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To: linux-kernel@vger.kernel.org
From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: PATCH: type safe(r) list_entry repacement: generic_out_cast
Date: Wed, 24 Jul 2002 01:04:53 +0000 (UTC)
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In article <200207240051.CAA16434@harpo.it.uu.se>,
Mikael Pettersson  <mikpe@csd.uu.se> wrote:
>On Tue, 23 Jul 2002 18:58:52 -0400, Kevin O'Connor wrote:
>>#define BackPtr(ptr, type, member) ({                                         \
>>        typeof( ((type *)0)->member ) *__mptr = (ptr);                        \
>>        ((type *)( (char *)__mptr - (unsigned long)(&((type *)0)->member) ));})
>
>I've seen this sort of code several times now in the Linux kernel,
>and I've never liked it. Is there some reason why you guys avoid
>offsetof() like the plague?

Trivial answer: offsetof() does not exist when you don't have system
header files. 

So it's not that it's getting avoided, it _has_ to be re-implemented
anyway.  And once you do that, you might as well do it right (ie what
the kernel wants is not offsetof, but a new pointer.

		Linus
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