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From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c,comp.lang.c++,comp.std.c
Subject: Re: references to dereferenced null pointers
Message-ID: <16179@haddock.ima.isc.com>
Date: 15 Mar 90 01:23:33 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <1990Mar12.175613.12082@utzoo.uucp> <1623@argus.UUCP> <1990Mar14.164539.23685@utzoo.uucp>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Followup-To: comp.lang.c
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 25

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 only
"dangerous" context (other than hacking with unions and such) is when a null
pointer constant is being passed as an argument to a function.  (In C++ and
ANSI C, any argument not covered by a prototype.  In old C, any function
argument at all.)  In particular, neither of the two calls
	execl("/bin/sh", "sh", "-i", 0);
	execl("/bin/sh", "sh", "-i", NULL);
is correct; it should be written as either of
	execl("/bin/sh", "sh", "-i", (char *)0);
	execl("/bin/sh", "sh", "-i", (char *)NULL);

But this problem can occur even without strange null pointers: such sloppy
code will already break on certain implementations where pointers and ints
have different lengths.

Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint
Followups to comp.lang.c.