Re: Question about Cygwin's select()

[Ken Brown <kbrown@cornell.edu>]

On 10/19/2011 10:49 AM, 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.

Sorry for the missing details.  Emacs uses a pseudo tty.  I'm currently 
using the latest Cygwin snapshot, but I'm pretty sure I saw the same 
behavior with 1.7.9.  I'll recheck that.  My workaround is to force 
emacs to check for process output earlier in the initialization phase. 
For reasons I don't understand, the select call used at that point 
always succeeds.

I think you've answered my basic question by saying that the EINTR 
sounds weird.  I'll try to make a testcase.

Ken


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