Sending signals to a subprocess

Ken Brown
Mon Oct 18 21:39:00 GMT 2010

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;
>>>>> #ifdef TIOCGPGRP
>>>>>       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.


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