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From: ncohen@watson.ibm.com (Norman H. Cohen)
Newsgroups: comp.lang.ada
Subject: Re: Easily-Read C++?
Date: 12 Oct 1994 13:49:51 GMT
Organization: IBM T.J. Watson Research Center
Distribution: world
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References: <1994Oct7.153254.29848@swlvx2.msd.ray.com>
 <374uke$8mo@delphi.cs.ucla.edu> <37bno4$ko4@gnat.cs.nyu.edu>
 <1994Oct11.090047.9190@sei.cmu.edu> <37emsq$smg@gnat.cs.nyu.edu>
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Date: 1994-10-12T13:49:51+00:00
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In article <37emsq$smg@gnat.cs.nyu.edu>, dewar@cs.nyu.edu (Robert Dewar)
writes: 

|> is it permissible for ++c to STORE a different value from what it returns?
...
|> Reading the C standard clearly does not answer my question, so we must
|> presume that the two sequences of code have quite a different effect.

Actually, the C standard clearly indicates that this can happen even in
circumstances that do not involve concurrency.  The second paragraph of
ANSI C section 3.3 (which I think of as the ANSI C answer to Ada RM
section 11.6, at least in its ability to keep language lawyers busy
trying to interpret it) states: 

        Between the previous and next sequence point an object shall have
   its stored value modified at most once by the evaluation of an
   expression.  Furthermore, the prior value shall be accessed only to
   determine the value stored.[34]

   -------

   [Footnote 34] This paragraph renders undefined statement expressions
   such as

      i = ++i + 1;

   while allowing

      i = i + 1;

Sequence points are defined syntactically.  They include, for example,
the points after the evaluation of the left operand of the &&, ||, and
comma operators and the ends of statements.  The definition appears to me
to presume a single-threaded model of execution.

I suspect that a large portion of the C code in the field has technically
undefined semantics because it violates the quoted paragraph.

--
Norman H. Cohen    ncohen@watson.ibm.com