Path: utzoo!attcan!uunet!ns-mx!umaxc.weeg.uiowa.edu!williams
From: williams@umaxc.weeg.uiowa.edu (Kent Williams)
Newsgroups: comp.lang.c
Subject: Re: references to dereferenced null pointers
Message-ID: <945@ns-mx.uiowa.edu>
Date: 15 Mar 90 14:30:43 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <1990Mar12.175613.12082@utzoo.uucp> <1623@argus.UUCP> <1990Mar14.164539.23685@utzoo.uucp> <16179@haddock.ima.isc.com>
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In article <16179@haddock.ima.isc.com> karl@haddock.ima.isc.com (Karl Heuer) writes:
>In article <1990Mar14.164539.23685@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>>There is absolutely nothing wrong with having a pointer representation in
>>which the bit pattern for a null pointer is not all zeros... except that
>>there are a lot of old, badly-written programs which will break.  Thus my
>>earlier comment that it is valid but unwise.
>
>Note that "p = 0", "p == 0", "!p", "char *f() { return 0; }" are *not*
>examples of such badly-written code; they may be bad style, but the compiler
>is required to generate correct code involving a true null pointer.

The bottom line in actual practice is that if NULL isn't a binary object 
of all zero bits, you can get into trouble porting programs.  Using 0 and
NULL interchangebly is an unfortunate but common practice -- see code
examples in Stroustroup's C++ book -- many assignments of 0 to
pointers.

In order to port a program to a not-all-zero-bits-NULL architecture
would be aided by a compiler that would flag ALL assignments of
non-pointer constants to pointer variables.  This wouldn't solve all
problems, since you can have a function like:

void assign_int_to_pointer(void **x, int y) { *x = (void *)y; }

Then your compiler would have to generate RUN-TIME checks of values
passed in.

To muddy the waters further, It isn't uncommon (and not terribly
unportable) to use a set of small integral constants, say 0 .. 10 as
sentinel values assigned to pointers, e.g.

		switch((int)ptr) {
		case 0:	do0(); break;
		case 1: do1(); break;
				.
				.
				.
		default:
				it_really_is_a_pointer(ptr);
		}

When discussing these issues, you do have to take into account actual
practice, whether actual practice is a good idea in the final analysis
or not.
--
                               
Kent Williams                  "We're One! All One! Exceptions Eternally?
williams@umaxc.weeg.uiowa.edu   None! Absolutely None!" - Dr. Bronner's Soap