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From: drw@cullvax.UUCP
Newsgroups: comp.lang.c
Subject: Pointer Comparison and Portability
Message-ID: <814@cullvax.UUCP>
Date: Thu, 19-Feb-87 11:07:56 EST
Article-I.D.: cullvax.814
Posted: Thu Feb 19 11:07:56 1987
Date-Received: Fri, 20-Feb-87 21:38:59 EST
Organization: Cullinet Software, Inc., Westwood, MA
Lines: 23

john@viper.UUCP (John Stanley) writes:
>   Tom, on segmented arcitecture machines you -could- have two distinctly
> different pointer values which point to the same memory space.  This is
> what Israel Pinkas was (I think) trying to point out.
> 
>   In the example he gave:
> 	  char p[16], q[16];
> (Details of 8086 memory addressing omitted.)
> The byte of memory addressed by q[0] and p[17] would
> be the exact same byte, but the two pointers 0002:0000 and 0001:0010 would be
> different.

Yes, but comparing &p[17] and &q[0] is comparing two addresses derived
from different allocations, and as the standard says, that is
implementation defined (i.e., may not work).  Note also that &p[17] is
a correct *pointer*, but attempting to fetch or store anything through
it is not.  &p[18] is not a correct pointer.

Dale
-- 
Dale Worley		Cullinet Software
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