Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: What C compilers have non-zero null pointers?
Message-ID: <1990Jul10.155941.11656@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <422@minya.UUCP>
Date: Tue, 10 Jul 90 15:59:41 GMT

In article <422@minya.UUCP> jc@minya.UUCP (John Chambers) writes:
>Consider that the C bible (page 192) says, concerning assignments
>to/from pointers and other types, "The assignment is a pure copy
>operation, with no conversion."  This means that in:
>	int i;
>	char*p;
>	i = 0;
>	p = i;
>the value assigned to p is the same bit pattern as i...

Reading the Old Testament (K&R1) and trying to apply it to modern C is
a mistake.  This code isn't even legal nowadays.  You need an explicit
cast to turn the int into a pointer, and there is no promise that that
cast doesn't do some sort of arcane conversion operation.

Actually, even the Old Testament continued with:  "This usage is
nonportable, and may produce pointers which cause addressing exceptions
when used.  However, it is guaranteed that assignment of the *constant*
0 to a pointer will produce a null pointer distinguishable from a
pointer to any object." [emphasis added]

The constant 0 in a pointer context has no relationship whatsoever to
the integer value 0; it is a funny way of asking for the null pointer,
which need not resemble the int value 0 in any way.
-- 
NFS is a wonderful advance:  a Unix    | Henry Spencer at U of Toronto Zoology
filesystem with MSDOS semantics. :-(   |  henry@zoo.toronto.edu   utzoo!henry