From: utzoo!decvax!cca!ima!ism780!jim
Newsgroups: net.unix-wizards
Title: Re: sending signals to print spoolers (Rich Wales)
Article-I.D.: ism780.3
Posted: Thu Sep  2 12:48:00 1982
Received: Wed Sep  8 05:28:13 1982

I thought everyone (except perhaps the designers of UNIX) was aware of
the fact that you can't use normal UNIX signals for IPC because, upon
receipt, they revert to the default action, viz., *kill this process*.
In our printer spooler (running on PWB, V7, and System III) we used
SIGTRAP (not reset when caught); since this was designed in, we never
encountered any "horrible problem".  The other problem with UNIX signals
for IPC is that there is no way to avoid race conditions, thereby missing
a signal.  We don't use sleep, which works poorly in conjunction with
alarm in V7, perhaps less so in System III.  We set an alarm for a minute
or so and then do a wait (if we have subprocesses running) or a pause,
with a check at the last possible moment for the flag which is set by the
signal-catch routine.  I suppose sigset is fine if you have it, but it
ain't UNIX!
				-- Jim Balter
				   INTERactive Systems

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