Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!xylogics!merk!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.std.c
Subject: Re: assignment to/from volatile execution sequence
Keywords: abstract machine
Message-ID: <16096@haddock.ima.isc.com>
Date: 6 Mar 90 00:42:06 GMT
References: <36669@mips.mips.COM>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 20

Taking the easy question first...

In article <36669@mips.mips.COM> lai@mips.COM (David Lai) writes:
>	a = *(b++) = c; /* will b be incremented twice? */

No.  Even if the abstract machine semantics require you to fetch back the
value from memory, the location of the object denoted by *b++ has already been
determined and should not be re-evaluated.  This is the same reason that
	*p++ += 1;
increments p only once.

>[What happens if the middle lvalue is volatile?]

Good question.  Is it intended to be the value that was stored there, or the
value that is read back?  These are equivalent for nonvolatiles, and the
Committee may have overlooked this situation.  I suggest you write the X3
Secretariat for an official interpretation.

Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint
(I also strongly disrecommend writing any code that depends on such nuances!)