return type and storage class.
function pointer:
A pointer that holds the address of a function.
function prototype:
A function declaration that includes a list of the names and types of formal
parameters in the parentheses following the function name.
global:
See "visibility."
graphics mode:
See "video mode."
header file:
An external source file that contains commonly used declarations and
definitions. The #include directive is used to insert the contents of a
header file into a C source file.
hexadecimal:
The base-16 numbering system whose digits are 0 through F. The letters A
through F represent the decimal numbers 10 through 15. It is often used in
computer programming because it is easily converted to and from binary, the
base-2 numbering system the computer itself uses.
HGC:
Hercules monochrome Graphics Card.
identifier:
A user-defined name in a C program. Identifiers name variables, functions,
macros, constants, and data types.
include file:
See "header file."
Incolor Card:
Hercules InColor Card, a 16-color version of the HGC+.
indirection:
Accessing a data object through a pointer, rather than directly by name.
initialize:
To assign a value to a variable, often at the time the variable is declared.
in-line assembler:
The part of QuickC that converts assembly-language instructions into machine
language.
in-line assembly code:
Assembly language instructions that appear within a QuickC source program.
input/output:
The processes involved in reading (input) and writing (output) data.
integer:
A whole number represented in the machine as a 16-bit two's-complement
binary number. A signed integer has a range of -32,768 to 32,767. An
unsigned integer has a range of 0 to 65,535. See "long integer."
I/O:
Abbreviation for input/output.
keyword:
A word with a special, predefined meaning for the C compiler.
label:
A unique name followed by a colon. Labels are used to denote statements to
which a goto statement can branch. See "case label."
library:
A file containing compiled modules. The linker extracts modules from the
library file and combines them with the user-created object file to form an
executable program.
lifetime:
The time, during program execution, that a variable or function exists. An
"automatic" variable has storage and a defined value only in the block where
it is defined or declared. A "static" variable exists for the duration of
the program.
line style:
An unsigned short integer (16 bits) that specifies the pattern with which
lines will be drawn. Each bit specifies whether a corresponding pixel in the
line will be displayed. The default line style is a solid line.
local:
See "visibility."
long integer:
A whole number represented inside the machine as a 32-bit two's-complement
binary number. A signed long integer has a range of -2,147,483,648 to
2,147,483,647. An unsigned long integer has a range of 0 to 4,294,967,295.
See "integer."
low-level input and output routines:
Run-time library routines that perform unbuffered, unformatted I/O
operations, for example, creat, read, write, and lseek.
lvalue:
An expression (such as a variable name) that refers to a memory location and
is required as the left-hand operand of an assignment operation, or as the
single operand of a unary operator.
machine language:
A series of binary numbers that a computer executes as program instructions.
macro:
An identifier defined in a #define preprocessor directive to represent
another series of characters.
main function:
The function with which program execution begins (the program's entry
point).
manifest constant:
See "symbolic constant."
MCGA (Multicolor Graphics Array):
The video subsystem integrated into the PS/2 Model 30. Also, Memory
Controller Gate Array, one of the components of the Model 30's video
subsystem.
member:
One of the elements of a structure or union.
member-of operator:
The dot operator (.), which is used with the name of a structure and one or
more fields to identify a structure member.
mode:
See "video mode."
monochrome display:
A computer monitor capable of showing only two colors─black and a second
color such as white, green, or amber. Some monochrome monitors can also show
the second color with higher intensity or with underlined text.
Monochrome Display Adapter (MDA):
A printed-circuit card that controls the display and can show text only at
medium resolution in one color.
newline character:
The character used to mark the end of a line in a text file, or the escape
sequence (\n) used to represent this character.
null character:
The ASCII character encoded as the value 0, represented as the escape
sequence (\0) in a source file. A null character marks the end of a string.
null pointer:
A pointer to nothing, expressed as the value 0.
one's complement:
The arithmetic operation in which all 1 bits are converted to 0 bits and
vice versa. The tilde character (~) is the one's-complement operator.
operand:
A constant or variable value that is manipulated in an expression.
operator:
One or more symbols that specify how the operand or operands of an
expression are manipulated. See "unary operator," "binary operator," and
"ternary operator."
origin:
The point on the screen at which the x and y coordinates are both equal to
0. On the physical screen, the origin is at the upper left corner.
palette:
The displayable colors for a given video mode. The CGA modes operate with a
set of predetermined palette colors. The EGA, VGA, and MCGA color modes
operate with a redefinable palette of colors.
parameter:
An identifier that receives a value passed to a function.
path:
The name that defines the location of a file or directory. A path may
include a drive name and one or more directory names.
PGA (Professional Graphics Adapter):
Another name for IBM's PGC.
physical coordinates:
The coordinate system defined by the hardware. The physical coordinate
system has the origin (0, 0) at the upper left corner of the screen. The
value of x increases from left to right, and the value of y increases from
top to bottom. See "viewport coordinates."
pixel:
A single dot on the screen. It is the smallest item that may be manipulated
with the graphics library, and it is the basic unit of the
viewport-coordinate system.
pointer:
A variable containing the address of another variable, function, or
constant.
pointer arithmetic:
The use of addition or subtraction to change a pointer's value. Pointer
arithmetic is typically used with array pointers, though it is not illegal
on other kinds of pointers.
pointer-member operator:
The -> operator, used with structure pointers to name a structure member.
pragma:
An instruction to the compiler to perform an action at compile time.
precedence:
The relative position of an operator in the hierarchy that determines the
order in which expressions are evaluated.
preprocessor:
A text processor that manipulates the contents of a C source file during the
first phase of compilation.
preprocessor directive:
See "directive."
prototype:
See "function prototype."
recursion:
The process by which a function calls itself.
register variable:
An integer variable that is placed in a machine register, which may cause
the program to be smaller and faster.
remapping:
The process of assigning new color values to color indexes. Remapping a
color index changes the screen color of any pixels that have been drawn with
that color index.
reserved word:
See "keyword."
return value:
The value that a function returns to the calling function.
run time:
The time during which a previously compiled and linked program is executing.
run-time library:
A file containing the routines needed to implement certain functions of the
Microsoft QuickC language.
scaling:
The mapping of real-window coordinates to viewport coordinates.
scope:
The parts of a program in which an item can be referenced by name. The scope
of an item may be limited to the file, function, block, or function
prototype in which it appears.
screen mode:
See "video mode."
single precision:
A real (floating-point) value that occupies four bytes of memory. Single-
precision values are accurate to seven decimal places.
sizeof operator:
A C operator that returns the amount of storage, in bytes, associated with
an identifier or a type.
source file:
A text file containing C language code.
standard error:
The device to which a program sends its error messages unless the error
output is redirected. In normal DOS operation, standard error is the
display. The predefined stream stderr is associated with standard error in
the C language.
standard input:
The device from which a program reads its input unless the input is
redirected. In normal DOS operation, standard input is the keyboard. The
predefined stream stdin is associated with standard input in the C language.
standard output:
The device to which a program sends its output unless the output is
redirected. In normal DOS operation, standard output is the display. The
predefined stream stdout is associated with standard output in the C
language.
static variable:
A variable that keeps its value even after the program exits the block in
which the variable is declared.
stream:
A sequence of bytes flowing into (input) or out of (output) a program.
stream functions:
Run-time library functions that treat data files and data items as "streams"
of individual characters.
string:
An array of characters, terminated by a null character (\0).
string constant:
A string of characters and escape sequences enclosed in double quotes ("").
Every string constant is an array of elements of type char. See "character
constant."
structure:
A set of elements, which may be of different types, grouped under a single
name.
structure member:
One of the elements of a structure.
structure pointer:
A pointer to a structure. Structure pointers identify structure members by
specifying the name of the structure, the pointer-member operator (->), and
the member name.
symbolic constant:
An identifier defined in a #define preprocessor directive to represent a
constant value.
tag:
The name assigned to a structure, union, or enumeration type.
ternary operator:
An operator used in ternary (three-part) expressions. C has one ternary
operator, the conditional operator (? :).
text:
Ordinary, readable characters, including the uppercase and lowercase letters
of the alphabet, the numerals 0-9, and punctuation marks.
text file:
A file of ASCII characters that you can read with the TYPE command or a word
processor.
text format:
A method of disk storage in which all data are converted to ASCII format.
text mode:
See "video mode."
text window:
A window defined in row and column coordinates where text output to the
screen will be displayed. Text printed beyond the edge of the text window is
not visible. The default text window is the whole screen.
two's complement:
A kind of base-2 notation used to represent positive and negative numbers in
which negative values are formed by complementing all bits and adding 1 to
the results.
type:
A description of a set of values. For example, the type char comprises the
256 values in the ASCII character set.
type cast:
An operation in which a value of one type is converted to a value of a
different type.
type checking:
An operation in which the compiler verifies that the operands of an operator
are valid, or that the actual arguments in a function call are of the same
types as the corresponding formal parameters in the function definition and
function prototype.
type declaration:
A declaration that defines the name and members of a structure or union
type, or the name and enumeration set of an enumeration type.
typedef declaration:
A declaration that defines a shorter or more meaningful name for an existing
C data type or for a user-defined data type. Names defined in a typedef
declaration are often referred to as "typedefs."
typeface:
The style of displayed text.
type name:
The name of a data type. See "type."
type qualifier:
The keywords short, long, signed, and unsigned, which modify a basic data
type.
type size:
A measure of the screen area occupied by individual characters in a font,
typically specified in pixels.
unary expression:
An expression consisting of a single operand preceded or followed by a unary
operator.
unary operator:
An operator that takes a single operand. Unary operators in the C language
are the complement operators (- ~ !), indirection operator (*), increment
(++) and decrement (- -) operators, address-of operator (&), and sizeof
operator. The unary plus (+) operator is legal but has no effect.
union:
A set of values of different types that occupy the same storage space.
vector-mapped font:
A font in which each character is defined in terms of lines and arcs.
VGA (Video Graphics Array):
Many users refer to the video subsystem integrated into the PS/2 Models 50,
60, and 80, as well as the IBM PS/2 Display Adapter, as the "VGA."
video adapter:
A printed-circuit card that generates video output. Well-known IBM PC video
adapters include the MDA, CGA, HGC, EGA, MCGA, and VGA Adapters.
video mode:
An integer that specifies the resolution and other characteristics of video
output. QuickC supports 17 different video modes, although some of them are
available only with certain video adapters.
viewport:
A clipping region in which the origin (0, 0) may be redefined. The initial
origin of a viewport is the upper left corner.
viewport coordinates:
The integer coordinate system defined by the programmer for a specific
viewport. By default, the viewport-coordinate system has the origin (0, 0)
at the upper left corner of the viewport, but this may be changed by a call
to _setvieworg.
visibility:
The parts of the program in which a particular variable or function can be
referenced by name. An item has global visibility if it is visible in all
source files constituting the program and local visibility if its use is
restricted.
white-space character:
A space, tab, line-feed, carriage-return, form-feed, vertical-tab, or
newline character.
window:
An imaginary rectangle on the screen where output takes place. See "text
window" and "window coordinates."
window coordinates:
The coordinate system defined by the programmer.