non blocking tcp connect with epoll

Question

My linux application is performing non-blocking TCP connect syscall and then use epoll_wait to detect three way handshake completion. Sometimes epoll_wait returns with both POLLOUT & POLLERR revents set for the same socket descriptor. I would like to understand what's going on at TCP level. I'm not able to reproduce it on demand. My guess is that between two calls to epoll_wait inside my event loop we had a SYN+ACK/ACK/FIN sequence but again I'm not able to reproduce it. Any clue ?

Regards, Seb

Answer 1

It is likely for this to happen if the connect has failed - for example with "connection timed out" (for sockets doing a non-blocking connect, POLLOUT becomes set when the connect operation has finished for both successful and unsuccessful outcomes).

When POLLOUT becomes set for the socket, use getsockopt(sock, SOL_SOCKET, SO_ERROR, ...) to check if the connect succeeded or not (the SO_ERROR socket option is 0 in this case, and otherwise indicates why the connect failed).