Mail Archives: cygwin/2010/10/18/17:32:13
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.
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.
Ken
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