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From: guy@gorodish.UUCP
Newsgroups: comp.lang.c
Subject: Re: Pointer comparison and Portability
Message-ID: <13651@sun.uucp>
Date: Thu, 19-Feb-87 14:54:38 EST
Article-I.D.: sun.13651
Posted: Thu Feb 19 14:54:38 1987
Date-Received: Fri, 20-Feb-87 22:12:40 EST
References: <4537@brl-adm.ARPA>
Sender: news@sun.uucp
Reply-To: guy@sun.UUCP (Guy Harris)
Organization: Sun Microsystems, Mountain View
Lines: 32

>Unfortunately, the Intel 8086 series provides another counter example.
>On this machine, address are in the form of segment and offset.  The
>Actual Address is 16*segment + offset.

Yes, but *if* you happen to construct two different long pointers that
point to the same address, that's just like double-mapping a location
with an MMU.

>So, if two pointers compare equal, they definately point at the same
>object.  However, the converse is not true.  Two pointers that do not
>compare equal do NOT (necessarily) point at different objects.

Yes, but this is an escape hatch for the benefit of e.g. systems
that have to do double mapping.  It is *not* intended to render
comparison of pointers for equality useless except when comparing
pointers to elements in the same array.

>Seems like when K & R says that pointer comparison is undefined except
>when the two are pointers to the same array, it should be taken to mean
>exactly that for all operators, including == and !=.

Yes, but the ANSI C standard explicitly separates pointer comparison
for (in)equality from relational comparison on pointers.  Anybody who
tried to sell *me* a C implementation where the same object had two
addresses *except* when something was explicitly doubly mapped, or in
similarly unusual cases (e.g. an 808[68] with no memory mapping,
where a segment, treated as a full 64KB segment, overlapped another
segment, but where the segment *really* isn't long enough to overlap
it) - i.e., an implementation where pointer equality wasn't
equivalent to object equality, except in some *very* specialized and
*explicitly*-documented cases - would be shown the door rather
quickly.