Cygwin select() issues and improvements

Corinna Vinschen
Mon Feb 15 12:57:00 GMT 2016

On Feb 14 03:09, john hood wrote:
> [I Originally sent this last week, but it bounced.]
> Various issues with Cygwin's select() annoyed me, and I've spent some
> time gnawing on them.
> * With 1-byte reads, select() on Windows Console input would forget
> about unread input data stored in the fhandler's readahead buffer.
> Hitting F1 would send only the first ESC character, until you released
> the key and another Windows event was generated.  (one-line fix, though
> I'm not sure it's appropriate/correct)

I think so, yes.  I applied this patch, thank you.

With the other patches I have two problems.

One of them is that they are not trivial enough to be acceptable without
copyright assignment (except patch 3, but see below).  Please have a
look at, the "Before you get started"
section.  There's a link to an "assign.txt" file with instructions.

The other one is just this:  Can you please describe each change in the
accompanying patch comment so that it's accessible from the git log?

> * Newer versions of Windows may return early on timers, causing select()
> to return early. (fixed, but other timers in Cygwin still have this problem)

It would be nice if we could discuss this a bit more in detail.  I wasn't
aware of this change.

> * The main loop in select() has some pretty tangled logic.  I improved
> that some.  There's room for more improvement


> but that would need
> changing fhandlers and related functions.

If it makes sense, sure.

> Windows scheduling in general seems to be rather poor for Cygwin
> processes, and there are scheduling differences between processes run in
> Windows console (which are seen as interactive by the scheduler) and
> processes run in mintty (which are not).  There doesn't seem to be any
> priority promotion for I/O as you see on most Unix schedulers.

I'm not aware of such a feature.

> I've attached plausible patches; they're also available on
> <>, branch 'microsecond-select'.
> I think it's all Windows XP compatible.  I've put some test programs up
> at <> too.

XP/2K3 compatibility is not required anymore.  If you think you need
a feature which is only available on Vista or later, feel free.  We just
have to inform the Cygwin mailing list, as promised.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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