Question

Suppose the code:

extern int foo(void);

static int foo(void)
{
        return 0;
}

Try to compile with GCC

$ gcc -Wall -std=c99 1.c 
1.c:3:12: error: static declaration of ‘foo’ follows non-static declaration
1.c:1:12: note: previous declaration of ‘foo’ was here
1.c:3:12: warning: ‘foo’ defined but not used [-Wunused-function]

So, how can I declare static function?

Answer 1

Why declaration by extern doesn’t work with static functions?

Because extern indicates external linkage while static indicates internal linkage. Obviously, You cannot have both on same function.

In simple words, when you use static on the function you tell the compiler please limit the scope of this function only to this translation unit and do not allow anyone to access this from another translation unit.
while function declarations are extern by default, when you specify extern explicitly you tell the compiler, Please allow everyone from other translation units to access this function.
Well obviously the compiler cannot do both.

So make a choice, whether you want the function to be visible only in the translation unit or not.
If former make it static and forget extern. If latter just drop the static.

Good Read:
What is external linkage and internal linkage?

Though above Q is for C++, most of what is discussed applies to C as well.