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From: mouse@mcgill-vision.UUCP (der Mouse)
Newsgroups: comp.unix.questions,comp.unix.wizards
Subject: Re: question - 4bsd ^Z handling and stopping a pipe
Message-ID: <827@mcgill-vision.UUCP>
Date: Fri, 26-Jun-87 05:29:33 EDT
Article-I.D.: mcgill-v.827
Posted: Fri Jun 26 05:29:33 1987
Date-Received: Sat, 4-Jul-87 23:06:49 EDT
References: <13907@teknowledge-vaxc.ARPA>
Organization: McGill University, Montreal
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Xref: mnetor comp.unix.questions:3018 comp.unix.wizards:3047

In article <13907@teknowledge-vaxc.ARPA>, mkhaw@teknowledge-vaxc.ARPA (Michael Khaw) writes:
> I've found that when I display a message in elm and ^Z at the "more"
> prompt, I'm hung.  [analysis]

> Are there any 4bsd gurus who can explain what a SIGTSTP handler needs
> to do to prevent this?  Currently Elm's SIGTSTP handler just restores
> default handling (SIG_DFL) and then does a kill(0, SIGTSTP).

Then I would say that elm is broken.  It should be doing a
kill(getpid(),SIGTSTP) instead of kill(0,SIGTSTP).  The latter form
sends the signal to the entire process group, all of which have already
received it from the user's keystroke and may misinterpret another
SIGTSTP.

But that's not why I think your elm is hanging.  The failure mode I
expect is occurring is that more, or the shell you saw running with the
-c option, probably takes over the process group of the terminal.  Then
when you type ^Z, the more and its shell get stopped.  The elm,
however, never sees the signal, 'cause its process group is different
(remember, someone stole the tty).  Now nobody is listening to your
keystrokes!  More and its shell are suspended, elm is waiting for the
shell it forked to exit (as opposed to just stop), and your top-level
shell is waiting for elm to do something (anything).  Killing the more
and shell wake the elm back up and everybody recovers.

					der Mouse

				(mouse@mcgill-vision.uucp)