Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!amdcad!cae780!leadsv!laic!darin
From: darin@laic.UUCP (Darin Johnson)
Newsgroups: comp.lang.c
Subject: Re: Volatile is Necessary
Message-ID: <193@laic.UUCP>
Date: 31 Mar 88 01:15:12 GMT
References: <12578@brl-adm.ARPA> <1988Mar25.172355.348@utzoo.uucp> <8107@sol.ARPA> <378@ma.diab.UUCP>
Organization: Lockheed AI Center, Menlo Park
Lines: 50

In article <378@ma.diab.UUCP>, pf@diab.UUCP (Per Fogelstr|m) writes:
> In article <7569@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
> >You need a hell of a lot more than "volatile" to properly
> >synchronize concurrent processes.
> 
> I don't beleive that "volatile" was intended to be a mechanisim for
> processor synchronization, rather a way to indicate that a "memory"
> location has side effects, for example when it's read. For those of
> You that don't normally do system and driver programming it's qite
> meaningless, but boy how nice it is to be able to use the -O switch
> when compiling a driver wich requires fast interrupt turnaround.

For a real life example...  There are quite a few languages in VMS that
have a 'volatile' keyword as a language extension.  I have actually
needed to use something like this to do the following (synopsis):

  volatile int op_cnt;
  .
  .
  void read_ast(.....) {
    .
    .
    op_cnt += z; 
    .
    .
  }

  .
  .
  sys$qio(.., read_ast, ..); /* sys$qio will call read_ast() asynchronously */
  .
  .
  /* now do some expression involving op_cnt, pass it to a function, 
     take its address(!), etc. */

If the word volatile is removed, I could not get consistent results
when optimization was turned on.  This was because op_cnt can get
changed at anytime, even between an add and a store instruction.
If I explicitly turned off optimization (it is on by default) then
everything would start working fine.  Of course, the above code is
bad programming, but you should get the general idea.

A #pragma could be used instead, but if any sort of similar asynchronous
routines get put into a standard (POSIX or otherwise) then 'volatile' will
have to be used instead of '#pragma' to make the code portable (or
else require '#pragma volatile' to be in every compiler you want to
port to).
-- 
Darin Johnson (...ucbvax!sun!sunncal!leadsv!laic!darin)
              (...lll-lcc.arpa!leadsv!laic!darin)
	All aboard the DOOMED express!