In C I did not notice any effect of the extern keyword used before function declaration. At first I thougth that when defining extern int f(); in a single file forces you to implement it outside of the files scope, however I found out that both

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

And

extern int f() {return 0;}

Compiles just fine, with no warnings from gcc. I used gcc -Wall -ansi, he 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.