Question about Cygwin's select()

Corinna Vinschen
Wed Oct 19 14:49:00 GMT 2011

On Oct 19 09:28, Ken Brown wrote:
> I'm trying to debug an emacs problem, and I'm running into something
> I don't understand involving Cygwin's select.  I'll try to make an
> STC if necessary, but I thought I'd start with a verbal description
> in case there's an easy answer.  Here's the situation:
> emacs creates a subprocess running gdb and sends a bunch of commands
> to gdb without immediately reading the resulting output.  emacs then
> goes into a loop in which it waits for keyboard input and
> periodically calls select to check for output from subprocesses.
> The first call to select has a 30 second timeout and *always* fails
> with EINTR.  As a result, emacs doesn't read the output from gdb
> right away and doesn't properly initialize the gdb buffer.
> Subsequent calls to select sometimes succeed and sometimes fail.
> When I'm running emacs under gdb and stepping through it, the buffer
> eventually gets initialized.  When I'm running emacs outside of gdb,
> the buffer doesn't get initialized until I press Return.
> I have a simple workaround for this, but I'd like to be sure there
> isn't some underlying bug that I'm masking with the workaround.  So
> my question is this: Is there some reason that I should expect that
> first call to select to consistently fail with EINTR, or might this
> indicate a bug?
> I realize that it might not be possible to answer the question based
> on the information I've provided, but I thought it was worth a try.

Some details are missing like the objects used to communicate with GDB.
Does Emacs use a pseudo tty or a pipe?  Is that with Cygwin from CVS or
with 1.7.9?  And what's your workaround?  The EINTR sounds weird.  A
testcase would be most helpful.


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

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