Elazar Leibovich Elazar Leibovich - 17 days ago 7
C Question

Effects of the extern keyword on C functions

In C, I did not notice any effect of the

extern
keyword used before function declaration.
At first, I thought that when defining
extern int f();
in a single file forces you to implement it outside of the file's scope. However I found out that both:

extern int f();
int f() {return 0;}


and

extern int f() {return 0;}


compile just fine, with no warnings from gcc. I used
gcc -Wall -ansi
; it wouldn't even accept
//
comments.

Are there any effects for using
extern
before function definitions? Or is it just an optional keyword with no side effects for functions.

In the latter case I don't understand why did the standard designers chose to litter the grammar with superfluous keywords.

EDIT: To clarify, I know there's usage for
extern
in variables, but I'm only asking about
extern
in functions.

Answer

We have two files, foo.c and bar.c.

Here is foo.c

#include <stdio.h>

volatile unsigned int stop_now = 0;
extern void bar_function(void);

int main(void)
{
  while (1) {
     bar_function();
     stop_now = 1;
  }
  return 0;
}

Now, here is bar.c

#include <stdio.h>

extern volatile unsigned int stop_now;

void bar_function(void)
{
   while (! stop_now) {
      printf("Hello, world!\n");
      sleep(30);
   }
}

As you can see, we have no shared header between foo.c and bar.c , however bar.c needs something declared in foo.c when it's linked, and foo.c needs a function from bar.c when it's linked.

By using 'extern', you are telling the compiler that whatever follows it will be found (non-static) at link time, don't reserve anything for it since it will be encountered later.

It's very useful if you need to share some global between modules and don't want to put / initialize it in a header.

Technically, every function in a library public header is 'extern', however labeling them as such has very little to no benefit, depending on the compiler. Most compilers can figure that out on their own. As you see, those functions are actually defined somewhere else.

In the above example, main() would print hello world only once, but continue to enter bar_function(). Also note, bar_function() is not going to return in this example (since it's just a simple example). Just imagine stop_now being modified when a signal is serviced (hence, volatile) if this doesn't seem practical enough.

Externs are very useful for things like signal handlers, a mutex that you don't want to put in a header or structure, etc. Most compilers will optimize to ensure that they don't reserve any memory for external objects, since they know they'll be reserving it in the module where the object is defined. However, again, there's little point in specifying it with modern compilers when prototyping public functions.

Hope that helps :)