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    SubjectRe: [patch/rfc] eventfd semaphore-like behavior
    On Mon, 30 Aug 2010, Michael Kerrisk wrote:

    > 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.

    Looks fine to me.


    - Davide
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