Why declaration by 'extern' doesn't work with static functions in C?

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?

"So, how can I declare static function?" `static int foo(void);` — Daniel Fischer, Dec 09 '12 at 13:57

score 21 · Accepted Answer · edited May 23 '17 at 10:29

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.

Why is the static declaration followed by non-static is allowed in your opinion? — Yug Singh, Jan 11 '20 at 20:01

score 4 · Answer 2 · answered Dec 09 '12 at 13:57

4

You declare it with static

static int foo(void);

static int foo(void)
{
        return 0;
}

answered Dec 09 '12 at 13:57

Olaf Dietsche

72,253
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score 1 · Answer 3 · answered Dec 09 '12 at 14:00

1

extern means the function is defined in a different translation unit (file). static means it is only used in the translation unit in which it is defined. The two are mutually exclusive.

answered Dec 09 '12 at 14:00

William Pursell

204,365
48
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score -1 · Answer 4 · edited Jul 28 '17 at 07:03

-1

Is n't it that static can be resolved with extern(al) linkage like File1.c

static int fx(void)
{
    return int;
}

File2.c

extern int fx(void);

/*call */
fx();

edited Jul 28 '17 at 07:03

Anh Pham

2,108
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answered Jul 28 '17 at 05:53

Angelo John

1

If static(s) are not found in symbol table they can be resolved by just putting extern in REDEFINITION. Linker will resolve it in most of the cases. Though the simple definition of static is that it has a internal linkage scope. – Angelo John Jul 31 '17 at 22:43

Why declaration by 'extern' doesn't work with static functions in C?

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