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From: karl@tut.cis.ohio-state.edu (Karl Kleinpaste)
Newsgroups: comp.unix.wizards
Subject: Re: tty hangs in ^S state was (Re: Killing the printer daemon)
Message-ID: <212@tut.cis.ohio-state.edu>
Date: Fri, 25-Sep-87 08:31:32 EDT
Article-I.D.: tut.212
Posted: Fri Sep 25 08:31:32 1987
Date-Received: Sun, 27-Sep-87 02:04:47 EDT
References: <2419@drivax.UUCP> <366@cogen.UUCP>
Organization: Ohio State Computer & Info Science
Lines: 20

alen@cogen.UUCP writes:
   I've had a similar problem before, the printer was on a serial
   interface and the printer died when the line was in a ^S state.
   The daemon process which was in an i/o pending state would not die
   until we attached a tty and typed ^Q at it!!!!

The reason for this race condition is due to the fact that the tty
driver is trying to close as the process exits.  Closing the tty
implies that its output q must be drained first.  Hence, if the output
q drain is prevented by a TTSTOP state with ^S, the close will not
complete.

A proper solution to this problem is to set a timeout for the close to
complete, preferably in proportion to the number of characters waiting
to be drained and the speed in use on the line; if the timeout
expires, force the tty to flush rather than drain, and allow the close
to complete.  I was going to do this once in a SysIII PDP-11, had it
all ready to test and integrate, but never actually finished the job.
-- 
Karl