Header/Include guards don't work?

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.

Answer 2

You have one definition in each translation unit (one in main.c, and one in thing.c). The header guards stop the header from being included more than once in a single translation unit.

You need to declare something in the header file, and only define it in thing.c, just like the function:

thing.c:

#include "thing.h"

int something = 0;

int increment(void)
{
    return something++;
}

thing.h:

#ifndef THING_H_
#define THING_H_

#include <stdio.h>

extern int something;

int increment(void);

#endif

Answer 3

The header guards will stop the file from being compiled more than once in the same compilation unit (file). You are including it in main.c and thing.c, so it will be compiled once in each, leading to the variable something being declared once in each unit, or twice in total.