epoll ET, what events should I subscribe to on listening socket?

Question

I have a fd:

socket(AF_INET6, SOCK_STREAM, ...)
bind(fd, ...)
listen(fd, ...)

And I have a epoll instance. I need to know, what events should I subscribe to via epoll_ctl ? I need Edge Triggered mode.

I have those flags atm: EPOLLET | EPOLLIN EPOLLET | EPOLLIN

Should I subscribe to EPOLLRDHUP , EPOLLOUT , EPOLLPRI ?

Should I handle EPOLLHUP , EPOLLERR ? Why they can occur?

Answer 1

You ask specifically about the passive socket used to accept incoming connection requests. The example in man epoll doesn't subscribe to EPOLLRDHUP , EPOLLOUT , EPOLLPRI for the listening socket, and understandably so:

• EPOLLOUT means: Writing now will not block. - There is no writing on the passive socket.
• EPOLLRDHUP applies to connections. There isn't a connection on the passive socket.
• EPOLLPRI means: There is urgent data to read . There's no urgent data on a passive socket.

Also the example doesn't handle EPOLLHUP , EPOLLERR for the listening socket. That's at least understandable for EPOLLHUP , since that applies to output only . I know of no case where EPOLLERR would be returned for the passive socket.