Question

For some reason, I’m getting multiple declarations of content within my header file even though I’m using header guards. My example code is below:

main.c:

#include "thing.h"

int main(){
    printf("%d", increment());

    return 0;
}

thing.c:

#include "thing.h"

int increment(){
    return something++;
}

thing.h:

#ifndef THING_H_
#define THING_H_

#include <stdio.h>

int something = 0;

int increment();

#endif

When I attempt to compile this, GCC says that I have multiple definitions of the something variable. ifndef should make sure that this doesn’t happen, so I’m confused why it is.

Answer 1

The include guards are functioning correctly and are not the source of the problem.

What happens is that every compilation unit that includes thing.h gets its own int something = 0, so the linker complains about multiple definitions.

Here is how you fix this:

thing.c:

#include "thing.h"

int something = 0;

int increment(){
    return something++;
}

thing.h:

#ifndef THING_H_
#define THING_H_

#include <stdio.h>

extern int something;

int increment();

#endif

This way, only thing.c will have an instance of something, and main.c will refer to it.