c - Global variables in header file

Question

I have a 2 modules (.c files) and one .h header file:

file1.c:

#include <stdio.h>
#include "global.h"

int main()
{
    i = 100;
    printf("%d
",i);
    foo();
    return 0;
}

file2.c

#include <stdio.h>
#include "global.h"

void foo()
{
    i = 10;
    printf("%d
",i);
}

global.h

int i;
extern void foo()

When I do gcc file1.c file2.c everything works fine and I get the expected output. Now, when I initialize variable 'i' in the header file to say 0 and compile again I get a linker error:

/tmp/cc0oj7yA.o:(.bss+0x0): multiple definition of `i'
/tmp/cckd7TTI.o:(.bss+0x0): first defined here

If I just compile file1.c (removing call to foo()) with the initialization in the header file i.e. gcc file1.c, everything works fine. What is going on?

Answer 1

There are 3 scenarios, you describe:

1. with 2 .c files and with int i; in the header.
2. With 2 .c files and with int i=100; in the header (or any other value; that doesn't matter).
3. With 1 .c file and with int i=100; in the header.

In each scenario, imagine the contents of the header file inserted into the .c file and this .c file compiled into a .o file and then these linked together.

Then following happens:

1. works fine because of the already mentioned "tentative definitions": every .o file contains one of them, so the linker says "ok".

2. doesn't work, because both .o files contain a definition with a value, which collide (even if they have the same value) - there may be only one with any given name in all .o files which are linked together at a given time.

3. works of course, because you have only one .o file and so no possibility for collision.

IMHO a clean thing would be

• to put either extern int i; or just int i; into the header file,
• and then to put the "real" definition of i (namely int i = 100;) into file1.c. In this case, this initialization gets used at the start of the program and the corresponding line in main() can be omitted. (Besides, I hope the naming is only an example; please don't name any global variables as i in real programs.)