Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!mailrus!tut.cis.ohio-state.edu!im4u!ut-sally!utah-cs!utah-gr!uplherc!sp7040!obie!wes
From: wes@obie.UUCP (Barnacle Wes)
Newsgroups: comp.lang.c
Subject: Re: Volatile is Necessary
Message-ID: <102@obie.UUCP>
Date: 30 Mar 88 18:28:06 GMT
References: <12578@brl-adm.ARPA> <1988Mar25.172355.348@utzoo.uucp> <8107@sol.ARPA>
Organization: the Well of Souls
Lines: 26
Summary: Volatile also useful in...

In article <8107@sol.ARPA>, crowl@cs.rochester.edu (Lawrence Crowl) writes:
> The `volatile' type qualifier of ANSI C has recently been criticized as a
> frill.  While this assertion is arguable for uniprocessors, it is not true for
> shared-memory multiprocessors.  Processes on such machines communicate via
> updates to a shared variables.  These updates may occur at arbitrary times
> relative to one of the observing processes.  If there is no mechanism to
> indicate when a variable referenced by one process may be modified by another
> process, the compiler must assume that all variables may be modified at any
> time.

The `volatile' qualifier is also a must for such things as I/O
processors that do not produce interrupts.  Typically with such a
device, you will poll the status register at a predetermined interval,
and when the operation is complete, you can go on to the next
operation on that device.

These types of devices are used quite a bit in imbedded systems where
the speed of the device is not critical, and the interrupt(s) are
already heavily used for speed-critical devices/operations.  You
certainly don't want the compiler to optimize away your check on the
status register!

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