Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!att!ulysses!andante!alice!debra
From: debra@alice.UUCP (Paul De Bra)
Newsgroups: comp.unix.wizards
Subject: Re: Invalid Pointers (was Re: Referencing NULL pointers)
Message-ID: <9622@alice.UUCP>
Date: 17 Jul 89 13:46:32 GMT
References: <20245@adm.BRL.MIL> <4348@eos.UUCP>
Reply-To: debra@alice.UUCP ()
Organization: AT&T, Bell Labs
Lines: 24

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?
}...
}So, the question is, how should NO_FOOBAR be defined?
}
}#define NO_FOOBAR ((struct foobar *) -1 )	is what I use, but...
}
}I note that malloc(3) returns NULL (0) on failure [on the sun], indicating
}that 0 could never be a valid address returned from malloc.

According to the (new) K&R book NULL (0) is indeed guaranteed not to be
a valid data (or text or stack) address.
Furthermore, the only meaningful comparisons between pointers are
comparison between pointers to elements of the same string and comparison
between a pointer and NULL.

Paul.
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