What happens when non-static function declaration

Question

The following compiles:

static int foo() { return 1; }
int foo();

But, will it always compile? Is the behavior in this case well defined? And what does it means when a non-static prototype follows a static declaration?

Answer 1

Yes it will compile and behavior is well defined. Since foo is declared static earlier than int foo();¹, foo has internal linkage.

C11: 6.2.2 Linkages of identifiers (p4):

For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,³¹⁾ if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. [...]

and the foot note states that:

31) As specified in 6.2.1, the later declaration might hide the prior declaration.

---

_{1. If no storage class is specified , the function is assumed to have external linkage. Standard says: If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern -- 6.2.2 (p5).}

Answer 2

By default functions are global. So making it

static int foo() { return 1; }

The function foo() is just visible within this file. Since you just have the declaration int foo(); this is fine and well defined if you have a definition for the same int foo(){ return 2;} then you will get redefinition error.

As stated by @haccks

6.2.1, the later declaration might hide the prior declaration.

Note the difference between declaration and definition.