Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752576AbdCFAbK convert rfc822-to-8bit (ORCPT ); Sun, 5 Mar 2017 19:31:10 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:35420 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752509AbdCFAbJ (ORCPT ); Sun, 5 Mar 2017 19:31:09 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Henrique de Moraes Holschuh Cc: Tomas Winkler , "linux-kernel\@vger.kernel.org" , sparse@vger.kernel.org Subject: Re: Arrays of variable length References: <20170305211254.GA3220@khazad-dum.debian.net> Date: Mon, 06 Mar 2017 00:31:03 +0000 In-Reply-To: <20170305211254.GA3220@khazad-dum.debian.net> (Henrique de Moraes Holschuh's message of "Sun, 5 Mar 2017 18:12:54 -0300") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1324 Lines: 33 Henrique de Moraes Holschuh writes: > On Sun, 05 Mar 2017, M?ns Rullg?rd wrote: >> Tomas Winkler writes: >> > Sparse complains for arrays declared with variable length >> > >> > 'warning: Variable length array is used' >> > >> > Prior to c99 this was not allowed but lgcc (c99) doesn't have problem >> > with that https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html. >> > And also Linux kernel compilation with W=1 doesn't complain. >> > >> > Since sparse is used extensively would like to ask what is the correct >> > usage of arrays of variable length >> > within Linux Kernel. >> >> Variable-length arrays are a very bad idea. Don't use them, ever. >> If the size has a sane upper bound, just use that value statically. >> Otherwise, you have a stack overflow waiting to happen and should be >> using some kind of dynamic allocation instead. >> >> Furthermore, use of VLAs generally results in less efficient code. For >> instance, it forces gcc to waste a register for the frame pointer, and >> it often prevents inlining. > > Well, if we're going to forbid VLAs in the kernel, IMHO the kernel build > system should call gcc with -Werror=vla to get that point across early, > and flush out any offenders. If it were up to me, that's exactly what I'd do. -- M?ns Rullg?rd