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.

c syntax standards
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