Question

I have an example code like this:

int var;
var = MACRO_A;

I expect the MACRO_A has been defined like this:

#define MACRO_A    1234

However, I can not find the MACRO_A defined anywhere in the source code, but I can build the code successfully, so what is the value of var at the end ?

The compiler is gcc.

Answer 1

That macro could have been passed during compilation with -D option.

#include <stdio.h>

int main()
{
    int i = MACRO_A;
    printf("MACRO_A : %d \n", i);
    return 0;
}

Output:

$ gcc macro.c -D MACRO_A=10
$ ./a.out 
MACRO_A : 10 
$