On Tue, Oct 19, 2010 at 09:32:34AM -0400, Ken Brown wrote:
>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:
>>>
>>> http://www.cygwin.com/ml/cygwin-patches/2009-q4/msg00045.html
>>
>> 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.
As I mentioned, my test case shows that when run on Linux, TIOCPGRP and
tcgetpgrp are the same. So, given that, if I implemented TIOCPGRP it
wouldn't solve your problem.
How about if you modify the test case that I provided and show me what's
different? Is this an issue with opening the wrong side of a pty, like
trying to run tcgetpgrp on the master rather than the slave or vice versa?
cgf
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