What's the difference between the macros “#define STR(x) #x” and “#define STR(x) VAL(x)” with “#defineVAL(x) #x”?

Question

When I use this code:

#include 
#define STR(x) #x

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


        

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.