Xref: utzoo comp.sys.ibm.pc:22870 comp.lang.c:15284 comp.std.c:641
Path: utzoo!utgpu!attcan!uunet!lll-winken!ames!haven!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.sys.ibm.pc,comp.lang.c,comp.std.c
Subject: Re: correct code for pointer subtraction
Message-ID: <9301@smoke.BRL.MIL>
Date: 7 Jan 89 01:35:45 GMT
References: <597@mks.UUCP> <3845@pt.cs.cmu.edu> <18123@santra.UUCP> <142@bms-at.UUCP> <6604@killer.DALLAS.TX.US> <PINKAS.89Jan3082456@hobbit.intel.com> <18683@santra.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 16

In article <18683@santra.UUCP> tarvaine@tukki.jyu.fi (Tapani Tarvainen) writes:
>>>   > >         static int a[30000];
>>>   > >         printf("%d\n",&a[30000]-a);
>I have been told that dpANS explicitly states that the address of
>"one-after-last" element of an array may be taken, and subtractions 
>like the above are legal and should give correct result.

Almost.  Address of "one after last" is legal for all data objects,
but of course cannot be validly used to access the pseudo-object.
All objects can be considered to be in effect arrays of length 1.
Pointers to elements of the same array object can be subtracted;
the result is of type ptrdiff_t (defined in <stddef.h>).
The example above assumes that ptrdiff_t is int,
which is not guaranteed by the pANS.
Casting to another, definite, integral type such as (long)
would make the result portably usable in printf() etc.