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From: smb@ulysses.homer.nj.att.com (Steven M. Bellovin)
Newsgroups: comp.unix.wizards
Subject: Re: Invalid Pointers (was Re: Referencing NULL pointers)
Message-ID: <11892@ulysses.homer.nj.att.com>
Date: 17 Jul 89 02:24:55 GMT
References: <20245@adm.BRL.MIL> <4348@eos.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 26

In article <4348@eos.UUCP>, jbm@eos.UUCP (Jeffrey Mulligan) writes:
> 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?

From the C Reference Manual, section 7.7, ``Equality Operators'':

	A pointer may be compared to an integer only if the integer is
	the constant 0.  A pointer to which 0 has been assigned is
	guaranteed not to point to any object and will appear to be
	equal to 0.  In conventional usage, such a pointer is
	considered to be null.

In other words, NO_FOOBAR should be defined as 0.  And this is a strong
hint to those who think that dereferencing 0 is legal.


		--Steve Bellovin