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

Answer 1

Here is some good information on non-blocking tcp connect().

When a socket error is detected (i.e. connection closed/refused/timedout), epoll will return the registered interest events POLLIN/POLLOUT with POLLERR. So epoll_wait() will return POLLOUT|POLLERR if you registered POLLOUT, or POLLIN|POLLOUT|POLLERR if POLLIN|POLLOUT was registered.

Just because epoll returns POLLIN doesn't mean there will be data available to read, since recv() may just return the error from the non-blocking connect() call. I think epoll returns all the registered events with POLLERR to make sure the program calls send()/recv()/etc.. and gets the socket error. Some programs never check for POLLERR/POLLHUP and only catch socket errors on the next send()/recv() call.