CODEWITHSUNDEEP
cstaticextern
user1150105
user1150105

Reputation:

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

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?

Upvotes: 6

Views: 12335

Answers (4)

Angelo John
Angelo John

Reputation: 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();

Upvotes: -1

Alok Save
Alok Save

Reputation: 206528

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.

Upvotes: 21

William Pursell
William Pursell

Reputation: 212248

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.

Upvotes: 1

Olaf Dietsche
Olaf Dietsche

Reputation: 74028

You declare it with static

static int foo(void);

static int foo(void)
{
        return 0;
}

Upvotes: 4

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