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From: tainter@ihlpg.UUCP (Tainter)
Newsgroups: net.lang.c
Subject: Re: Indirect comma assigned from side effects
Message-ID: <2401@ihlpg.UUCP>
Date: Mon, 25-Aug-86 16:57:28 EDT
Article-I.D.: ihlpg.2401
Posted: Mon Aug 25 16:57:28 1986
Date-Received: Tue, 26-Aug-86 18:48:19 EDT
References: <792@navajo.STANFORD.EDU> <3047@umcp-cs.UUCP> <796@navajo.STANFORD.EDU>
Organization: AT&T Bell Laboratories
Lines: 33

> In article <3047@umcp-cs.UUCP>, chris@umcp-cs.UUCP (Chris Torek) writes:
> > In article <792@navajo.STANFORD.EDU> I write:
> > >	int lhs, rhs, index; int *pointers[9];
> > >	foo() { *( index=lhs, pointers[index] ) = sideeffects(); }
> > >The question is, does C allow for the possibility that the order
> > >of evaluation might be "index=lhs" then "sideeffects()" then
> > >"*pointers[index]=<result>"?
> > No.  The compiler can, however, effectively generate either of the
> > following:
> Can someone second this, or show me the appropriate reference?
> It's Microsoft 4.0 that's putting the sideeffects() call between
> the two parts of the comma expression, and sideeffects() modifies
> index . . . you get the idea.  I could find nothing in K&R, but
> I'm only been hacking C for a year or so.
> 
> I just tested under 4.3bsd; cc also puts the sideeffects() in the
> middle.  Comments, anyone?
> 
> Thanks!   -tom

As far as I can tell the only restriction on order of evaluation is 
index=lhs before pointers[index].
Lets meta a bit:
*(a,b)= c;
can be evaluated:
c a b
a c b
a b c
If c plays with something also played with in a or b then you are in the
same boat as the people counting on order of evaluation in:
*a++ = *a + gunga_din;

--j.a.tainter