linux-kernel - Re: [patch/rfc] eventfd semaphore-like behavior

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Message-ID: <AANLkTim_seWr0LZJyC+dPBy-N+_NkzTx21vte7qrwunC@mail.gmail.com>
Date:	Mon, 30 Aug 2010 07:21:05 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Thu, Feb 5, 2009 at 2:02 AM, Davide Libenzi <davidel@...ilserver.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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