Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Value of a null pointer
Message-ID: <1988Apr24.090957.8517@utzoo.uucp>
Organization: U of Toronto Zoology
References: <4728@cup.portal.com>
Date: Sun, 24 Apr 88 09:09:57 GMT

> 1. An uninitialized external is set to zero bits...

Wrong.  This was never made entirely clear in K&R, but ANSI C makes it
quite specific that an uninitialized external is set as if it were
explicitly initialized to 0.

> 2. One can use calloc to allocate an array of pointers. The initial value of
>    this array must be null pointers. calloc sets the area to zero bits.

Wrong.  Calloc sets its area to zero bits.  That is not guaranteed to be
zeros of any data type.  Well, if you read the fine print it's probably
guaranteed to be integer zeros, given that integers are pretty tightly
defined to be binary, but it is *not* guaranteed to be pointer or
floating-point zeros.

> Also, does the new standard require that a cast of a literal zero always
> does the right thing? That is, are
>     ptr_type_var = (ptr_type)0;     /* assigns null pointer */
>     ptr_type_var = 0;               /* assigns null pointer */
>     func( (ptr_type)0 );            /* function expecting pointer type */

Yes.  This was true in K&R and it is true in ANSI C.


Mind you, given the quantity of sloppy C code around, machine designers
interested in C and/or Unix will think twice about making NULL anything
but all-zero-bits, but they are within their rights to do so.
-- 
"Noalias must go.  This is           |  Henry Spencer @ U of Toronto Zoology
non-negotiable."  --DMR              | {ihnp4,decvax,uunet!mnetor}!utzoo!henry