SIGIO vs epoll for Linux sockets

Question

The socket documentation for linux ( man 7 socket ) says that you can set your socket to be O_ASYNC and then receive a signal when the socket is ready for read/write.

However, it seems most people use epoll instead. What is the reason for using epoll rather than this asynchronous signaling system?

Answer 1

If you have a central loop where you catch all kind of events makes it very easy to write a single threaded application and you don't have to take care about all the synchronization problems which may occur if you are running with different execution contexts.

If you use a signal handler you must take care that you never call a non-reentrant function from the signal handler context. There is a list of Async-signal-safe functions you are allowed to call. And as you can see, it is a short list! As a result your signal handler can not do much, maybe only set a flag or send a message and the real work must be done "somewhere". In fact, signal handlers are very limited.

And using signal handlers in multi threaded applications is also not so easy as it looks in the first place, as the handler is per task and not per thread. Read more: signal handler function in multithreaded environment