Sender:	nate AT cartsys DOT com
Message-ID:	<36AD59D1.8CB03C29@cartsys.com>
Date:	Mon, 25 Jan 1999 21:59:45 -0800
From:	Nate Eldredge <nate AT cartsys DOT com>
X-Mailer:	Mozilla 4.08 [en] (X11; I; Linux 2.0.36 i586)
MIME-Version:	1.0
To:	djgpp AT delorie DOT com
Subject:	Re: Strange pointer manipulation
References:	<36AD11AA DOT 3876 AT erols DOT com>
Reply-To:	djgpp AT delorie DOT com

John S. Fine wrote:
> 
> I want a decent way to make a macro, which I call undot,
> that does something like the reverse of the "." operator.
[delete application note]
>   The heart of the problem is representing the offset of b
> within FOO in some reasonable GCC syntax.  Clearly the
> compiler knows the value, since it uses it every time you
> do something like (xxx->b).  But, I don't know any good
> way to represent it.
> 
>   I don't need portable C;  A GCC specific kludge would be
> fine.  Using a GCC specific kludge, I came close:
[delete example]
>   Is there some C or GCC feature that I am overlooking that
> provides an easier way?

Yes; the `offsetof' macro.  `offsetof(type, member)' equals the offset
of `member' within `type'.  So

struct foo { int a, b, c; };
offsetof(struct foo, b)
   => 4

It's in <stddef.h>, so I assume it's ANSI.  Interestingly, GCC defines
it as

((size_t) &((TYPE *)0)->MEMBER)

IOW, the address of MEMBER of a TYPE at address 0.
-- 

Nate Eldredge
nate AT cartsys DOT com

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