Question

When I use this code:

#include <stdio.h>
#define STR(x) #x

int main(void)
{
    printf(__FILE__ STR(__LINE__) "hello!\n");
    return 0;
}

it prints

hello.c__LINE__hello!

but when I use this:

#include <stdio.h>
#define STR(x) VAL(x)
#define VAL(x) #x

int main(void)
{
    printf(__FILE__ STR(__LINE__) "hello!\n");
    return 0;
}

it prints

hello.c7hello!

what’s the difference between

#define STR(x) #x

and

#define STR(x) VAL(x)
#define VAL(x) #x

Answer 1

Arguments to macros are themselves macro-expanded, except where the macro argument name appears in the macro body with the stringifier # or the token-paster ##.

In the first case, the argument of STR is not macro-expanded, and so you just get the name of the LINE macro.

In the second case, the argument of STR is macro-expanded when it is substituted into the definition of VAL, and so it works — you get the actual line number because the LINE macro is expanded.