[c] Global variables in header file

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\n",i);
    foo();
    return 0;
}

file2.c

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

void foo()
{
    i = 10;
    printf("%d\n",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?

The currently-accepted answer to this question is wrong. C11 6.9.2/2:

If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

So the original code in the question behaves as if file1.c and file2.c each contained the line int i = 0; at the end, which causes undefined behaviour due to multiple external definitions (6.9/5).

Since this is a Semantic rule and not a Constraint, no diagnostic is required.

Here are two more questions about the same code with correct answers:

• Tentative definitions in C and linking
• How do I use extern to share variables between source files?

You should not define global variables in header files. You can declare them as extern in header file and define them in a .c source file.

(Note: In C, int i; is a tentative definition, it allocates storage for the variable (= is a definition) if there is no other definition found for that variable in the translation unit.)

Don't initialize variables in headers. Put declaration in header and initialization in one of the c files.

In the header:

extern int i;

In file2.c:

int i=1;

Dont define varibale in header file , do declaration in header file(good practice ) .. in your case it is working because multiple weak symbols .. Read about weak and strong symbol ....link :http://csapp.cs.cmu.edu/public/ch7-preview.pdf

This type of code create problem while porting.

@glglgl already explained why what you were trying to do was not working. Actually, if you are really aiming at defining a variable in a header, you can trick using some preprocessor directives:

file1.c:

#include <stdio.h>

#define DEFINE_I
#include "global.h"

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

file2.c:

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

void foo()
{
    i = 54;
    printf("%d\n",i);
}

global.h:

#ifdef DEFINE_I
int i = 42;
#else
extern int i;
#endif

void foo();

In this situation, i is only defined in the compilation unit where you defined DEFINE_I and is declared everywhere else. The linker does not complain.

I have seen this a couple of times before where an enum was declared in a header, and just below was a definition of a char** containing the corresponding labels. I do understand why the author preferred to have that definition in the header instead of putting it into a specific source file, but I am not sure whether the implementation is so elegant.