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Preprocessors Directives in C programming

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      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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