Only locking the close() is not enough. Another thread could still get the same file descriptor through whatever syscall while you loop around the close(). You would need to lock all syscalls that create or close a file descriptor which is a) infeasable and b) way too slow.

In other words: it's a race condition between allocating and deallocating a fd and not between two deallocations.

Which mutex? What about threads you don't 'own', inside libraries for instance? I don't think you will be able to find "one ring to rule them all"

Oh if they use the close direct, too bad. But libraries that launch the starships behind your back with no control left to you are also not the best gift.

Standard's bug aside, I wonder how the practical implementations behave. Is this condition reproducible ?