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From: guy@gorodish.UUCP
Newsgroups: comp.lang.c
Subject: Re: Query: Implementation with non-zero NULL
Message-ID: <20412@sun.uucp>
Date: Fri, 5-Jun-87 03:26:58 EDT
Article-I.D.: sun.20412
Posted: Fri Jun  5 03:26:58 1987
Date-Received: Sat, 6-Jun-87 07:19:56 EDT
References: <158@delftcc.UUCP> <3673@gitpyr.gatech.EDU> <414@sds.UUCP>
Sender: news@sun.uucp
Lines: 36
Keywords: C, NULL, portability

> I thought the C language guaranteed that there was an integral type
> large enough to hold any pointer type.  From p. 210 of the Apocrypha
> (== Appendix A of K&R :-)
> 	
> 	A pointer may be converted to any of the integral types
> 	large enough to hold it.  Whether an int or long is
> 	required is machine dependent.
> 
> Although the word "guarantee" doesn't appear here (as it does elsewhere),
> the second sentence seems to disallow the first sentence from being
> vacuously true.

More importantly, the next paragraph says:

	   An object of integral type may be explicitly converted to a
	pointer.  The mapping always carries an integer converted
	from a pointer back to the same pointer, but is otherwise
	machine dependent.

which guarantees that the pointer->integer conversion must not
discard any bits that are of any significance; i.e., any such bits
must not participate in pointer comparisons, and it must be possible
to regenerate those bits if necessary.  I don't know if these apply
to the 48-bit pointers on the Prime or not.

Fortunately, the current ANSI C draft withdraws the guarantee
extended by K&R; see the discussion in section 3.2.2.3 of the
Rationale, where it says "Since pointers and integers are now
considered incommensurate, the only integer that can be safely
converted to a pointer is the constant 0."  (This kind of bit-banging
is generally non-portable anyway; if some architecture renders it
awkward to provide a C implementation that permits this to be done in
the "straightforward" way, some non-standard way can be provided.)
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com