Date: Thu, 16 Oct 2003 11:39:41 +0200 From: Boris Nikolaus <boris@cs.tu-berlin.de> To: John-Mark Gurney <gurney_j@efn.org>, John Polstra <jdp@polstra.com>, freebsd-bugs@freebsd.org Subject: Re: kern/45291: kevent(2) ignores timeout if nevents == 0 Message-ID: <20031016093941.GB23308@fiesta.cs.tu-berlin.de> In-Reply-To: <20031013194716.GP533@funkthat.com> References: <200310130749.h9D7nWF9038579@freefall.freebsd.org> <XFMail.20031013093759.jdp@polstra.com> <20031013194716.GP533@funkthat.com>
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On Mon, Oct 13, 2003 at 12:47:16PM -0700, John-Mark Gurney wrote:
> John Polstra wrote this message on Mon, Oct 13, 2003 at 09:37 -0700:
> > On 13-Oct-2003 John-Mark Gurney wrote:
> > > Synopsis: kevent(2) ignores timeout if nevents == 0
> > >
> > > State-Changed-From-To: open->feedback
> > > State-Changed-By: jmg
> > > State-Changed-When: Mon Oct 13 00:46:53 PDT 2003
> > > State-Changed-Why:
> > > to quote the man page: If timeout is a non-NULL pointer, it specifies
> > > a maximum interval to wait for an event,
> > >
> > > The behavior is correct since you wanted to wait for 0 events, and since
> > > it has delivered all the events it can (none), it returns. Unless provide
> > > with good reason (or maybe a patch to the manpage to document this feature?)
> > > I will close the PR in 5 days.
I still disagree:
To quote the poll and select man pages:
poll: If timeout is neither zero nor INFTIM (-1), it specifies a maximum
interval to wait for any file descriptor to become ready, in
milliseconds.
select: If timeout is a non-nil pointer, it specifies the maximum interval to
wait for the selection to complete.
In both cases, it is a very similar text to the kevent man page, but for
poll and select, the behaviour is to wait if nfds = 0 (and so no event
can be returned).
> Simple fix, you wait for a single event, adding a struct kevent such as:
> struct kevent ke;
> ret = kevent(qfd, NULL, 0, &ke, 1, &ts);
>
> makes it wait the full 10 seconds. so, are you suggesting that we change
> the behavior to wait for the timeout to expire before we return ANY
> events? (because that is what your suggesting). Special caseing when
> someone passes in 0 is not good IMO... it will probably brake lots of
> code that may in advertantly set as ts, but expect immediate return
> because of not specifing any return events.
I think, you misunderstood me: I do not want kevent to wait for the timeout
to expire before returning ANY events, I want kevent to wait for the timeout
when NO event has signalled! This is not a special case, I think the current
implementation handles nevent = 0 as special case, as it returns before the
timeout has expired although no event has been signalled.
Your fix to call kevent with nevents = 1 will remove an event from the event
queue. This may be a solution for the trivial example program of the PR
(as there are no events registered for the queue) but not the expected
behaviour in a real main loop.
To summarize, I think kevent should not return until
- it has found any signalled events, or
- the timeout expires, or
- a (non-ignored) signal arrives
as this is the behaviour of poll() and select() and IMHO the intuitive
behaviour.
Greetings,
Boris
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