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Comment by rwmj | Hacker News Reader

It's weird that (according to this document) you can epoll Unix domain sockets but not sockets created by socketpair(2). I thought socketpair created essentially two pre-connected Unix domain sockets.

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Hmm, I don't think that's it says (unless they edited it since your post?). It mentions socketpair explicitly as something that is epoll-friendly, and which you can use to communicate with another thread, in the case where you must create a thread to perform some blocking task but still want to get completion notification in the main thread via epoll.

Indeed, I am all but certain you can epoll on socketpairs. That sounds like a mistake in the article.

You can and they do. The article says:

    There are four epoll-able ways for processes or threads to communicate:
    - UNIX socket pairs: man socketpair.

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