From: Nick Piggin <piggin@cyberone.com.au>
To: William Lee Irwin III <wli@holomorphy.com>
Cc: Darren Williams <dsw@gelato.unsw.edu.au>,
Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: VM code question
Date: Tue, 14 Oct 2003 11:52:36 +1000 [thread overview]
Message-ID: <3F8B56E4.1060902@cyberone.com.au> (raw)
In-Reply-To: <20031014014427.GL16158@holomorphy.com>
William Lee Irwin III wrote:
>On Tue, Oct 14, 2003 at 11:32:27AM +1000, Darren Williams wrote:
>
>>I have a small question wrt some VM code.
>>source file is include/linux/kernel.h
>>#define container_of(ptr, type, member) ({ \
>> const typeof( ((type *)0)->member ) *__mptr = (ptr); \
>> (type *)( (char *)__mptr - offsetof(type,member) );})
>>what is the use of the 0 (zero) in the typeof? I am thinking
>>that we are casting 0 to (type *) then referencing 'member' of
>>'type', however why do we require the 0 ?
>>Just curious
>>
>
>It's an address calculation method. We subtract the address of the
>start of the structure from the address of the member inside the
>structure.
>
AFAIKS the 0 is not part of the address calculation method though. It
is only used in the argument to the typeof operator. I think 0 is used
simply because its as good a place as any, right?
next prev parent reply other threads:[~2003-10-14 1:52 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-10-14 1:32 VM code question Darren Williams
2003-10-14 1:44 ` William Lee Irwin III
2003-10-14 1:52 ` Nick Piggin [this message]
2003-10-14 2:03 ` William Lee Irwin III
2003-10-14 5:07 ` Matt Mackall
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