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Date:	Wed, 20 Oct 2010 13:20:53 +0100
Message-ID:	<AANLkTinoRNBvUpiCGpVWXKLh4kYtyF=USrVhSuOX7_18@mail.gmail.com>
Subject:	Re: Sending signals to a subprocess
From:	Andy Koppe <andy DOT koppe AT gmail DOT com>
To:	cygwin AT cygwin DOT com
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On 20 October 2010 12:21, Ken Brown wrote:
> On 10/20/2010 1:09 AM, Andy Koppe wrote:
>>> Emacs creates a subprocess that runs an interactive bash shell. =C2=A0E=
macs
>>> wants
>>> to get the PGID of the foreground process group associated to the tty of
>>> this shell, and it does this on Linux via TIOCGPGRP (or equally well
>>> tcgetpgrp). =C2=A0I think it uses the file descriptor of the master of =
the pty
>>> for this purpose. =C2=A0If you (or some other programmer reading this) =
could
>>> give
>>> me the code for setting all this up, I could play with it and try to
>>> figure
>>> out why I'm seeing a difference between Linux and Cygwin here. =C2=A0I =
just
>>> don't
>>> know how to create a subprocess, give it a terminal, etc.
>>
>> Here's a test along those lines that does show a difference between
>> Linux and Cygwin:
>>
>> #include<stdio.h>
>> #include<pty.h>
>>
>> int main(void)
>> {
>> =C2=A0 int pid, fd;
>> =C2=A0 pid =3D forkpty(&fd, 0, 0, 0);
>> =C2=A0 if (!pid)
>> =C2=A0 =C2=A0 sleep(2);
>> =C2=A0 else {
>> =C2=A0 =C2=A0 sleep(1);
>> =C2=A0 =C2=A0 printf("pid=3D%i fd=3D%i pgrp=3D%i\n", pid, fd, tcgetpgrp(=
fd));
>> =C2=A0 }
>> }
>
> Thanks, Andy. =C2=A0I had no idea how to do this.

D'oh, I'd actually written a very similar test before, except there
I'd looked at the slave side of the pty only:
http://www.cygwin.com/ml/cygwin-developers/2009-10/msg00101.html

So here's a test trying tcgetpgrp on both sides:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pty.h>

int main(void) {
  int pid, master_fd, slave_fd;
  openpty(&master_fd, &slave_fd, 0, 0, 0);
  pid =3D fork();
  if (!pid) {
    close(master_fd);
    login_tty(slave_fd);
    sleep(2);
  }
  else {
    sleep(1);
    printf("pid=3D%i tcgetpgrp(master_fd)=3D%i tcgetpgrp(slave_fd)=3D%i\n",
           pid, tcgetpgrp(master_fd), tcgetpgrp(slave_fd));
  }
}

Cygwin 1.5:
pid=3D1072 tcgetpgrp(master_fd)=3D1072 tcgetpgrp(slave_fd)=3D1072

Cygwin 1.7:
pid=3D2440 tcgetpgrp(master_fd)=3D0 tcgetpgrp(slave_fd)=3D-1

Linux:
pid=3D23238 tcgetpgrp(master_fd)=3D23238 tcgetpgrp(slave_fd)=3D-1

I think the luit/tcsh problem that triggered the change from 1.5 to
1.7 only concerned the slave side.


>> On Linux, where it requires -lutil to link, this gives:
>>
>> pid=3D13308 fd=3D3 tcgetpgrp(fd)=3D13308
>
> I can confirm this on my Linux system. =C2=A0I mention this because appar=
ently
> tcgetpgrp isn't the same on all Linux systems. =C2=A0See below.
>
>> On Cygwin:
>>
>> pid=3D268 fd=3D3 tcgetpgrp(fd)=3D0
>
> Corinna made tcgetpgrp return 0 instead of -1 in some circumstances (see
> http://www.cygwin.com/ml/cygwin-patches/2009-q4/msg00045.html) because she
> saw Linux doing that. =C2=A0But when I run Corinna's test on my Linux sys=
tem, I
> get -1 where she got 0. =C2=A0So not all Linuxes agree on what tcgetpgrp =
should
> do.

Hmm, Corinna's test calls tcgetpgrp(master) in the parent only before
the child is forked and after it exited, so it's correct to report
that there's no foreground process.

I wonder which Linux it was that returned 0 in case of failure. I've
tried it on a recent Opensuse, an old Redhat with a 2.6.9 kernel, and
also a Debian with a 2.4 kernel, and got -1 on all of those.

Andy

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