From owner-freebsd-hackers@FreeBSD.ORG  Fri Nov  7 19:27:49 2008
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id D5D90106564A
	for <freebsd-hackers@freebsd.org>; Fri,  7 Nov 2008 19:27:49 +0000 (UTC)
	(envelope-from sem@FreeBSD.org)
Received: from mail.ciam.ru (mail.ciam.ru [212.34.63.72])
	by mx1.freebsd.org (Postfix) with ESMTP id 953C08FC08
	for <freebsd-hackers@freebsd.org>; Fri,  7 Nov 2008 19:27:49 +0000 (UTC)
	(envelope-from sem@FreeBSD.org)
Received: from dhcp250-210.yandex.ru ([87.250.250.210])
	by mail.ciam.ru with esmtpa (Exim 4.x) id 1KyWgK-000NcD-1z
	for freebsd-hackers@freebsd.org; Fri, 07 Nov 2008 22:07:44 +0300
Message-ID: <491491E5.8090805@FreeBSD.org>
Date: Fri, 07 Nov 2008 22:07:17 +0300
From: Sergey Matveychuk <sem@FreeBSD.org>
User-Agent: Thunderbird 2.0.0.17 (Windows/20080914)
MIME-Version: 1.0
To: freebsd-hackers@freebsd.org
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Subject: LIST_REMOVE problem
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 07 Nov 2008 19:27:49 -0000

Hi!

I wonder how this example from queue(3) works:

      while (!LIST_EMPTY(&head)) {            /* List Deletion. */
              n1 = LIST_FIRST(&head);
              LIST_REMOVE(n1, entries);
              free(n1);
      }

If we'll take a look at code, LIST_REMOVE don't change a head pointer if 
you remove the first element:

#define LIST_FIRST(head)        ((head)->lh_first)
...
#define LIST_REMOVE(elm, field) do {                        \
         if (LIST_NEXT((elm), field) != NULL)                \
                 LIST_NEXT((elm), field)->field.le_prev =    \
                     (elm)->field.le_prev;                   \
         *(elm)->field.le_prev = LIST_NEXT((elm), field);    \
} while (0)

So n1 in the example above will point to removed (and freed) element.

-- 
Dixi.
Sem.