Question about Cygwin's select()

Ken Brown
Wed Oct 19 20:15:00 GMT 2011

On 10/19/2011 1:32 PM, Ken Brown wrote:
> On 10/19/2011 12:22 PM, Christopher Faylor wrote:
>> On Wed, Oct 19, 2011 at 04:49:10PM +0200, Corinna Vinschen wrote:
>>> 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.
>> It isn't clear if you're getting the EINTR by looking at strace output
>> or from the code. If it's strace then be aware that sometimes the
>> output just reports the current errno regardless of whether there is an
>> error or not.
> I'm getting the EINTR by examining errno while running the program under
> gdb. But thanks for the heads-up about strace. I didn't know that.

I don't have a testcase yet, but I have a clearer idea of what's 
happening.  It actually has nothing to do with the gdb subprocess, but 
rather is a problem that can occur whenever emacs is running its main 
command loop.  emacs polls for keyboard input while also using select to 
check for output from subprocesses.  It's in this setting that select 
often fails with EINTR, even when there are no subprocesses running.  I 
wonder if the keyboard polling is doing something that interrupts the 
select call.

Unfortunately, the keyboard code (in src/keyboard.c) is full of alarms 
and timers and other things I know nothing about, so it's not going to 
be easy for me to come up with an STC.  I'll see if I can get help on 
the emacs-devel list.


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