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From: bill@twwells.com (T. William Wells)
Newsgroups: comp.std.c
Subject: Re: a[], *p: if 0 <= p - a < sizeof(a), must p point to an element of a?
Message-ID: <1990Jan5.040710.23691@twwells.com>
Date: 5 Jan 90 04:07:10 GMT
References: <875@stealth.acf.nyu.edu>
Distribution: usa
Organization: None, Ft. Lauderdale, FL
Lines: 17

In article <875@stealth.acf.nyu.edu> brnstnd@stealth.acf.nyu.edu (Dan Bernstein) writes:
: Basically, I want to have a guaranteed test of whether p points to an
: element of a, with foo a[N] and foo *p. I know that *if* p points to an
: element of a, *then* 0 <= p - a < sizeof(a); is the reverse true? (If
: yes, please explain the logic.)

No. In general, if p does not point to a member of a, p - a is
undefined.

: The standard should define pointer subtraction more carefully.

It has defined this most explicitly. Since my fingers are tired,
I won't type in the appropriate text. But it is in section 3.3.6.

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