Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!ico!ism780c!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Adding two pointers
Message-ID: <13004@haddock.ima.isc.com>
Date: 8 May 89 23:05:41 GMT
References: <2765@buengc.BU.EDU> <563@lzaz.ATT.COM> <4093@ficc.uu.net> <17340@mimsy.UUCP> <4646@freja.diku.dk>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 21

In article <4646@freja.diku.dk> njk@freja.diku.dk (Niels J|rgen Kruse) writes:
>The word "array" as used when saying that two pointers must point into the
>same array in order to subtract them is a bit foggy to me.  What about
>	struct { char c; double d; } x[10];
>Are (& x[3].d) and (& x[7].d) not pointing into x?

Not in this sense.  They point into (different) arrays of type `double [1]'.

>Is (&x[3].d - &x[7].d) undefined?

Yes (even after I fixed the typo).  On some machines, there does not exist an
integer k such that (&x[1].d == &x[0].d + k), so it would be meaningless to
attempt to evaluate (&x[1].d - &x[0].d) to any integer.

>What array is (char *)x pointing into?

An array of type `char [sizeof(x)]' (or equivalently, `char [10*sizeof(S)]'
where S is the struct type) whose existence is supposedly implied by the pANS
statement that `objects are composed of bytes'.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint