Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!portal!cup.portal.com!Kevin_P_McCarty
From: Kevin_P_McCarty@cup.portal.com
Newsgroups: comp.lang.c
Subject: Re: detecting invalid pointers
Message-ID: <16039@cup.portal.com>
Date: 20 Mar 89 06:47:02 GMT
References: <15495@cup.portal.com> <11998@haddock.ima.isc.com> <3011@nunki.usc.edu> <12043@haddock.ima.isc.com> <3092@nunki.usc.edu>
Distribution: usa
Organization: The Portal System (TM)
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In <3092@nunki.usc.edu>, jeenglis@nunki.usc.edu (Joe English) writes:
>Since in 'large'
>model all arrays have to fit in the same segment, any
>two pointers calculated from the base of the array can
>be compared just by their offsets.  In normal usage,
>this isn't a problem; however, it is possible for two
>pointers into *different* arrays to test equal

Not true.  I just tried it.  The <, <=, >= and > comparisons
use only the offset parts of the pointers, but == looks
at both segment and offset when pointers are 32 bits.
This is also documented in the manual (User's Guide v2.0, p.346).

It is possible however to have two pointers point to the same storage
location but which compare unequal.  A one-to-one mapping between
pointers and storage locations is not required.
Not only is it true that pointers are not ints, they don't even
behave like ints.

Kevin McCarty