"private function" doesn't mean "you can't know about this", it means "you shouldn't rely on this as a stable interface to my code".
Just because you can use the information you have to call a given function, doesn't mean you aren't violating an interface.
my point was that f() had been defined static then you can't access it from outside the translation unit it is defined in - in other words, it is "private". i'm afraid i'm unclear what your point is.