Xref: utzoo comp.lang.c:6382 comp.sys.m68k:666
Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!bu-cs!polygen!pablo
From: pablo@polygen.uucp (Pablo Halpern)
Newsgroups: comp.lang.c,comp.sys.m68k
Subject: Re: C machine
Message-ID: <25@polygen.UUCP>
Date: 20 Jan 88 02:23:10 GMT
References: <461@auvax.UUCP> <9961@mimsy.UUCP> <166@teletron.UUCP> <6936@brl-smoke.ARPA> <147@ateng.UUCP> <39aca826.7f32@apollo.uucp> <7092@brl-smoke.ARPA> <708@umbc3.UMD.EDU>
Reply-To: pablo@polygen.UUCP (Pablo Halpern)
Organization: Polygen Corporation, Waltham, MA
Lines: 54

In article <708@umbc3.UMD.EDU> alex@umbc3.UMD.EDU (Alex S. Crain) writes:
>>>The result of subtracting two pointers is *defined* (K&R) to yield
>>>a result of type int.
>>
>>That's another of the things that ANSI C intends to fix.
>
>
>Fix? How pray tell would you fix it? make it machine dependent? or just
>arbitrary? 

The ANSI Draft that I have (Oct 1, 1986) describes the header file <stddef.h>
which defines two types.  As quoted from the document:

	The types are

		ptrdiff_t

	which is the signed integral type of the result of subtracting
	two pointers; and

		size_t

	which is the unsigned integral type of the result of the sizeof
	operator.

(end quote)
A machine with a short word but long pointer type (e.g. 68000, 8086) would
have definitions like

	typedef long		ptrdiff_t;
	typedef unsigned long	size_t;

in <stddef.h>.  Machines with short pointers (e.g. 8080, 6502) would
have definitions like

	typedef int		ptrdiff_t;
	typedef unsigned	size_t;

or better yet

	typedef long		ptrdiff_t;
	typedef unsigned	size_t;

The reason for the long ptrdiff_t definition is that the difference
between two unsigned, 16-bit pointers can excede the -32768 - +32767
range of a 16-bit two's compliment signed integer.  Machines with long
words and long pointers could use any of the above pairs of
declarations.

So the short answer to the question is: the difference of two pointers
has a type that is machine (and compiler) dependent.  But this is no
worse (or better) than making the definition of an int machine dependent.

Pablo Hapern,  Polygen Corp.  UUCP: {bu-cs,princeton}!polygen!pablo