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 20It tells the compiler to undefine the existing LIMIT and set it as 20.
#ifndef LIMIT
#define LIMIT 50
#endifIt tells the compiler to define LIMIT only if it isn't already defined.
#ifdef LIMIT
/* Your statements here */
#endifIt means the compiler processes the statements enclosed if the LIMIT is defined.
