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From: libes@nbs-amrf.UUCP
Newsgroups: mod.computers.vax
Subject: Is it possible to do critical regions in VMS?
Message-ID: <8608060216.AA06958@seismo.CSS.GOV>
Date: Tue, 5-Aug-86 22:16:12 EDT
Article-I.D.: seismo.8608060216.AA06958
Posted: Tue Aug  5 22:16:12 1986
Date-Received: Thu, 7-Aug-86 06:15:07 EDT
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Is it possible to do critical regions in VMS?  I would prefer
non-privileged code but, if necessary, some weird privs would be
acceptable.

My problem is as follows:

int x = 0;
main() {
	while (1) {
		increment();
		decrement();
	}
}

increment() {x++;}
decrement() {x--;}

The user can call subroutines increment and decrement in strictly
alternating sequence (as is done here in main()).  However, he may
^c or exit (via forcex() or the debugger) at any time.  I would
like to ensure that x is 0.

The reason I can't just set it to 0, is that x is actually shared
with several other processes (all doing the same thing).

I'm at a loss on how to do this.  I'm already using an exit
handler, but the exit handler can't tell if it needs to decrement x
or not (x actually takes on values > 1, so it can't tell if we've
incremented x a different number of times than we've decremented
it).

No matter how I write it, there is always a window in which
I will have a problem.  If I could do critical regions in VMS I
would like to do something like this:

	increment() {
		enter_critical_region(); /* block ^c, ^y, forcex(), etc */
		x++;
		incremented = TRUE;
		exit_critical_region();  /* ok, now I'm willing to exit */
	}

	exit_handler() {
		if (incremented) x--;
	}

(In case it's important, I'm using Fortran (patching someone else's
code).  I'm assuming a language-independent VMS-dependent solution.)
I would appreciate it if anyone has a solution to this seemingly
simple problem.  Thanks.

(If anyone knows how to do critical regions in general in VMS, I
would like to know, as all the people around here that I asked said
that they had run into this problem before to no avail.)

Don Libes       {seismo,umcp-cs}!nbs-amrf!libes		(301) 921-2171