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From: karl@haddock.UUCP
Newsgroups: comp.lang.c
Subject: Arithmetic on NULL (was: char (*a)[])
Message-ID: <1151@haddock.ISC.COM>
Date: Wed, 16-Sep-87 20:36:42 EDT
Article-I.D.: haddock.1151
Posted: Wed Sep 16 20:36:42 1987
Date-Received: Sat, 19-Sep-87 06:04:10 EDT
References: <1459@cullvax.UUCP> <48400001@tub.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 18

In article <48400001@tub.UUCP> cabo@tub.UUCP writes:
>While I agree that "char *p = 0; p++;" should not be allowed by the standard,
>I see some benign applications for constant expressions involving null
>pointers, e.g.
>	(char *)&((struct foo *)0)->bar - (char *)&((struct foo *)0)->baz

In ANSI C, the correct way to write the above is "offsetof(struct foo, bar) -
offsetof(struct foo, baz)".  Since this is guaranteed to work, your example is
not evidence of a need for NULL pointer arithmetic.

Now, it may well be the case that "offsetof" is defined in <stddef.h> to
expand into the above mess, on machines where it happens to work.  That's
okay; the use is portable even though the implementation is not.

I expect that machines with non-flat address spaces and weird representations
of NULL may have to implement "offsetof" as a builtin.  That's okay too.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint