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From: jimp@cognos.uucp (Jim Patterson)
Newsgroups: comp.lang.c
Subject: Re: To comment 48: Xenix
Message-ID: <641@cognos.UUCP>
Date: Fri, 1-May-87 17:42:18 EDT
Article-I.D.: cognos.641
Posted: Fri May  1 17:42:18 1987
Date-Received: Sat, 2-May-87 14:23:08 EDT
References: <77200001@uiucdcsp> <6464@mimsy.UUCP>
Reply-To: jimp@cognos.UUCP (Jim Patterson)
Organization: Cognos Incorporated, Ottawa, Canada
Lines: 23

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.
>(DG MV series and PR1ME series have 48 bit char pointers, but 32
>bit word (integer, structure, &c) pointers.)

I've no experience on PRIME, but I do know that the DG MV series
uses 32 bit pointers exclusively (for both char and word pointers).
They are represented differently (a char pointer is the corresponding
word pointer shifted one bit left, with the low-order bit
addressing one char of the corresponding word).  However, the
representation of NULL is the same, 0 in both cases.  In fact, because
an int is also 32 bits with DG's C compiler, there are very few reasonable
ways of defining NULL that won't work over function calls (I think that
all of the alternatives that have flown by this discussion would
in fact work).
-- 
Jim Patterson
Cognos Inc.