On Sun, Oct 09, 2011 at 07:58:15PM -0400, Christopher Faylor wrote: >On Sat, Oct 08, 2011 at 05:39:20PM -0400, Ken Brown wrote: >>Attached is a slight modification of the STC, in which I set stdin for >>the bash subprocess to /dev/null. With this modification, the program >>works as expected (and as on Linux) with cygwin-1.7.9, but the same >>problem as before occurs with the latest snapshot. > >I can duplicate this problem. I'll take a look. > >Thanks for the STC.
As far as I can tell, this is arguably a bug in bash. It's also an annoying compatibility problem between Linux and Cygwin. When the tty layer in Cygwin was first developed, the model (either in my head or in reality) was "If you don't have a tty and open a tty, that becomes your controlling tty". But, apparently that model changed over time to something more sensical where you have to explicitly use ioctl(TIOCSCTTY) to set up a controlling terminal. On Linux systems that would presumably be done via login(1). We don't normally run login on Cygwin, though. The problem is that when there is no controlling tty, bash uses a fallback mechanism to find the name of the tty by opening the tty associated with fd 0. It does this without setting the O_NOCTTY flag and, so, the act of opening the fd assigns a controlling terminal. This is not a problem on Linux (or FreeBSD for that matter) but it is a problem on Cygwin: it causes the aforementioned hang. So, bash could be modified to work around this issue but Cygwin is likely the only system left with this problem. So, I've modified Cygwin so that a controlling tty is not assigned if the fd being opened is > 2. There are still pathological situations which will cause problems with that but, for this particular problem, it seems to make bash behave better. Oh, and the reason why bash was "hanging" is because, thanks to the intracies of UNIX job control, it had received a SIGTSTP when an attempt was made to output to a tty for which bash was not the process group leader cgf -- 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
