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From: James Kuyper <jameskuyper@verizon.net>
Newsgroups: comp.lang.c
Subject: Re: Size of an arraylist in bytes
Date: Sat, 04 Feb 2012 17:32:18 -0500
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:16689

On 02/04/2012 04:25 PM, Joe keane wrote:
...
> int g(int x)
> {
>     int a[4][5];
>     int *p;
>     int *q;
>     int j;
> 
>     p = &a[0][5];
>     q = &a[1][0];
>     if (p != q)
> 	barf();
>     *p = 2;;
>     *q = 3;;

What's with the double semi-colons?

Because dereferencing p has undefined behavior, a conforming
implementation of C can do anything it wants with this code. However, in
particular, it need not pay any attention to the fact that p and q point
at the same location. It could therefore rearrange the order of the
previous two lines, which would produce unexpected results on the
following line:

>     j = *p;
>     if (j != 3)
> 	puke();
> }
> 
> I'm looking for 'reductio ad absurdum'.
> 
> i.e., chapter and verse for 'this can't possibly not work'

It can work; but it's not required to work - which is the point.