__FILE__ In .h what does it resolve to

Question

Is there a specification on how the __FILE__ macro will be expanded if it is in a .h?

If I define in foo.h

#define MYFILE __FILE__

And include it in foo.c

#include "foo.h"

int main(){
  printf("%s",MYFILE);
  ....
}

Does this output foo.h or foo.c? (Yes I realize this is a stupid example)

Sorry for what should be a simple question. The documentation on the web seems conflicting. For what it is worth VS2008 comes back as foo.c which is what I would expect....I think. I am just trying to confirm if this is defined behavior.

Answer 1

The advice given in yan's answer is 'generally correct'. That is, the value of __FILE__ is the name of the current source file when the macro is used, not when the macro is defined. However, it is not absolutely correct - and here is a counter-example:

$ cat x.h
static void helper(void)
{
    printf("%s:%d helper\n", __FILE__, __LINE__);
}
$ cat x.c
#include <stdio.h>
#include "x.h"

int main(void)
{
    helper();
    printf("%s:%d\n", __FILE__, __LINE__);
    return 0;
}

$ make x
cc -Wall -Wextra -std=c99 -g x.c -o x
$ ./x
x.h:3 helper
x.c:7
$

This is a contrived example; in C, you very seldom put actual code into a header as I did here — unless you are using inline functions. But the output shows that there are circumstances where the name of the header can be the correct name that __FILE__ expands to.