From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chase Douglas Subject: Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots Date: Sat, 04 Feb 2012 19:21:36 +0100 Message-ID: <4F2D7730.3090704@gmail.com> References: <1328036403-2827-1-git-send-email-rydberg@euromail.se> <4F2B24AC.1020408@gmail.com> <20120203072725.GA6914@polaris.bitmath.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from mail-pz0-f46.google.com ([209.85.210.46]:36934 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753209Ab2BDSUg (ORCPT ); Sat, 4 Feb 2012 13:20:36 -0500 In-Reply-To: <20120203072725.GA6914@polaris.bitmath.org> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Henrik Rydberg Cc: Dmitry Torokhov , linux-input@vger.kernel.org, linux-kernel@vger.kernel.org On 02/03/2012 08:27 AM, Henrik Rydberg wrote: > Hi Chase, > >>> +#define INPUT_MT_REQUEST(num_slots) \ >>> + { \ >>> + __u32 code; \ >>> + __s32 values[num_slots]; \ >> >> I think this assumes a userspace C compiler that can handle variable >> length arrays. This would require only compiling in C source code at the >> C99 standard or later. It looks like C++ doesn't even allow variable >> length arrays, though gcc handles it. According to: >> >> http://www.cplusplus.com/forum/beginner/1601/ >> >> it looks like Borland c++ compilers may not be able to compile this :(. > > This is resolved on the preprocessor level, so C99 or not does not > enter the problem. Compile-time constant, as you can see in the code > example in the patch summary. You're right, I didn't catch that. It will be compatible with all C compilers if you use a static number of slots. However, it will break if you use a non-C99 C compiler and the code wants to do dynamic number of slots calculations. I imagine most callers would do: EVIOCGABS call on ABS_MT_SLOT; int num_slots = ABS_MT_SLOT.max - ABS_MT_SLOT.min struct INPUT_MT_REQUEST(num_slots) req; This will break on non-C99 C compilers and other language compilers. It also will lead to head-scratcher bugs when someone compiles it just fine in their C99 project, copies the code to another project with a different compiler, and is confronted with the issue. I think this issue should be enough to rethink the interface. -- Chase