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From: xsimon@its63b.ed.ac.uk (Simon Brown)
Newsgroups: comp.unix.questions
Subject: Re: SIGUSR1, SIGUSR2 are pretty useless
Message-ID: <462@its63b.ed.ac.uk>
Date: Thu, 4-Jun-87 08:39:56 EDT
Article-I.D.: its63b.462
Posted: Thu Jun  4 08:39:56 1987
Date-Received: Tue, 9-Jun-87 03:18:05 EDT
References: <444@its63b.ed.ac.uk> <1179@ius2.cs.cmu.edu>
Reply-To: xsimon@its63b.ed.ac.uk (Simon Brown)
Organization: Computer Science Department, Edinburgh University
Lines: 47

In article <1179@ius2.cs.cmu.edu> edw@ius2.cs.cmu.edu (Eddie Wyatt) writes:
>
>One, do you know about the system calls
>signal and sigvec?  As soon as you set up a handler then SIGUSR1 and
>SIGUSR2, they have real meaning and usefulness.  I'm not going to go into
>all the things you can do.

I think you have missed the entire point of what I was trying to say. Sure,
you can trap SIGUSR[12], but using them is dangerous and restrictive because:
      a) They are "global" (like all signals), so there can be no
	 guarantee that some bastard process won't send them to a
	 process that is making some special use of them. This is
	 reasonable for signals which have a well-defined meaning, but
	 not for these.
      b) Some not-very-vanilla System-V's have BSD sockets bolted on, with 
     	 "#define SIGURG SIGUSR1" in <signal.h>. However, BSD's SIGURG is 
	 ignored if set to SIG_DFL, whereas this fake nasty SysV SIGURG (USR1)
	 causes termination. This can be very embarassing in a process which
	 has a socket open unknown to it (inherited from its parent) - it
	 doesn't know it's got a socket open and so is in danger of getting a
	 SIGURG, so it probably won't bother to trap SIGURG (ie, SIGUSR1).
	 Of course, this isn't a very meaningful objection, since I shouldn't
	 be using such a cruddy system in the first place, I suppose :-)
      c) There are only two of them, which isn't enough.

>
>   Two, there are sychronous and asynchronous messages passing conventions.
>What do you think the SIGURG is used for.  There are limitations like the
>number of bytes you may send out of band (asychronously) and you must do a
>few strange things to enable interrupts to be generated on the socket but
>the facility exist, at least in Berkley Unix.
>
I was talking 'bout System V, really, and the msgop IPC. But yes, I suppose 
SIGURG does the job for the Berkeley heap.


>			Eddie Wyatt
>e-mail: edw@ius2.cs.cmu.edu


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