Path: utzoo!attcan!uunet!husc6!mailrus!ames!amdahl!amdcad!crackle!tim
From: tim@crackle.amd.com (Tim Olson)
Newsgroups: comp.lang.c
Subject: Re: pointer comparison question
Message-ID: <23537@amdcad.AMD.COM>
Date: 14 Nov 88 16:38:10 GMT
References: <185@umigw.MIAMI.EDU>
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Reply-To: tim@crackle.amd.com (Tim Olson)
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Organization: Advanced Micro Devices, Inc. Sunnyvale CA
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In article <185@umigw.MIAMI.EDU> steve@umigw.miami.edu (steve emmerson) writes:
| The question is this: is the following expression guaranteed to return 
| true if and only if "TestPtr" points within the array:
| 
| static struct foo	array[NUM];
| static struct foo	BegPtr	= array[0];
| static struct foo	BegPtr	= array[NUM-1];
| extern struct foo	*TestPtr;				/* could also come from a
|								   function call */
| 
| TestPtr >= BegPtr && TestPtr <= EndPtr;		/* guaranteed to do what
|							   I want? */
| 
| Or is it "implementation defined" or "undefined".  Note the "if and only
| if" condition, which means that if the expression is false, then "TestPtr" 
| *must* point "outside" the array.

It is undefined.  The "iff" condition also means that if the expression
is true, then "TestPtr" must point *inside* the array, which is the
problem here. 

Section 3.3.8 of the ANSI Draft says:

	"... If the objects pointed to are not members of the same
	aggregate or union object, the result is undefined."


	-- Tim Olson
	Advanced Micro Devices
	(tim@crackle.amd.com)