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From: mystone@mondo.engin.umich.edu (Dean Yu)
Newsgroups: comp.sys.mac.programmer
Subject: Re: StripAddress and pointer arithmetic
Message-ID: <1990May25.233110.1715@caen.engin.umich.edu>
Date: 25 May 90 23:31:10 GMT
References: <1990May25.194025.9751@csrd.uiuc.edu>
Sender: news@caen.engin.umich.edu (CAEN Netnews)
Reply-To: mystone@caen.engin.umich.edu
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In article <1990May25.194025.9751@csrd.uiuc.edu> bruner@sp15.csrd.uiuc.edu (John Bruner) writes:
>I've just read the technical note on StripAddress (#213, April 1990).
>On the second page under the (sub)heading "Ordered Address Comparison"
>it says
>
> [Passage from TN 213 deleted]
>
>Taken literally, this means that it is unsafe to write
>
>	struct something *p, things[64];
>
>	for (p = things; p < things + 64; p++)
>		...
>
>because one can't rely upon the comparison "p < things+64" working.
>This type of expression is very common in C (and is sanctioned by the
>ANSI standard).  I don't really believe this will fail, because the
>high bits in the expressions "things" and "things + 64" will almost
>certainly be the same (unless it would wrap in a 24-bit address
>space).
>

  Actually, this would work.  The Tech Note was working under the assumption
that the pointers/handles you were sorting were unrelated, and might have their
high byte in different states.  (Some locked, others purgeable, etc...)  Since
you're assigning p to thing, the two are refererencing the same address, so
comparisons, sorts, and all that fun stuff would work fine.

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