C Preprocessors

The preprocessor is a program invoked by the compiler that modifies the source code before the actual composition takes place. This tutorial describes C Preprocessor Directives.

To use any preprocessor directives, first, you have to prefix them with the # (hash) symbol

The following section lists all preprocessor directives:

Category Directive Description
Macro substitution division #include File includes.
#define
#undif 		Macro define. Macro undefine.
#ifdef
#ifndef 		If Macro defined. If Macro not defined.
File inclusion division #if
#elif
#else
#endif 		If, Else if, Else, End if.
Compiler control division #line
#error
#pragma 		Set line number, Abort compilation, Set compiler option.

C Preprocessors Examples

Syntax:

#include <stdio.h>

/* #define macro_name character_sequence */
#define LIMIT 10

int main()
{
    int counter;
    for(counter =1; counter <=LIMIT; counter++)
    {
        printf("%d\n",counter);
    }
return 0;
}

In the above example for loop will run ten times.

#include <stdio.h>
#include "header.h"

#include <stdio.h> tell the compiler to add stdio.h file from System Libraries to the current source file, and #include "header.h" tells the compiler to get header.h from the local directory.

#undef  LIMIT
#define  LIMIT 20

It tells the compiler to undefine the existing LIMIT and set it as 20.

#ifndef LIMIT
   #define LIMIT 50
#endif

It tells the compiler to define LIMIT only if it isn't already defined.

#ifdef LIMIT
   /* Your statements here */
#endif

It means the compiler processes the statements enclosed if the LIMIT is defined.