Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!mcvax!ukc!eagle!icdoc!cam-cl!am
From: am@cl.cam.ac.uk (Alan Mycroft)
Newsgroups: comp.lang.c
Subject: offsetof (was Arithmetic on NULL)
Message-ID: <1076@jenny.cl.cam.ac.uk>
Date: Mon, 12-Oct-87 07:00:38 EDT
Article-I.D.: jenny.1076
Posted: Mon Oct 12 07:00:38 1987
Date-Received: Thu, 15-Oct-87 20:08:18 EDT
References: <1459@cullvax.UUCP> <48400001@tub.UUCP> <1151@haddock.ISC.COM>
Reply-To: am@cl.cam.ac.uk (Alan Mycroft)
Organization: U of Cambridge Comp Lab, UK
Lines: 20

In article <1151@haddock.ISC.COM> karl@haddock.ima.isc.com (Karl Heuer) wrt:
>>I see some benign applications for constant expressions involving null
>>	(char *)&((struct foo *)0)->bar - (char *)&((struct foo *)0)->baz
>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 have a concern about this.  ANSI (Oct 86) specifically disallows
unary-& from appearing in constant expressions (no doubt
except in sizeof() context).  Thus offsetof cannot expand into
the above mess since it has to be a constant expression, e.g. for
initialisers.

My current hack is to take the view that &((struct foo *)0)->bar)
is a constant expression (pace ANSI) to allow offsetof() to be definable
by macro at all.

Apart from the arguably worse suggestion that offsetof expands into 'funny'
reserved words beginning with '_' does anyone have a ANSI-legal version of
offsetof() for any machine (e.g. vax-like)?  I suppose I reckon it is
impossible, but requires much legalistic reading of dpANS.