Preprocessors Directives in C programming

The C preprocessor is a collection of special statements, called directives that are executed at the beginning of compilation process. Preprocessors directives usually appear at the beginning of a program. A preprocessor directive may appear anywhere within a program. Preprocessor directives follow special syntax rules that are different from the normal C syntax. They all begin with the symbol # in column one and do not require a semicolon at the end. We have already used the directives #define and #include to a limited extent. A set of commonly used preprocessor directives and their functions are listed below:

Directives Functions

#define Defines a macro substitution

#undef Undefines a macro

#include Species the files to be included

#ifdef Test for a macro definition

#endif Specifies the end of #if

#ifndef Tests whether a macro is not defined

#if Tests a compile-time condition

#else Specifies alternatives when #if test fails.

These directives can be divided into three categories:

1. Macro substitution directives

2. File inclusion directives

3. Compiler control directives

Macro substitution directives:

Macro substitution is a process where an identifier in a program is replaced by a predefined string composed of one or more tokens. The general format is:

#define identifier string

#define PI 3.1415926

#define FALSE 0

#define COUNT 100

#define CAPITAL “KATHMANDU”

File inclusion directives

We use File inclusion in order to include a file where functions or macros are defined.

#include <filename> or #include “filename”

Compiler control directives:

In order find files based switch or (on or off) particular line or groups of lines in a program, we use conditional compilation. For that, we use following preprocessor directives such as #ifdef, #endif, #ifndef, #if, #else.

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