There's at least one compiler that adds visibility into the name mangling (is it gcc? Seems like the kind of thing they'd do), so if you unhide functions in this way you still won't necessarily be able to call them.

(The compiler is also free to change the object layout when you do this, too, though I've no idea whether any of them do.)

C has no such keywords. Why the need for this hackery? Curious also if putting that in a header and including it in a C++ program even compiles without leading to undefined behaviour...