variable defined in header file shared?

Question

In header file "foo.h", there is the following statement,

int foo;

In source file "a.c" and "b.c", the header file is included.

#include "foo.h"

Are there two independent foo's in two places or there is only one foo shared across "a.c" and "b.c"? I see this happens in a shared piece of code from others. Thanks in advance if you can understand this.

Answer 1

You will get an error by defining an external object multiple times.

Use extern int foo; in the header file and int foo; in exactly one .c file.

The extern specifier (if there is no initializer) at file-scope declares without defining.