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From: roger@celtics.UUCP (Roger Klorese)
Newsgroups: comp.lang.c
Subject: Re: To comment 48: Xenix
Message-ID: <1526@celtics.UUCP>
Date: Wed, 6-May-87 15:00:30 EDT
Article-I.D.: celtics.1526
Posted: Wed May  6 15:00:30 1987
Date-Received: Sat, 9-May-87 02:08:40 EDT
References: <77200001@uiucdcsp> <6464@mimsy.UUCP> <641@cognos.UUCP>
Reply-To: roger@celtics.UUCP (Roger Klorese)
Organization: CELERITY (Northeast Area), Framingham, MA
Lines: 23

In article <641@cognos.UUCP> jimp@cognos.UUCP (Jim Patterson) writes:
>In article <6464@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>>>But I think that declaring: #define NULL (char *)0
>>>is expensive but will always do the trick for all models (small,large,huge)
>>
>>This is wrong:  It will not always work.  In fact, it will not work
>>on at least two existing machines, both of which have C compilers.
>
>I've no experience on PRIME...

Well, I DO... let's talk about it.

The Prime 50 series uses a segmented architecture.  The null pointer is
defined as a pointer to segment 7777(octal), location 0.  NULL must be
defined as 0, NOT (char *)0, for the compiler and runime environment to
test this properly.

By the way, character pointers are indeed a 48-bit quantity, containing 
segment number, protection ring, location (word), and byte-offset.
-- 
 ///==\\   (No disclaimer - nobody's listening anyway.)
///        Roger B.A. Klorese, CELERITY (Northeast Area)
\\\        40 Speen St., Framingham, MA 01701  +1 617 872-1552
 \\\==//   celtics!roger@seismo.CSS.GOV - seismo!celtics!roger