This is the mail archive of the cygwin mailing list for the Cygwin project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

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.

Sorry, I missed the point. In view of the use of TIOCGPGRP in emacs, there must be some unix-like systems (BSD?) where TIOCGPGRP is not subject to that limitation. Is it necessary to keep the limitation in Cygwin? I guess it boils down to how important Linux compatibility is, given that POSIX (as far as I know) is silent about TIOCGPGRP.

This is not a big deal from my point of view. As I said in my first message, I have a workaround for the most important uses in emacs. I think it might be more of an issue for mintty.


-- Problem reports: FAQ: Documentation: Unsubscribe info:

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]