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Asked: 2026-06-12T09:01:24+00:00

I’m using epoll to get notifications about incoming data. It’s not hard because all

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I’m using epoll to get notifications about incoming data. It’s not hard because all events returned by epoll_wait() indicates, that I can read data from epoll_event.data.fd (socket descriptor).

But now I want get both types of notification: receiving and sending (socket is available for send). But I can’t to do it because:

  1. epoll_event.events which is returned by epoll_wait() is the same as I pass in epoll_ctl(). So it contains both EPOLLIN and EPOLLOUT in my case.
  2. Also if i trying to twice add one socket in epoll (as EPOLLIN and as EPOLLOUT event) I’ll get a EEXIST.

How can I solve this problem without manually calling select() every time I get notification?

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1 Answer

  1. Editorial Team

    man epoll_wait clearly states that “the events member will contain the returned event bit field.”. Therefore, if you are getting EPOLLIN | EPOLLOUT in epoll_event.events, then your socket must be ready for both reading and writing.

    If you only want to be notified when the socket changes state, use EPOLLET for edge-triggered operation.

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