Question

I’m reading external C code, and I found the following line of code:

#define __MAIN_C__

where #define is given only one “argument” (namely __MAIN_C__).

Is this just a placeholder, or can it have a function?

Answer 1

That #define will define __MAIN_C__ to no value (thanks to n.m. and a simple test program). It’s a shortcut when you need something #defined when you don’t care what it’s defined to. In that case, somewhere in your code, you’d probably just see:

#ifdef __MAIN_C__

And that doesn’t care what specific value it has, as long as it’s defined (and perhaps nonzero).

You’ll very often see this for include guards, like so:

#ifndef MYHEADER_H
#define MYHEADER_H

// stuff here

#endif

This is similar to doing this in the compile flags:

gcc -D__MAIN_C__ main.c

If you want to see that it’s defined to nothing, try to compile this and watch it fail:

#include <stdio.h>

#define TEST

int main(int argc, char *argv[])
{
    printf("%d\n", TEST);
    return 0;
}