Xref: utzoo comp.lang.c++:6773 comp.std.c:2583
Newsgroups: comp.lang.c++,comp.std.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: references to dereferenced null pointers
Message-ID: <1990Mar11.015305.28264@utzoo.uucp>
Organization: U of Toronto Zoology
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <52081@microsoft.UUCP> <25F8D2FB.10981@paris.ics.uci.edu>
Date: Sun, 11 Mar 90 01:53:05 GMT

In article <25F8D2FB.10981@paris.ics.uci.edu> rfg@ics.uci.edu (Ronald Guilmette) writes:
>...noted the obscure restriction that a null pointer must be
>something created from "an integral constant expression with the value 0"
>(see ANSI C standard 3.2.2.3).  This could be an issue for both C and
>C++, since it would seem to place in doubt the validity of the following:
>
>	int nil ()
>	{
>		return 0;
>	}
>
>	...
>
>		void *vp;
>
>		vp = (void *) nil ();

This code is illegal in C, and always has been.  There is no general
relationship between the integer value 0 and the null pointer.  An
integer *constant* value equal to zero in a pointer context is automatically
converted to a null pointer; this may be a non-trivial conversion, and is
guaranteed valid only at compile time.  The result of converting the
integer value 0 to a pointer at run time is implementation-defined.
-- 
MSDOS, abbrev:  Maybe SomeDay |     Henry Spencer at U of Toronto Zoology
an Operating System.          | uunet!attcan!utzoo!henry henry@zoo.toronto.edu