This is similar to the question 1 but is about pipes. If epoll_wait returns EPOLLERR for the write end of a pipe, how do I distinguish a general error condition from the read end of the pipe been closed? For the sockets case the answer was to use “use getsockopt and SO_ERROR to get the pending error” and compare that with EPIPE. But what API should I use for the pipe as the pipe created by pipe(2) on Linux or returned by opening a named pipe is not a socket?
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Answer
Other than the other end being closed, there aren’t any errors on a write end of a pipe which could be detected by epoll. The few errors that are possible on an actual call to write() could not be known in advance (e.g., if you pass an invalid buffer pointer), so epoll cannot detect them. Therefore, if epoll tells you there’s an error, it is EPIPE.
(OK, there is actually another possible error condition, but it can be triggered only by a programming error: if you close the file descriptor and then use epoll_wait while it is on the list – I don’t know how epoll will react to that).