A very nice feature of epoll is that an epoll file descriptor (which monitors a set of file descriptors) can itself be monitored via epoll/select/poll. This mea

A very nice feature of epoll is that an epoll file descriptor (which monitors a set of file descriptors) can itself be monitored via epoll/select/poll. This means that if epoll is available, a library can abstract all its I/O via a single file descriptor. Think of a complex operation that involves multiple sockets and timer events. All the user needs to do is monitor this one file descriptor in his event loop, and call a function in the library which determines which of its own file descriptors have become ready, and reacts appropriately, possibly by calling callbacks if the user needs to be notified. Unfortunately if you're trying to be portable, you can't do this, and instead have to implement a complete event notification abstraction (unless you know you will only ever be dealing with this one file descriptor). E.g. user of the library needs to implement functions like addWait(fd, io_type, callback), delWait(wait_id), addTimeout(milliseconds, callback), delTimeout(timeout_id).