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From: volpe@camelback.crd.ge.com (Christopher R Volpe)
Newsgroups: comp.std.c
Subject: Re: Function Argument Evaluation
Message-ID: <17983@crdgw1.crd.ge.com>
Date: 28 Mar 91 14:36:31 GMT
References: <3461@inews.intel.com> <1991Mar26.181821.22912@cs.ucla.edu> <17936@crdgw1.crd.ge.com> <15607@smoke.brl.mil>
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Reply-To: volpe@camelback.crd.ge.com (Christopher R Volpe)
Lines: 26

In article <15607@smoke.brl.mil>, gwyn@smoke.brl.mil (Doug Gwyn) writes:
|>In article <17936@crdgw1.crd.ge.com> volpe@camelback.crd.ge.com
(Christopher R Volpe) writes:
|>>The only reason the output of the program in question can be
|>>"200 200" is because the BEHAVIOR of the PROGRAM (right Doug?) is
|>>undefined because the program violates the "shall" rule in paragraph
|>>2 of 3.3, ...
|>
|>NO -- my original argument did NOT rely on undefined behavior.  It
|>relied on the unspecified order of evaluation of the subexpressions.
|>Even without the cited "shall" in the standard, "200 200" would have
|>been a valid output (but not "42 3.1416").

But the only reason you were able to produce an order of evaluation that
resulted in "200 200" is because the program did something it should never
have done in the first place, which was to modify p twice between sequence
points. I would be very interested in seeing a program whose behavior
was *not* undefined, yet could arbitrarily produce either of two
drastically different outputs depending solely on the unspecified
order of evaluation. (By "drastically different" I mean something
like "100 200" vs. "200 200". I don't mean something like "12.427" 
vs. "12.426999".)          
==================
Chris Volpe
G.E. Corporate R&D
volpecr@crd.ge.com