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From: haug@almira.uucp (Brian R Haug)
Newsgroups: comp.lang.c++,comp.std.c
Subject: Re: references to dereferenced null pointers
Message-ID: <1990Mar11.222634.2701@almira.uucp>
Date: 11 Mar 90 22:26:34 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <52081@microsoft.UUCP> <25F8D2FB.10981@paris.ics.uci.edu> <1990Mar11.015305.28264@utzoo.uucp>
Reply-To: haug@Columbia.NCR.COM (Brian Haug)
Organization: Personal machine, Columbia, SC
Lines: 58

In article <1990Mar11.015305.28264@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>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.

Henry,
    I beg to differ.  I see where you are coming from with the wording:
"an integral constant expression with the value 0", but in K&R First edition,
page 97, there is an explicit example which 0 is returned by a function
returning a pointer to a character.  Also, from the text "C guarantees that no
pointer that validly points at data will contain zero, so a return value of
zero can be used to signal an abnormal event,..."
    Also, from Appendix A, section 7.7 (Equality operators) "A pointer may be
compared to an integer, but the result is machine dependent unless the integer
is the constant 0.  A pointer to which 0 has been assigned is guaranteed no to
point to any object, and will appear to be equal to 0; in conventional usage,
such a pointer is considered to be null."  This implies to me that I can
compare a pointer to any arbitrary expression which evaluates to 0 and have
machine independent behavior.  And from section 7.14, "it is guaranteed that
the assignment of the constant 0 to a pointer will produce a null pointer
distinguishable from a pointer to any object."  Since the "BNF" for this
section lists "lvalue = expression," I would assume that if expression
evaluates to 0 then the pointer will produce a null pointer distinguishable
from a pointer to any object.
    As far as ANSI C goes, I do not have easy access to the standard at the
current time, nor the experience reading it.  However, I would have expected
this behavior to be brought forward (hopefully not a silly idea, but we'll
see).
     This was a quick scan through K&R, so I hope I have not taken anything
out of context.  The only reason this objection stuck in my mind is that I had
once thought of doing an implementation where NULL != 0, but after further
reading convinced myself the implementation would be invalid.

			Share and Enjoy!

			      Brian Haug

Disclaimer:  These opinions are mine alone.