Newsgroups: comp.lang.c
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!psuvax1!psuvm!cunyvm!uupsi!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Subject: Re: NULL question not in FAQ
Message-ID: <1991Mar27.194101.1685@grebyn.com>
Organization: Grebyn Timesharing
References: <1991Mar26.235643.4498@ux1.cso.uiuc.edu>
Date: Wed, 27 Mar 1991 19:41:01 GMT

In article <1991Mar26.235643.4498@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>Given that the compiler is supposed to translate the constant "0" to the
>appropriate value for a NULL pointer on the machine type, how does one
>get a pointer value whose representation happens to be all zeroes, but
>is a non-NULL pointer?

void some_func(void) {
	int **ip;

	ip = (int **) calloc(sizeof(int *), 1);

	...

And now, *ip is a pointer whose representaiton is all-bits-zero, which
of course is not guaranteed to be NULL, but *probably is on your
implementation.  Most architectures are not capable of distinguishing
between a pointer representation of all-bits-zero, and a null pointer.
But you as a programmer should make this distinction.

And, if I might restate the rest, for brevity...

I am comfortable with the idea that 0 may be transformed into the null
pointer value.  However, the FAQ says that the *compiler* translates
the integer 0 *constant* into the null pointer.  It says nothing about
transforming, at run time, integer zero values which may be in an object.
Therefore, the following is not guaranteed to give me a null pointer:

	int i = 0;
	char *p = (char *)i;

So is it correct to interpret this code as unportable?
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