How to use global variables in C functions included in other files

Question

I have a simple program like:

int velocity=0;

#include "extra.h"

int main()
{
    extra();
    return 0;
}

where extra.h is:

void extra(){
   velocity += 1;
}

yet when I compile this, I get the error:

extra.h:5:5: error: 'velocity' was not declared in this scope

Obviously, my code in extra.h can't "see" the variables in main.c, but why is this? How do I fix this?

http://stackoverflow.com/questions/10422034/when-to-use-extern-in-c — willll, Sep 19 '16 at 17:13
@OP: Why do you _define_ a function in a header, in first place? — Sourav Ghosh, Sep 19 '16 at 17:14
What compiler are you using? This worked as is with clang 3.4. — Robert Prévost, Sep 19 '16 at 17:14

score 0 · Answer 1 · edited Sep 19 '16 at 19:42

You could add the following declaration to extra.h:

extern int velocity;

However, extra() shouldn't be defined in extra.h in the first place. This will cause problems if extra.h is included by multiple .c files in the same binary. The following is what you should have:

extra.h:

void extra();

extra.c:

#include "extra.h"

static int velocity = 0;

void extra() {
   velocity += 1;
}

main.c:

#include "extra.h"

int main()
{
    extra();
    return 0;
}

How to use global variables in C functions included in other files

1 Answers1