kern/45291: kevent(2) ignores timeout if nevents == 0

Boris Nikolaus boris at
Thu Oct 16 02:42:25 PDT 2003

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
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


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