c preprocessor ifndef

C #ifndef:

The #ifndef is a preprocessor directive in C which is used to execute a piece of code, only if the specified macro is not defined by #define.

Syntax:

#ifndef MACRO

// code to be executed if macro is not defined

#endif

OR

#ifndef MACRO

// code to be executed if macro is not defined

#else

// code to be executed if macro is defined

#endif

Example:

#include<stdio.h>  
#define E 2.718
void main()
{    
#ifndef E
printf("E: %f", E);
 
#else  
printf("E is not defined.");  
 
#endif    	 
}

Output

E is not defined.

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