Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!oliveb!bu-cs!bzs
From: bzs@bu-cs.BU.EDU (Barry Shein)
Newsgroups: comp.unix.wizards
Subject: Re: Invalid Pointers (was Re: Referencing NULL pointers)
Message-ID: <34785@bu-cs.BU.EDU>
Date: 17 Jul 89 02:57:32 GMT
References: <20245@adm.BRL.MIL> <4348@eos.UUCP>
Organization: Boston U. Comp. Sci.
Lines: 34
In-reply-to: jbm@eos.UUCP's message of 16 Jul 89 20:44:47 GMT


From: jbm@eos.UUCP (Jeffrey Mulligan)
>It has been pointed out that there should be no assumptions
>about what addresses are valid; is there any way to get
>a guaranteed INVALID address?
>
>I commonly do this sort of thing:
>
>struct foobar { /* some stuff */};
>
>static struct foobar *fb1=NO_FOOBAR;

>...

>So, the question is, how should NO_FOOBAR be defined?
>
>#define NO_FOOBAR ((struct foobar *) -1 )	is what I use, but...

A perfectly good solution is:

	struct foobar no_foobar;
	#define NO_FOOBAR (&no_foobar)

being as no_foobar will be the only thing assigned this address it is
unique and solves your problem. This of course works for your example
where you test for a return value of NO_FOOBAR, doing it by causing a
signal to occur (eg. SEGV) is another problem entirely.
-- 
	-Barry Shein

Software Tool & Die, Purveyors to the Trade
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