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Re: Sending signals to a subprocess

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:


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