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Re: Sending signals to a subprocess

On 10/18/2010 4:18 PM, Christopher Faylor wrote:
On Mon, Oct 18, 2010 at 03:40:21PM -0400, Ken Brown wrote:
On 10/18/2010 2:34 PM, Christopher Faylor wrote:
On Sat, Oct 16, 2010 at 02:06:56PM -0400, Ken Brown wrote:
On 10/16/2010 1:17 PM, Ken Brown wrote:
I could use some help fixing a longstanding bug in the Cygwin build of
emacs, in which emacs is unable to send signals to subprocesses.  A
symptom from the user's point of view is that one cannot interrupt a
process in shell mode by typing C-c C-c.  I've found a workaround that
handles that case (SIGINT), as well as SIGQUIT and SIGTSTP.  But as long
as I'm fixing this, I'd like to do it right and figure out how to handle
all signals.

This boils down to finding the right process group ID to pass to 'kill'.
On systems that have TIOCGPGRP, emacs uses the following code (in
src/process.c) to get this ID:

/* Return the foreground process group for the tty/pty that
       the process P uses.  */
static int
emacs_get_tty_pgrp (p)
         struct Lisp_Process *p;
      int gid = -1;

      if (ioctl (p->infd, TIOCGPGRP,&gid) == -1&&    ! NILP (p->tty_name))
          int fd;
          /* Some OS:es (Solaris 8/9) does not allow TIOCGPGRP from the
	 master side.  Try the slave side.  */
          fd = emacs_open (SDATA (p->tty_name), O_RDONLY, 0);

          if (fd != -1)
	  ioctl (fd, TIOCGPGRP,&gid);
	  emacs_close (fd);
#endif /* defined (TIOCGPGRP ) */

      return gid;

What's the right way to do this in Cygwin?

I guess it's clear from the context, but I should have said that the problem only arises when emacs has to communicate with the subprocess through a tty that is not the controlling tty of emacs. So tcgetpgrp() doesn't work.

I am a little confused as to the difference between tcgetpgrp and TIOCGPGRP given this man page description from "man 4 tty_ioctl" on linux:

         TIOCGPGRP pid_t *argp
                When successful, equivalent to *argp = tcgetpgrp(fd).
                Get the process group ID of the foreground process group on this terminal.

         TIOCSPGRP const pid_t *argp
                Equivalent to tcsetpgrp(fd, *argp).
                Set the foreground process group ID of this terminal.

Do you have a simple test case which demonstrates the difference between
the calls?  It seems odd that TIOCGPGRP would allow more access to a tty
than tcgetpgrp.

The difference is that, according to POSIX, tcgetpgrp is required to fail unless fd references the controlling terminal of the calling process. Ironically, Cygwin's tcgetpgrp used to succeed in this situation until Corinna fixed it a year ago:

Yes, I got that but TIOCGPGRP seems to have that same limitation on Linux. That's why I quoted the above man page. A simple test case (tm) seems to bear out the fact that the two are the same.

I just tried an experiment, and now I'm thoroughly confused. I inserted "#undef TIOCGPGRP" into process.c in the emacs source and rebuilt it on Linux. [Technical note if anyone wants to try to reproduce this: I also inserted "#undef SIGNALS_VIA_CHARACTERS", since SIGNALS_VIA_CHARACTERS provides an alternate method of sending signals to processes; this is in fact my workaround on Cygwin.] Then trying to kill a process running in an emacs shell with C-c C-c fails the same way it fails in Cygwin. So somehow TIOCGPGRP is doing the right thing under Linux in the emacs code above, in spite of its limitations. I don't understand why. When I get a chance (not today), I'll try running emacs under gdb to see if I can figure out what's going on.

I guess this should mean that if you implement TIOCGPGRP in Cygwin and make it emulate Linux, it should work for emacs in Cygwin too. I can also try to see if tcgetpgrp works instead of TIOCGPGRP. I'm embarrassed to say that I didn't actually try this before, because my understanding of the documentation was that it wouldn't work. You can see I don't think like a programmer.


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