Path: utzoo!attcan!uunet!tektronix!tekig5!larryha
From: larryha@tekig5.PEN.TEK.COM (Larry Hattery)
Newsgroups: comp.sys.mac.programmer
Subject: Re: what the heck is "reentrant"?
Message-ID: <3111@tekig5.PEN.TEK.COM>
Date: 5 Aug 88 20:25:08 GMT
References: <730049@hpcilzb.HP.COM> <104700046@p.cs.uiuc.edu>
Reply-To: larryha@tekig5.PEN.TEK.COM (Larry Hattery)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 25


Reentrant routines only become an issue in a multitasking OS where a
routine may get suspended before it is finished and the same routine
gets called again by another task.  The same is true for routines
called by interrupt handlers.  Any time a routine can be called from
more than one place, conceptually, at the same time, it should be
reentrant.  This means that it must provide mutual exclusion around
any global resources it is using to insure that subsequent calls to
the same routine will not screw things up for the call that hasn't yet
completed.  Here, global also means static variables in the routine
itself since these refer to the same location from call to call.  Of
course, any stack based variables are private to a particular instance
of the call and do not need protection.  Mutual exculsion is generally
handled with semaphores, or more radically by disabling interrupts,
while in critical sections of a routine which would fail if the routine
were reentered at that point.

I'm excluding rescursive routines because they are by definition,
and presumably be design, reentrant.

-- 
Larry E. Hattery                        Tektronix Inc.
larryha@penguin.PEN.TEK.COM             M/S 47-704
Portable Instruments Division           PO Box 500
(503)627-1225 days                      Beaverton, OR 97077