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Date:	Wed, 20 Oct 2010 06:09:35 +0100
Message-ID:	<AANLkTikUc9i0h-g3QHGzqKKRGoJf0-xSPgu6wy1U+zZ=@mail.gmail.com>
Subject:	Re: Sending signals to a subprocess
From:	Andy Koppe <andy DOT koppe AT gmail DOT com>
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On 20 October 2010 04:17, Ken Brown <kbrown AT cornell DOT edu> wrote:
> Emacs creates a subprocess that runs an interactive bash shell. =C2=A0Ema=
cs 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 th=
e pty
> for this purpose. =C2=A0If you (or some other programmer reading this) co=
uld give
> me the code for setting all this up, I could play with it and try to figu=
re
> out why I'm seeing a difference between Linux and Cygwin here. =C2=A0I ju=
st 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)
{
  int pid, fd;
  pid =3D forkpty(&fd, 0, 0, 0);
  if (!pid)
    sleep(2);
  else {
    sleep(1);
    printf("pid=3D%i fd=3D%i pgrp=3D%i\n", pid, fd, tcgetpgrp(fd));
  }
}

On Linux, where it requires -lutil to link, this gives:

pid=3D13308 fd=3D3 tcgetpgrp(fd)=3D13308

On Cygwin:

pid=3D268 fd=3D3 tcgetpgrp(fd)=3D0

Neither of those looks POSIX-compliant to me, because tcgetpgrp should
return -1 since fd 3 isn't the controlling terminal of the calling
process, but the Linux behaviour is rather useful. Perhaps they
decided to apply that restriction only to the slave side?

Andy

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