Macro Value Changes

February 2, 2022

I need to explain for the MACRO define and if else conditions.

When I using the below function I get CHANGE value to 5

#include <stdio.h>

#define CHANGE var
int var;
int main()
{
    var = 5;
    printf("%d",CHANGE);

}

Output: 5

But if I use like this:

#include <stdio.h>

#define CHANGABLE_VALUE var
int var;
int main()
{
    var = 5;
    #if CHANGABLE_VALUE == 5
    printf("%d",CHANGABLE_VALUE);
    #else
    printf("There is no value");
    #endif



}

Output: There is no value

Why the #if statements not working ?

>Solution :

Preprocessor definitions (aka Macros) are resolved during pre-compilation time, not during compilation time, and most certainly not during runtime, which seems to be what you’re expecting.

Hence #if CHANGABLE_VALUE == 5 is replaced with #if var == 5 and then resolved as something which is false, so the actual code inside it is not even compiled (let alone executed).