Effects of the extern keyword on C functions?

Question

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 1

I can show you 2 instances which might help you crystallize the answer with your own logic.

file a.h

extern int func1(int);

file a.c

#include "a.h"

main()

{

func1(100);

}

Second instance:

file a.c :

int func1 (int a)

{

actual contents of function func1

}

file b.c

extern int func1(int a);

main()

{

func1(100);

}