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From: karish@forel.stanford.edu (Chuck Karish)
Newsgroups: comp.unix.wizards
Subject: Re: Referencing NULL pointers
Message-ID: <3492@portia.Stanford.EDU>
Date: 11 Jul 89 17:29:50 GMT
References: <19367@paris.ics.uci.edu> <1891@auspex.auspex.com> <32UP02Eg3d=801@amdahl.uts.amdahl.com>
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Reply-To: karish@forel.stanford.edu (Chuck Karish)
Organization: Mindcrfat, Inc.
Lines: 26

In article <32UP02Eg3d=801@amdahl.uts.amdahl.com> vohra@amdahl.uts.amdahl.com
(KC) wrote:
>In article <1891@auspex.auspex.com> guy@auspex.auspex.com (Guy Harris) writes:
[ It's not valid to dereference a NULL pointer ]

>Try a rewrite: "x= ((struct somestruct *)0)->somefield".

>It can be convenient to make the first page (or whatever unit)
>inaccessible to cause errors for structures that are smaller
>than that unit.  Maybe Sunos does this...

>Anyway, it is the operating system that makes the action that the
>process takes valid or invalid.

Neither the draft C standard nor Kernighan and Ritchie define the
format of an executing program.  Accessing an arbitrarily-chosen
address in memory is thus completely non-portable; there's no way to
predict whether 0 is a valid address.  The `0' value for NULL is a
token with a special meaning, not an address.

Whether non-portable usages are `valid' depends on the way the program
is to be used.  If the program is to have a long life or should be
usable on more than one architechure, don't dereference NULL pointers.

	Chuck Karish		{decwrl,hpda}!mindcrf!karish
	(415) 493-7277		karish@forel.stanford.edu