Advanced C 1992

Part III • Working with Others

The stdlib.h File (ANSI)

The stdlib.h header file contains the definitions and declarations for commonly used library functions. These library functions are usually found in the stdlib.h header file because they do not fit elsewhere or cannot be placed in other header files for technical reasons. This section describes some of the functions in the stdlib.h header file.

String Conversion

The string conversion functions convert strings to their numeric equivalent. The conversion process is performed in accordance with the locale settings, so that the thousands separator (a comma in the United States) may be recognized. The three ANSI functions for string conversion are strtod(), strtol(), and strtoul().

Memory Allocation

If a compiler does not have a separate memory allocation header file (such as malloc.h), the memory allocation functions are defined in stdlib.h.

Random Numbers

The C random number function is called rand(). The random numbers may be seeded using srand(). The sequence of random numbers repeats, however, if srand() receives the same seed each time. A common trick is to seed the random numbers with the current time or the elapsed time between two (or more) keystrokes.

Communications with the Operating System

You can issue operating system commands with the system() function. This function’s behavior is not defined by ANSI C, with two exceptions. If the system() function is called with a NULL parameter, it returns TRUE if a command interpreter is present. If the system() parameter is called with NULL and signal() returns zero, you cannot issue commands to the operating system.

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You can use the atexit() function to tell the operating system which functions should be called when the program ends. Although you can include functions, they cannot be removed after installation.

You can use abort() to end your program. Regardless of the use of the SIGABORT signal error handler, abort() will not return to the caller.

Search Functions

The bsearch() function is often defined in stdlib.h. Why bsearch() is defined here and qsort() is not is beyond me.

Integer Math

ANSI C has two integer math functions. These functions enable you to divide two integer values, and provide both a quotient and a remainder in one call. As well, these functions produce more predictable results than the division operator.

Multibyte Characters

Because ANSI C supports multibyte character sets (useful for languages that do not use the roman character set), there are functions to support character sets that have more than 256 characters. These characters are represented with a two-byte sequence, and their values depend on both the compiler and the language. You must read your compiler’s documentation to determine which functions are provided and how to use them.

The string.h File (ANSI)

The various string manipulation functions are defined in the string.h header file. This file also contains various memory functions (such as the ones found in the memory.h header file).

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Part III • Working with Others

The time.h File (ANSI)

The time functions are in the time.h header file. ANSI C added the strftime() function to the C language, which helps you format a time string.

The varargs.h File

In most C compilers, the varargs.h header file has been replaced with the stdarg.h header file. You usually cannot mix these two header files in the same source file.

Summary

In this chapter, you learned about the ANSI standard header files.

•Although there are standards for header files, most compilers add header files that offer additional functionality (such as graphics) or split the functionality of existing ANSI header files into several new files (such as Microsoft’s use of the memory.h header file).

•The varargs.h and the stdarg.h files both define the passing of a variable number of arguments to a function. In general, the stdarg.h header file is considered the ANSI standard way to pass a variable number of arguments.

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Part IV

Documenting the

Differences

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Advanced C

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ANSI C’s Library

Functions

This chapter presents the ANSI C standard functions. I have not included functions that are not ANSI—there are several hundred additional non-ANSI functions that vary both in function and implementation between different compilers. For the nonANSI functions, it is preferable to use the documentation that is supplied with the compiler because there may be differences between different implementations of a non-ANSI function that you would not expect.

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Part IV • Documenting the Differences

Functions

The ANSI C libarary functions are presented below in alphabetical order.

abort()

abs()

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Note: Sets errno to EDOM if the parameter passed is outside the allowed range.

asctime()

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Part IV • Documenting the Differences

asin()

assert()

atan()

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