From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262183AbTJNFHd (ORCPT ); Tue, 14 Oct 2003 01:07:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262186AbTJNFHc (ORCPT ); Tue, 14 Oct 2003 01:07:32 -0400 Received: from waste.org ([209.173.204.2]:50852 "EHLO waste.org") by vger.kernel.org with ESMTP id S262183AbTJNFHb (ORCPT ); Tue, 14 Oct 2003 01:07:31 -0400 Date: Tue, 14 Oct 2003 00:07:13 -0500 From: Matt Mackall To: Nick Piggin Cc: William Lee Irwin III , Darren Williams , Linux Kernel Subject: Re: VM code question Message-ID: <20031014050712.GY5725@waste.org> References: <20031014013227.GA20406@cse.unsw.EDU.AU> <20031014014427.GL16158@holomorphy.com> <3F8B56E4.1060902@cyberone.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3F8B56E4.1060902@cyberone.com.au> User-Agent: Mutt/1.3.28i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 14, 2003 at 11:52:36AM +1000, Nick Piggin wrote: > > > 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? It could be simplified to: ((type *)((char *)(ptr) - offsetof(type, member))) The other bit is just there to throw errors if you cast in a pointer of the wrong type. To do this, we've got to create a pointer of the same type as &type.member so that assigning to it without casting will throw a warning if ptr isn't of the right type. But we can't do typeof(&type.member), as type is a type name and not an object. So 0 is simply the shortest, safest thing to cast to a (type *). -- Matt Mackall : http://www.selenic.com : Linux development and consulting