Date: Fri, 13 Apr 2007 10:11:53 -0400
From: Bob Rossi <bob_rossi AT cox DOT net>
To: cygwin AT cygwin DOT com
Subject: Re: SIGTSTP and select
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On Fri, Apr 13, 2007 at 03:25:01PM +0200, Corinna Vinschen wrote:
> On Apr  3 10:13, Bob Rossi wrote:
> > On Mon, Apr 02, 2007 at 08:37:53PM -0400, Christopher Faylor wrote:
> > > On Mon, Apr 02, 2007 at 08:07:23PM -0400, Bob Rossi wrote:
> > > >When ctrl-z is typed, CGDB receives a SIGTSTP on both linux and cygwin.
> > > >When CGDB is at the select loop and this happens on linux select returns
> > > >-1 and errno is set to EINTR. My code simple does a 'continue' when this
> > > >happens and the select loop is reentered. All works well. On cygwin,
> > > >select does not return with -1. (I didn't check the return value but I
> > > >can, I just compare to -1 in an if statement). In fact, select also
> > > >detects that input is ready on stdin. This causes CGDB to get to a read
> > > >system call (which is non blocking) and the read system call fails with
> > > >errno set to EAGAIN. This causes CGDB to exit.
> > > >
> > > >The main loop looks something like this,
> > > >  if (select (max + 1, &rset, NULL, NULL, NULL) == -1)
> > > >    { 
> > > >      if (errno == EINTR)
> > > >        continue;
> > > >      ..
> > > >    }
> > > >
> > > >  if (FD_ISSET (STDIN_FILENO, &rset)) {
> > > >    handle_stdin
> > > >  }
> > > >
> > > >So, my question is, is there a bug with select on cygwin? Is select
> > > >working properly and I should handle the read call differently? Why does
> > > >it act differently than linux?
> > > 
> > > You say that something changed between different releases but you don't
> > > mention what those releases are.  Is this releases of Cygwin?  If so,
> > > what releases?
> > > 
> > > If this is as easy to demonstrate as you say, then a simple test case
> > > is definitely called for.
> > 
> > Hi Christopher,
> > 
> > Attached is the test case. If I run it under linux, and then type ctrl-z, I 
> > never get into the user_input_loop call. If I run it in cygwin, I do.
> > 
> > Hopefully I'm doing something wrong here. Please advise!
> 
> I'm a bit puzzled.  I don't see any difference in behaviour on Linux and
> Cygwin related to your testcase.  I have no problems to trigger the
> user_input_loop call on Linux and Cygwin.  After I press ctrl-z, I don't
> get into it on both systems.  After unsuspending the process, I get into
> user_input_loop on both systems again.  Either your testcase is wrong,
> or you should exactly specify what has to be typed to trigger the
> problem.  I tested this with Cygwin 1.5.24 and Linux 2.6.20.5, btw.

Hmmm, maybe I don't understand cygwin well enough. Does it require me to
have CYGWIN=tty to have this functionality work properly?

If I use the exact same test case I just sent in, and set CYGWIN=tty, I
still get different results than on linux, but it seems better. On
cygwin, select returns with value -1 and errno set to EINTR. On
linux, as I just posted, that doesn't seem to happen.

Should I assume that if cygwin users want to use cgdb that they must set
CYGWIN=tty, or is that a bad assumption to make?

Thanks,
Bob Rossi

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