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From: bruner@sp15.csrd.uiuc.edu (John Bruner)
Newsgroups: comp.sys.mac.programmer
Subject: StripAddress and pointer arithmetic
Message-ID: <1990May25.194025.9751@csrd.uiuc.edu>
Date: 25 May 90 19:40:25 GMT
Sender: usenet@csrd.uiuc.edu (news)
Reply-To: bruner@sp15.csrd.uiuc.edu (John Bruner)
Organization: CSRD, University of Illinois
Lines: 32

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

	If you need to sort by address or do any other kind of
	ordered address comparison, you need to call _StripAddress
	on each address before doing any ordered comparisons
	(>, <, >=, <=).  Remember, even though the CPU only uses
	the lower 24 bits in 24-bit mode, it still uses all 32 bits
	when performing arithmetic operations.

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).

I believe I understand what will and will not behave as expected, but
I think it should be spelled out better.  The way I read this
technical note the Macintosh cannot (efficiently) support an
ANSI-compliant implementation of C.
--
John Bruner	Center for Supercomputing R&D, University of Illinois
	bruner@uicsrd.csrd.uiuc.edu	(217) 244-4476