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SubjectRe: select() on /dev/input/eventX not level-triggered?
On Mon, May 23, 2011 at 10:06:22PM +0200, Pavel Machek wrote:
> On Mon 2011-05-23 11:13:12, Dmitry Torokhov wrote:
> > On Fri, May 20, 2011 at 01:50:51AM +0200, Pavel Machek wrote:
> > > Hi!
> > >
> > > I'm debugging strange behaviour on /dev/input/eventX ... it seems that
> > > select is not behaving level-triggered as it apparently should. I can
> > > reproduce it when hitting windows & alt keys.
> > >
> > > Now, I really should rewrite it into C, first, but perhaps someone has
> > > an idea?
>
> (Ok, so we agree that select() should mark descriptor as ready as
> long as data are available... right?)
>
> > Is this with next or with mainline? In next we try not to signal that FD
> > is ready unless we have full packet in the buffer...
>
> 2.6.39-rcX mainline.
>
> > FWIW I see python indeed not reading the tail of events (btw the format
> > should be 'llhhi' and the size on 64 bit arches is 24, not 16) but when
> > I hacked evtest to use select and non-blockign read it all worked
> > properly.
>
> It was on x32, but I'll fix that, thanks.
>
> Difference was I'm not using non-blocking read().

Does not matter.

> It is unusual but
> should work AFAICT. Just read one packet when select shows its ready;
> if there's more than one, don't loop around read, but rely on select
> returning immediately.
>
> I did verify it on strace, so it should not be python artefact.

It is artefact of your program, you buffering your input. Use:

file = open("/dev/input/event3", "rb", 0)

Thanks.

--
Dmitry


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