From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262131AbTJNBl2 (ORCPT ); Mon, 13 Oct 2003 21:41:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262139AbTJNBl2 (ORCPT ); Mon, 13 Oct 2003 21:41:28 -0400 Received: from holomorphy.com ([66.224.33.161]:62344 "EHLO holomorphy") by vger.kernel.org with ESMTP id S262131AbTJNBl1 (ORCPT ); Mon, 13 Oct 2003 21:41:27 -0400 Date: Mon, 13 Oct 2003 18:44:27 -0700 From: William Lee Irwin III To: Darren Williams Cc: Linux Kern Subject: Re: VM code question Message-ID: <20031014014427.GL16158@holomorphy.com> Mail-Followup-To: William Lee Irwin III , Darren Williams , Linux Kern References: <20031014013227.GA20406@cse.unsw.EDU.AU> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20031014013227.GA20406@cse.unsw.EDU.AU> Organization: The Domain of Holomorphy User-Agent: Mutt/1.5.4i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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. -- wli