From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 23067C6786F for ; Sun, 28 Oct 2018 11:46:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D3D482075D for ; Sun, 28 Oct 2018 11:46:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D3D482075D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lip6.fr Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727462AbeJ1UbK (ORCPT ); Sun, 28 Oct 2018 16:31:10 -0400 Received: from mail3-relais-sop.national.inria.fr ([192.134.164.104]:25385 "EHLO mail3-relais-sop.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726354AbeJ1UbK (ORCPT ); Sun, 28 Oct 2018 16:31:10 -0400 X-IronPort-AV: E=Sophos;i="5.54,436,1534802400"; d="scan'208";a="283549915" Received: from 89-157-201-244.rev.numericable.fr (HELO hadrien) ([89.157.201.244]) by mail3-relais-sop.national.inria.fr with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 28 Oct 2018 12:46:42 +0100 Date: Sun, 28 Oct 2018 12:46:42 +0100 (CET) From: Julia Lawall X-X-Sender: jll@hadrien To: Himanshu Jha cc: Sasha Levin , Shayenne da Luz Moura , Greg Kroah-Hartman , Hans de Goede , Michael Thayer , devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org, outreachy-kernel@googlegroups.com Subject: Re: [Outreachy kernel] [RESEND PATCH 2/2] staging: vboxvideo: Use unsigned int instead bool In-Reply-To: <20181028112011.GA5157@himanshu-Vostro-3559> Message-ID: References: <211701e4ae42acd95afb24713314bce5a4c58ecf.1540580493.git.shayenneluzmoura@gmail.com> <20181026204225.GH2015@sasha-vm> <20181028075209.GA1938@himanshu-Vostro-3559> <20181028112011.GA5157@himanshu-Vostro-3559> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 28 Oct 2018, Himanshu Jha wrote: > On Sun, Oct 28, 2018 at 09:47:15AM +0100, Julia Lawall wrote: > > > The "possible alignement issues" in CHECK report is difficult to figure > > > out by just doing a glance analysis. :) > > > > > > Linus also suggested to use bool as the base type i.e., `bool x:1` but > > > again sizeof(_Bool) is implementation defined ranging from 1-4 bytes. > > > > If bool x:1 has the size of bool, then wouldn't int x:1 have the size of > > int? But my little experiments suggest that the size is the smallest that > > fits the requested bits and alignment chosen by the compiler, regardless of > > the type. > > Yes, correct! > And we can't use sizeof on bitfields *directly*, nor reference it using a > pointer. > > It can be applied only when these bitfields are wrapped in a structure. > > Testing: > > #include > #include > > struct S { > bool a:1; > bool b:1; > bool c:1; > bool d:1; > }; > > int main(void) > { > printf("%zu\n", sizeof(struct S)); > } > > Output: 1 > > If I change all bool to unsigned int, output is: *4*. > > So, conclusion is compiler doesn't squeeze the size less than > native size of the datatype i.e., if we changed all members to > unsigned int:1, > total width = 4 bits > padding = 4 bits > > Therefore, total size should have been = 1 byte! > But since sizeof(unsigned int) == 4, it can't be squeezed to > less than it. This conclusion does not seem to be correct, if you try the following program. I get 4 for everything, meaning that the four unsigned int bits are getting squeezed into one byte when it is convenient. #include #include struct S1 { bool a:1; bool b:1; bool c:1; bool d:1; char a1; char a2; char a3; }; struct S2 { unsigned int a:1; unsigned int b:1; unsigned int c:1; unsigned int d:1; char a1; char a2; char a3; }; int main(void) { printf("%zu\n", sizeof(struct S1)); printf("%zu\n", sizeof(struct S2)); printf("%zu\n", sizeof(unsigned int)); } > Well, int x:1 can either have 0..1 or -1..0 range due implementation > defined behavior as I said in the previous reply. > > If you really want to consider negative values, then make it explicit > using `signed int x:1` which make range guaranteed to be -1..0 The code wants booleans, not negative values. julia