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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: comp.lang.c
Subject: Re: offsetof (was Arithmetic on NULL)
Message-ID: <2500@mmintl.UUCP>
Date: Tue, 20-Oct-87 20:28:58 EDT
Article-I.D.: mmintl.2500
Posted: Tue Oct 20 20:28:58 1987
Date-Received: Sat, 24-Oct-87 16:34:29 EDT
References: <1459@cullvax.UUCP> <48400001@tub.UUCP> <1151@haddock.ISC.COM> <1076@jenny.cl.cam.ac.uk>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT.
Lines: 20

In article <1076@jenny.cl.cam.ac.uk> am@cl.cam.ac.uk (Alan Mycroft) writes:
|In article <1151@haddock.ISC.COM> karl@haddock.ima.isc.com (Karl Heuer) wrt:
|>>	(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.
|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.

I don't have a copy of the proposed standard available, but I would guess
that it does *not* disallow unary & in constant expressions.  I would guess
that, instead, it merely does not require that use of unary & be supported.
So a compiler writer would be perfectly free to support it, and then
implement offsetof in this way.
-- 

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108