Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!ptsfa!ames!oliveb!sun!gorodish!guy
From: guy@gorodish.UUCP
Newsgroups: comp.unix.questions
Subject: Re: SIGUSR1, SIGUSR2 are pretty useless
Message-ID: <19926@sun.uucp>
Date: Thu, 28-May-87 14:19:25 EDT
Article-I.D.: sun.19926
Posted: Thu May 28 14:19:25 1987
Date-Received: Sat, 30-May-87 05:04:51 EDT
References: <444@its63b.ed.ac.uk>
Sender: news@sun.uucp
Lines: 25

> It would seem that System V's "user definable" signals SIGUSR1 and SIGUSR2

And 4.3BSD's...

> are really not so useful after all. A signal may only be safely sent to a
> process if its "meaning" in the target process can be guarenteed. (eg, if
> However, SIGUSR[12] have no such fixed purpose, so the only way it is
> completely safe to use these is for a process to kill itself (or possibly
> its children or its parent, if it can be sure they're executing from the
> same image - havn't done an exec).

Nope.  Two processes can be running programs that agree on the meaning
of SIGUSR1 or SIGUSR2.  (No, the fact that one of those processes
might do an "exec" isn't relevant - if they agree on the meaning of
one of these signals, they should also agree not to "exec" any
programs that don't also agree.)

However, there is a problem with them that renders them dangerous to
use on S5RN for N < 3; they're signals, which means that there is a
window between the arrival of the signal and the call to "signal"
inside the handler to set the action to SIG_IGN where the arrival of
another such signal will kill the process.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com