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From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <7939@think.UUCP>
Date: Sat, 29-Aug-87 17:18:21 EDT
Article-I.D.: think.7939
Posted: Sat Aug 29 17:18:21 1987
Date-Received: Sun, 30-Aug-87 09:19:51 EDT
References: <493@its63b.ed.ac.uk> <6061@brl-smoke.ARPA> <3812@spool.WISC.EDU> <483@mtxinu.UUCP> <572@l.cc.purdue.edu>
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Reply-To: barmar@godot.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
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Xref: mnetor comp.lang.c:4032 comp.unix.wizards:3975

In article <572@l.cc.purdue.edu> cik@l.cc.purdue.edu (Herman Rubin) writes:
>In article <483@mtxinu.UUCP>, ed@mtxinu.UUCP (Ed Gould) writes:
> 
>> It's also not legal in the proposed ANSI C standard.  Pointers
>> may be subtracted *only* if they point to members of the same
>> array of elements.  
>
>The fact that some `gurus' cannot see the uses of this construct, as well
>as others such as goto's, forcing inline, etc., is no more appropriate
>than prohibiting the use of any tools developed since 1800 to sculptors.
>You have no way of knowing how I can use the power of the machine; I may
>very well find a new way of doing some things tomorrow that I do not see
>today.  Let us remove unnecessary restrictions from the languages.

This is not an unnecessary restriction.  It is there because the
construct is non-portable, and the purpose of the C standard (indeed,
ANY language standard) is to define a language in which portable
programs may be written.  No one is prohibiting you from subtracting
pointers to your heart's delight on machines where it makes sense;
just be aware that the standard doesn't specify what the result will
be, so your program may behave differently on different architectures.
In fact, I know of an architecture where it may return different
results for pointers to the same two objects at different times: the
Symbolics Lisp Machine.  It has a garbage collector that moves objects
around in memory, so the addresses may change, and therefore the
difference may change.  (Note: I've never used their C compiler, so I
don't know it will do this; however, I also believe that their
architecture allows them to detect comparisons of pointers to
different arrays).


---
Barry Margolin
Thinking Machines Corp.

barmar@think.com
seismo!ththers' arta