Question

There are three files:
source1.c
source2.c
header.h

The two source-files includes the header.

This is the code of the header:

struct
{
    int a;
    int b
} x;

What happens now is that the struct becomes global and the two source-files now shares the struct called x. Why does this happen?

I know that if you write the following code it will make two global variables. One for each of the source-files. (they don’t share the globals)

int x = 0;

The last piece of code makes sense to me but i really don’t understand the one with the struct..

EDIT:
Hmm everybody here think I should get linker errors. My current code is for an embedded system (nxtOSEK). I’ll try to convert it to a regular C program later.

EDITEDIT:
I’m back with examples in regular C. As you can see it is not only possible with structs but also with regular variables.

source1.c

#include "header.h"

int main(void)
{
    f();
    x = 1;
    f();
}

source2.c

#include "header.h"

void f()
{
    printf("source2: %i\n", x);
}

header.h

#include <stdio.h>

int x;

Output

source2: 0
source2: 1

Note that x must not be declared for it to work or it gives a linker error like everyone here said. (I don’t know why it work with the embeded system..)

It also looks like I misread Eric Postpischil’s answer which looks correct.

Answer 1

An external declaration of an object identifier at file scope that has an initializer is a definition. The declaration int x = 0; is a definition because x is initialized.

An external declaration of an object identifier at file scope that does not have an initializer is a tentative definition. The declaration struct {…} x; is a tentative definition because x is not initialized.

Multiple definitions at link time cause an error.

Multiple tentative definitions at link time may be coalesced to a single definition, which is initialized with zero. This was traditional Unix behavior and was the default behavior with GCC prior to version 10; GCC marked tentative definitions as “common” symbols by default. With GCC version 10 and later, multiple-definition errors may result. The old behavior can be selected with the -fcommon command-line switch to GCC.

If you change int x = 0; to int x;, you will not a get a link error while using build tools that treat tentative definitions as common symbols. If you change struct {…} x; to struct {…} x = {0};, you will get a link error.