On 20 October 2010 12:21, Ken Brown wrote: > On 10/20/2010 1:09 AM, Andy Koppe wrote: >>> Emacs creates a subprocess that runs an interactive bash shell. Emacs >>> wants >>> to get the PGID of the foreground process group associated to the tty of >>> this shell, and it does this on Linux via TIOCGPGRP (or equally well >>> tcgetpgrp). I think it uses the file descriptor of the master of the pty >>> for this purpose. If you (or some other programmer reading this) could >>> give >>> me the code for setting all this up, I could play with it and try to >>> figure >>> out why I'm seeing a difference between Linux and Cygwin here. I just >>> don't >>> know how to create a subprocess, give it a terminal, etc. >> >> Here's a test along those lines that does show a difference between >> Linux and Cygwin: >> >> #include<stdio.h> >> #include<pty.h> >> >> int main(void) >> { >> int pid, fd; >> pid = forkpty(&fd, 0, 0, 0); >> if (!pid) >> sleep(2); >> else { >> sleep(1); >> printf("pid=%i fd=%i pgrp=%i\n", pid, fd, tcgetpgrp(fd)); >> } >> } > > Thanks, Andy. I had no idea how to do this.
D'oh, I'd actually written a very similar test before, except there I'd looked at the slave side of the pty only: http://www.cygwin.com/ml/cygwin-developers/2009-10/msg00101.html So here's a test trying tcgetpgrp on both sides: #include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <pty.h> int main(void) { int pid, master_fd, slave_fd; openpty(&master_fd, &slave_fd, 0, 0, 0); pid = fork(); if (!pid) { close(master_fd); login_tty(slave_fd); sleep(2); } else { sleep(1); printf("pid=%i tcgetpgrp(master_fd)=%i tcgetpgrp(slave_fd)=%i\n", pid, tcgetpgrp(master_fd), tcgetpgrp(slave_fd)); } } Cygwin 1.5: pid=1072 tcgetpgrp(master_fd)=1072 tcgetpgrp(slave_fd)=1072 Cygwin 1.7: pid=2440 tcgetpgrp(master_fd)=0 tcgetpgrp(slave_fd)=-1 Linux: pid=23238 tcgetpgrp(master_fd)=23238 tcgetpgrp(slave_fd)=-1 I think the luit/tcsh problem that triggered the change from 1.5 to 1.7 only concerned the slave side. >> On Linux, where it requires -lutil to link, this gives: >> >> pid=13308 fd=3 tcgetpgrp(fd)=13308 > > I can confirm this on my Linux system. I mention this because apparently > tcgetpgrp isn't the same on all Linux systems. See below. > >> On Cygwin: >> >> pid=268 fd=3 tcgetpgrp(fd)=0 > > Corinna made tcgetpgrp return 0 instead of -1 in some circumstances (see > http://www.cygwin.com/ml/cygwin-patches/2009-q4/msg00045.html) because she > saw Linux doing that. But when I run Corinna's test on my Linux system, I > get -1 where she got 0. So not all Linuxes agree on what tcgetpgrp should > do. Hmm, Corinna's test calls tcgetpgrp(master) in the parent only before the child is forked and after it exited, so it's correct to report that there's no foreground process. I wonder which Linux it was that returned 0 in case of failure. I've tried it on a recent Opensuse, an old Redhat with a 2.6.9 kernel, and also a Debian with a 2.4 kernel, and got -1 on all of those. Andy -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple