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c++clinuxepoll

Is it valid to add a file descriptor to epoll with events set to zero?

Is it valid to add a file descriptor to epoll with epoll_event.events set to zero? What can be expected to happen when calling epoll_wait? See abridged example:

struct epoll_event event = {};
event.data.fd = fd;
event.events = 0;

epoll_ctl(efd, EPOLL_CTL_ADD, fd, &event);
epoll_wait(efd, &event, 1, -1);

What semantics can I rely on from the epoll_wait call in this instance? Are there any events that are still delivered even when events = 0?

Solution

  • It should be valid, and the EPOLLERR and EPOLLHUP are always included even if you're not requesting them, so setting events = 0 will still respond on those 2 events. Remember to check the return value of epoll_ctl though.

    The documentation states:

    The events member is a bit mask composed by ORing together zero or more of the following available event types:

    EPOLLERR Error condition happened on the associated file descriptor. This event is also reported for the write end of a pipe when the read end has been closed. epoll_wait(2) will always report for this event; it is not nec‐ essary to set it in events.

    EPOLLHUP Hang up happened on the associated file descriptor. epoll_wait(2) will always wait for this event; it is not necessary to set it in events.