It's subtler than that. It's returning immediately when the number of file descriptors that it fails to add to a kqueue (that is actually going to do the work

It's subtler than that. It's returning immediately when the number of file descriptors that it fails to add to a kqueue (that is actually going to do the work and that it is setting up) is the same as the total number of file descriptors passed, which it is when they are both zero.