Static keyword in function declaration can be missing in function definition?

Question

I want to have a static function which I declare in my .c file before defining it:

//file a.c version 1
static int foo();
...
static int foo()
{
...
}
         


        

Answer 1

7.1.1/7:

The linkages implied by successive declarations for a given entity shall agree. That is, within a given scope, each declaration declaring the same object name or the same overloading of a function name shall imply the same linkage.

7.1.1/6: (Thanks Steve - this is also needed for the answer to be clear)

A name declared in a namespace scope without a storage-class-specifier has external linkage unless it has internal linkage because of a previous declaration and provided it is not declared const. Objects declared const and not explicitly declared extern have internal linkage.

Yes, those two are the same.

This however is invalid:

int foo();

static int foo() {
  return 0;
}