From owner-freebsd-current@FreeBSD.ORG  Thu Jul 10 02:24:11 2003
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 5D3C837B401
	for <current@freebsd.org>; Thu, 10 Jul 2003 02:24:11 -0700 (PDT)
Received: from mailhub.fokus.fraunhofer.de (mailhub.fokus.fraunhofer.de
	[193.174.154.14])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 26DC643FAF
	for <current@freebsd.org>; Thu, 10 Jul 2003 02:24:08 -0700 (PDT)
	(envelope-from brandt@fokus.fraunhofer.de)
Received: from beagle (beagle [193.175.132.100])h6A9O2Q27345;
	Thu, 10 Jul 2003 11:24:02 +0200 (MEST)
Date: Thu, 10 Jul 2003 11:24:02 +0200 (CEST)
From: Harti Brandt <brandt@fokus.fraunhofer.de>
To: Luigi Rizzo <rizzo@icir.org>
In-Reply-To: <20030710011956.A61125@xorpc.icir.org>
Message-ID: <20030710111332.C30571@beagle.fokus.fraunhofer.de>
References: <20030710011956.A61125@xorpc.icir.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
cc: current@freebsd.org
Subject: Re: what is the suggested way to do void * arithmetic ?
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 10 Jul 2003 09:24:11 -0000

On Thu, 10 Jul 2003, Luigi Rizzo wrote:

LR>
LR>Hi,
LR>in several places in ipfw2.c i have to move pointers across
LR>structures of variable length (lists of ipfw2 instructions
LR>returned by the getsockopt()), and i use the following type of code:
LR>
LR>	void *next;
LR>	foo *p;
LR>
LR>	...
LR>
LR>	next = (void *)p + len;
LR>	...
LR>	foo = (foo *)p + len;
LR>
LR>When using WARNS=5, the compiler in -current flags them with 'Warning
LR>void * arithmetic'.
LR>
LR>What is the best way to do the above given that i do need to use
LR>these variable-size structures ?

Because sizeof(char), sizeof(unsigned char) and sizeof(signed char) all
are defined to be 1 by the standard, any of them'll do it. Arithmetic on
void pointers is undefined by the standard.

next_foo = (struct foo *)((char *)this_foo + this_foo->len);

provided that the object pointed to by this_foo is at least this_foo->len
bytes long.

harti
-- 
harti brandt,
http://www.fokus.fraunhofer.de/research/cc/cats/employees/hartmut.brandt/private
brandt@fokus.fraunhofer.de, harti@freebsd.org