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Date: Mon, 30 Aug 2010 13:23:13 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Subject: Re: /dev/windows and select() [was Re: Slow response to keypresses in xorg-server-1.8.0-1]
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On Aug 29 14:39, Jon TURNEY wrote:
> On 08/08/2010 12:04, Andy Koppe wrote:
> >On 7 August 2010 23:07, Jon TURNEY wrote:
> >>Hmmm, looking again at the implementation of select(), I don't immediately
> >>see that when waiting on /dev/windows, it checks that the message queue has
> >>old messages on it before waiting.  The MSDN documentation for
> >>MsgWaitForMultipleObjects() seems to says that messages which had arrived
> >>before the last PeekMessage() etc. aren't considered new and so don't end
> >>the wait?
> >
> >I think you're right, a call to PeekMessage is needed for proper
> >select() semantics: it shouldn't block if data is available for
> >reading.
> 
> Attached is a small test-case which seems to demonstrate this problem.
> 
> Run ./dev-windows-select-test and observe select() blocks for the
> full timeout, despite the fact that the /dev/windows fd is ready for
> reading (and it reported as such as the end of the timeout)
> 
> If you run './dev-windows-select-test -skip' to skip the
> PeekMessage(), select() returns immediately, indicating the
> /dev/windows fd is ready for reading.

Again, thanks for the testcase.  I applied a patch to Cygwin which
should make select on /dev/windows fully functional.

The blessed solution is to replace the call to MsgWaitForMultipleObjects
with a call to MsgWaitForMultipleObjectsEx with the MWMO_INPUTAVAILABLE
flag set.  This flag is defined to do exactly what we need.

The only downside is that this flag does not exist on NT4 and its usage
results in an "invalid argument" error.  So, for NT4, I added the
workaround I described in my yesterday's soliloquy.

I'm planning to release Cygwin 1.7.7 tomorrow at the latest, so please
give it a test as soon as possible.  Here's a binary DLL for testing
(build w/o optimization, so it's probably a bit slow):

  http://cygwin.de/cygwin-177/new-cygwin1.dll.bz2
  (md5sum: 7e07fd9eafd021697a0861c1ae4fa94e)


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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