/dev/windows and select() [was Re: Slow response to keypresses in xorg-server-1.8.0-1]

Corinna Vinschen corinna-cygwin@cygwin.com
Sun Aug 29 14:50:00 GMT 2010

On Aug 29 16:17, Corinna Vinschen wrote:
> 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?
> > >[...]
> Thanks for the testcase.  I examined this and I think I have a
> workaround.  MSDN states that there's a flag QS_ALLPOSTMESSAGE for
> MsgWaitForMultipleObjects, which is not cleared by PeekMessage, if the
> wMsgFilterMin and wMsgFilterMax arguments are not both 0.  So, what I
> did was to add the QS_ALLPOSTMESSAGE flag to the
> MsgWaitForMultipleObjects call in select.cc, and to change the
> PeekMessage call in select.cc:peek_windows() from
>   PeekMessage (&m, (HWND) h, 0, 0, PM_NOREMOVE)
> to
>   PeekMessage (&m, (HWND) h, 1, UINT_MAX, PM_NOREMOVE)
> Same in your above test application.  This appears to do the trick.
> However, I'm not exactly sure if that's a valid fix.  Patch below.

Hmm, this ignores the potential WM_NULL message, afaics.  For some
reason, using

  PeekMessage (&m, (HWND) h, 0, UINT_MAX, PM_NOREMOVE)

results in MsgWaitForMultipleObjects hanging, too.  OTOH, using

  PeekMessage (&m, (HWND) h, 0, 16, PM_NOREMOVE)
  && PeekMessage (&m, (HWND) h, 17, UINT_MAX, PM_NOREMOVE)

does not.  Go figure.


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

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