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    SubjectRe: [patch/rfc] eventfd semaphore-like behavior
    On Thu, Feb 5, 2009 at 2:02 AM, Davide Libenzi <davidel@xmailserver.org> wrote:
    > On Thu, 5 Feb 2009, Michael Kerrisk wrote:
    >
    >> > Dunno.  Probably try the syscall and see if it returned -EINVAL.  Does
    >> > that work in this case?
    >>
    >> As youll have seen by now, Ulrich and I noted that it works.
    >>
    >> > If so, it would be sensible to mention this in
    >> > the description somewhere as the approved probing method and to
    >> > maintain it.
    >>
    >> I'll add something to the man page, as this patch progresses.
    >
    > I see we already have stuff like this inside the man pages:
    >
    > O_CLOEXEC (Since Linux 2.6.23)
    >          Enable the close-on-exec flag for the new file descriptor.
    >          ...
    >
    > Maybe a similar note for the new flag?

    It took a while, but here's the new text in the eventfd.2 (will be in
    man-pages-2.36). Could you please ACK, Davide?

    EFD_SEMAPHORE (since Linux 2.6.30)
    Provide semaphore-like semantics for reads from
    the new file descriptor. See below.
    ...
    * If EFD_SEMAPHORE was not specified and the
    eventfd counter has a nonzero value, then a
    read(2) returns 8 bytes containing that value,
    and the counter's value is reset to zero.

    * If EFD_SEMAPHORE was specified and the eventfd
    counter has a nonzero value, then a read(2)
    returns 8 bytes containing the value 1, and the
    counter's value is decremented by 1.

    Thanks,

    Michael

    --
    Michael Kerrisk
    Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
    Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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