Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pacbell!att!cuuxb!dlm
From: dlm@cuuxb.ATT.COM (Dennis L. Mumaugh)
Newsgroups: comp.std.c
Subject: Re: volatile registers (was: The world is not ready for 'volatile')
Summary: volatiler is for kernel hackers only
Keywords: volatile
Message-ID: <2337@cuuxb.ATT.COM>
Date: 4 Jan 89 03:52:18 GMT
References: <141@bms-at.UUCP> <275@twwells.uucp> <15166@mimsy.UUCP> <9236@smoke.BRL.MIL> <15171@mimsy.UUCP> <9316@ihlpb.ATT.COM>
Reply-To: dlm@cuuxb.UUCP (Dennis L. Mumaugh)
Organization: ATT Data Systems Group, Lisle, Ill.
Lines: 44

In      article      <9316@ihlpb.ATT.COM>       nevin1@ihlpb.UUCP
(55528-Liber,N.J.)  makes  some comments about volatile that seem
to reflect many people's understanding.

The concept "volatile" should not occur in any general C program.
This  feature  exists only for the stand alone, the real time and
the kernel hacker.

Its purpose is to state that each  and  every  reference  to  the
volatile  object  [location, structure, register, or whatever] is
necessary and not to optimize such references in any way.

Its only use in a UNIX environment *should* be  with  respect  to
shared  memory  or  mmaped files where more than once process may
access the same location "simultaneously".  I have seen its  need
only  in  writing  drivers  where  a  location actually maps to a
device register and each  referrence  to  the  location  actually
results in the device doing something.  For example:

volatile int foo;
...
foo = 'H';
foo = 'e';
...
foo = 'd';
foo = '.';

might print "Hello world." on the console of the computer if  the
location  "foo"  were  the  output  port for a device controller.
Without the volatile the compiler or the optimizer is allowed  to
reduce the above to

	foo = '.';

In this sense the constuct of a "volatile" register doesn't  make
sense  in the C language as it is not possible to bind a variable
to any specific register [in the instruction set sense].

The question of aliasing is orthoganol to volatile.  It  just  so
happens  that  the  side-effects of declaring a volatile location
result in anti-aliasing of pointers to volatile locations.
-- 
=Dennis L. Mumaugh
 Lisle, IL       ...!{att,lll-crg}!cuuxb!dlm  OR cuuxb!dlm@arpa.att.com