From owner-freebsd-hackers@FreeBSD.ORG Wed Nov 23 17:32:09 2005 Return-Path: X-Original-To: freebsd-hackers@freebsd.org Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 65F0E16A41F for ; Wed, 23 Nov 2005 17:32:09 +0000 (GMT) (envelope-from dr2867@pacbell.net) Received: from smtp106.sbc.mail.mud.yahoo.com (smtp106.sbc.mail.mud.yahoo.com [68.142.198.205]) by mx1.FreeBSD.org (Postfix) with SMTP id A368843D5F for ; Wed, 23 Nov 2005 17:32:08 +0000 (GMT) (envelope-from dr2867@pacbell.net) Received: (qmail 24011 invoked from network); 23 Nov 2005 17:32:07 -0000 Received: from unknown (HELO ?192.168.0.190?) (dr2867.business@pacbell.net@68.126.211.246 with plain) by smtp106.sbc.mail.mud.yahoo.com with SMTP; 23 Nov 2005 17:32:07 -0000 Message-ID: <4384A799.30809@pacbell.net> Date: Wed, 23 Nov 2005 09:32:09 -0800 From: Daniel Rudy User-Agent: Mozilla/5.0 (X11R6; UNIX; FreeBSD/i386 5.4-RELEASE-p7; en-US; ja-JP; rv:1.7.12) Gecko/20050915 MultiZilla/1.6.2.0c Mnenhy/0.7.2.0 X-Accept-Language: en-us, en, ja MIME-Version: 1.0 To: rookie@gufi.org References: <4383F0CA.2030609@pacbell.net> <3bbf2fe10511230323xafaea9cr@mail.gmail.com> In-Reply-To: <3bbf2fe10511230323xafaea9cr@mail.gmail.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: freebsd-hackers@freebsd.org Subject: Re: getdirentries_args and other kernel syscall structures X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Nov 2005 17:32:09 -0000 At about the time of 11/23/2005 3:23 AM, rookie stated the following: > 2005/11/23, Daniel Rudy : > >>Ok, I'va got a little question here. In the structure >>getdirentries_args, there seems to be duplicated fields that I'm not >>entirely sure what they do. Here's the definition of a structure >>verbatim from sys/sysproto.h: >> >>struct getdirentries_args { >> char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)]; >> char buf_l_[PADL_(char *)]; char * buf; char buf_r_[PADR_(char *)]; >> char count_l_[PADL_(u_int)]; u_int count; char >>count_r_[PADR_(u_int)]; >> char basep_l_[PADL_(long *)]; long * basep; char >>basep_r_[PADR_(long *)]; >>}; >> >>Now my question is what does the l and r variables do? It seems that >>they do something with padding the data based on the endian of the >>machine? I look through this header file, and I see all the structures >>have similar constructs. Is it something that can be safely ignored > > > It just pads in the right way (according with endianism) the structure > to the right word. For example, x86 gots sizeof(long *) == 4. If you > want to have a syscall structure like that: > > struct example_sys > { > char f; > short p; > int g; > }; > > it is misaligned. In order to get a proper padded structure (all 32 > bits entries) to speed-up accesses to the members, this little trick > is used. > > Attilio > > -- > Peace can only be achieved by understanding - A. Einstein > Ah, a performance trick. I get it now. Thanks. -- Daniel Rudy