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    SubjectRe: epoll design problems with common fork/exec patterns
    Marc Lehmann a écrit :
    > On Sat, Oct 27, 2007 at 10:23:17AM +0200, Eric Dumazet <> wrote:
    >>> In this case, the parent process works fine until the child closes fds,
    >>> after which the fds become unarmed in the parent too. This works as
    >> I have no idea what exact problem you have.
    > Well, I explained it rather succinctly, I think. If you tell me whats unclear
    > I can explain...
    >> But if the child closes some
    >> file descriptor that were 'cloned' at fork() time, this only decrements a
    >> refcount, and definitely should not close it for the 'parent'.
    > It doesn't. It removes it from the epoll set, though, so the parent will not
    > receive events for that fd anymore.
    >> I have some apps that are happily using epoll() and fork()/exec() and have
    > The problem I described is fork/close/exec. close being the explicit
    > syscall.
    >> no problem at all. I usually use O_CLOEXEC so that all close() are done at
    >> exec() time without having to do it in a loop. epoll continues to work as
    >> expected in the parent process.
    > This is because epoll doesn't behave like documented: It removes the fd
    > from the parents epoll set only on an explicit close() syscall, not on an
    > implicit close from exec.
    >>> fd sets. This would explain the behaviour above. Unfortunately (or
    >>> fortunately?) this is not what happens: when the fds are being closed by
    >>> exec or exit, the fds do not get removed from the epoll set.
    >> at exec() (granted CLOEXEC is asserted) or exit() time, only the refcount
    >> of each file is decremented. Only if their refcount becomes NULL, files are
    >> then removed from epoll set.
    > Yes. But thats obviously not the only way to close fds.
    >>> Is epoll really designed to be so incompatible with the most commno fork
    >>> patterns? Shouldn't epoll do refcounting, as is commonly done under
    >>> Unix? As the fd space is not shared between rpocesses, why does epoll
    >>> try? Shouldn't the epoll information be copied just like the fd table
    >>> itself, memory, and other resources?
    >> Too many questions here, showing lack of understanding.
    > You already said you don't the problem. No need to get insulting :(
    >> epoll definitly is not useless. It is used on major and critical apps.
    >> You certainly missed something.
    > Well, it behaves like documented, which is the problem. You admit you
    > don't understand the problem or the documentation, so again, no need to
    > insult me.

    Hum... I will update my english vocabulary and mark "missed" as an insult.

    I have no problem with epoll nor its documentation.

    >> Please provide some code to illustrate one exact problem you have.
    > // assume there is an open epoll set that listens for events on fd 5
    > if (fork () = 0)
    > {
    > close (5);
    > // fd 5 is now removed from the epoll set of the parent.
    > _exit (0);
    > }

    It doesnt on every kernels I had played with. And I played with *lot* of
    kernels you know.

    If such a bug exists on your kernel, please fill a complete bug report, giving

    Thank you

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      Last update: 2007-10-27 11:25    [W:0.024 / U:25.804 seconds]
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