C programming Interview questions and answers: #ifndef in c example

Directive #ifndef is just opposite to the directive #ifdef . In this case if identifier has not defined then #ifndef is true and if identifier has defined then #ifndef condition will false.

Syntax :

#ifndef <Identifier>

--------------

#else

--------------

#endif

Example 1:

#include<stdio.h>

#define int ‘A’

int main(){

char num = int;

#ifndef int

printf("Please define int");

#else

printf("%d",int);

#endif

return 0;

}

Output: 65

Explanation: Macro constant int has been defined so #else directive will execute. Its intermediate file will look like:

int main(){

char num = ‘A’;

printf("%d",’A’);

return 0;

}

ASCII value of ‘A’ is 65.

Example 2:

#include<stdio.h>

int main(){

#ifndef __TIME__

Wow! We can write any thing. Exact help.

< * 9 j x %% && ## ++ (( ))

#else

printf("%s",__TIME__);

#endif

return 0;

}

Output: It will print current system time.

Explanation: __TIME__ is global identifier. It has been defined in the header file stdio.h. Compiler doesn’t compile the c codes which are inside the any conditional preprocessor directive if its condition is false. So we can write anything inside it.

Its intermediate file will look like:

int main(){

printf("%s",__TIME__);

return 0;

}

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