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From: Davide Libenzi <davidel@xmailserver.org>
To: "Chris \"ク\" Heath" <chris@heathens.co.nz>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>,
	David Schwartz <davids@webmaster.com>,
	dada1@cosmosbay.com,
	"Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
	linux-man@vger.kernel.org
Subject: Re: epoll design problems with common fork/exec patterns
Date: Wed, 27 Feb 2008 11:35:11 -0800 (PST)	[thread overview]
Message-ID: <Pine.LNX.4.64.0802271131180.377@alien.or.mcafeemobile.com> (raw)
In-Reply-To: <1204075804.5238.7.camel@linux.heathens.co.nz>

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On Tue, 26 Feb 2008, Chris "ã~B¯" Heath wrote:

> On Tue, 2008-02-26 at 10:51 -0800, Davide Libenzi wrote:
> > 
> > Yes, you can't add the same fd twice. Think about a DB where "file*,fd" is 
> > the key.
> 
> To clarify, the key appears to be file* plus the user-space integer that
> represents the fd.

Yes, that's what I said.



> > > c) It is possible to add duplicated file descriptors referring to the same
> > > underlying open file description ("file *").  As you note, this can be a
> > > useful filtering technique, if the two file descriptors specify different
> > > masks.
> > > 
> > > Assuming that is all correct, for man-pages-2.79, I've reworked the text
> > > for Q1/A1 as follows:
> > > 
> > >        Q1     What  happens  if you add the same file descriptor
> > >               to an epoll set twice?
> > > 
> > >        A1     You will probably get EEXIST.  However, it is pos-
> > >               sible   to   add  a  duplicate  (dup(2),  dup2(2),
> > >               fcntl(2) F_DUPFD, fork(2)) descriptor to the  same
> > >               epoll  set.   This  can  be a useful technique for
> > >               filtering events, if the duplicate  file  descrip-
> > >               tors are registered with different events masks.
> > > 
> > > Seem okay Davide?
> > 
> > Looks sane to me.
> 
> I think fork(2) should not be in the above list.  fork(2) duplicates the
> kernel's fd, but the user-space integer that represents the fd remains
> the same, so you will get EEXIST if you try to add the fd that was
> duplicated by fork.

Good catch, fork(2) should not be there.



- Davide


  reply	other threads:[~2008-02-27 19:35 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-10-27  6:22 Marc Lehmann
2007-10-27  8:23 ` Eric Dumazet
2007-10-27  8:51   ` Marc Lehmann
2007-10-27  9:22     ` Eric Dumazet
2007-10-27  9:34       ` Marc Lehmann
2007-10-27 10:23         ` Eric Dumazet
2007-10-27 10:46           ` Marc Lehmann
2007-10-27 16:59     ` Davide Libenzi
2007-10-27 17:38       ` Willy Tarreau
2007-10-27 18:01         ` Davide Libenzi
2007-10-29 22:36         ` Mark Lord
2007-10-28  4:47       ` David Schwartz
2007-10-28  9:33         ` Eric Dumazet
2007-10-28 21:04           ` David Schwartz
2007-10-29 18:55             ` Davide Libenzi
2008-02-26 15:13               ` Michael Kerrisk
2008-02-26 18:51                 ` Davide Libenzi
2008-02-27  1:30                   ` Chris "ク" Heath
2008-02-27 19:35                     ` Davide Libenzi [this message]
2008-02-28 13:12                       ` Michael Kerrisk
2008-02-28 13:23                         ` Michael Kerrisk
2008-02-28 19:34                           ` Davide Libenzi
2008-02-28 19:23                         ` Davide Libenzi
2008-02-29 15:46                           ` Michael Kerrisk
2008-02-29 19:19                             ` Davide Libenzi
2008-02-29 19:54                               ` Michael Kerrisk
2008-03-02 15:11                                 ` Sam Varshavchik
2008-03-02 21:44                                   ` Davide Libenzi
2007-10-28 18:48         ` Davide Libenzi

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