The preprocessor will process directives that are inserted into the C source code. These directives allow additional actions to be taken on the C source code before it is compiled into object code. Directives are not part of the C language itself.
Preprocessor directives begin with a pound (#) symbol and may have several arguments.
The following are some of the preprocessor directives that you can use in your source code:
Directive Description Example
#include Include another C file into the current file at the location of the #include statement prior to compiling the source code. #include <stdio.h>
#define Define a macro which can be used as a constant throughout the source code. #define AGE 50
#undef Clear a macro which was previously defined. #undef AGE
#if Conditional expresssion which can be used to include source code for compilation. #if AGE > 50
#ifdef Allows the inclusion of source code if the provided macro identifier has been defined. Equivalent to #if defined(identifier). #ifdef SOLARIS
#ifndef Allows the inclusion of source code if the provided macro identifier has not been defined. #ifndef WINDOWS
#elif Provides an alternate inclusion of source code when used with the #if, #ifdef, or #ifndef directives and the #elif condition evaluates to true. #elif YEARS_OLD > 10
#else Allows the inclusion of source code if the preceeding #if, #ifdef, or #ifndef directive expression evaluates to false. #else
#endif Signals the end of a #if, #ifdef or #ifndef condition . #endif
#warning Report a warning message and continue preprocessing. #warning Non-critical error found
#error Report error and stop preprocessing
