From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030644Ab2I2Czo (ORCPT ); Fri, 28 Sep 2012 22:55:44 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:50546 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030612Ab2I2CzQ convert rfc822-to-8bit (ORCPT ); Fri, 28 Sep 2012 22:55:16 -0400 X-Originating-IP: 217.70.178.132 X-Originating-IP: 50.43.46.74 Date: Fri, 28 Sep 2012 19:55:07 -0700 From: Josh Triplett To: Daniel Santos Cc: LKML , Andi Kleen , Andrea Arcangeli , Andrew Morton , Christopher Li , David Daney , David Howells , Joe Perches , Konstantin Khlebnikov , linux-sparse@vger.kernel.org, Michel Lespinasse , Paul Gortmaker , Pavel Pisa , Peter Zijlstra , Steven Rostedt Subject: Re: [PATCH 8/10] bug.h: Make BUILD_BUG_ON generate compile-time error Message-ID: <20120929025506.GA3183@leaf> References: <1348874411-28288-1-git-send-email-daniel.santos@pobox.com> <1348874411-28288-9-git-send-email-daniel.santos@pobox.com> <20120929003239.GA14293@jtriplet-mobl1> <50664E30.7050809@att.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <50664E30.7050809@att.net> User-Agent: Mutt/1.5.21 (2010-09-15) Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 28, 2012 at 08:26:08PM -0500, Daniel Santos wrote: > On 09/28/2012 07:32 PM, Josh Triplett wrote: > > On Fri, Sep 28, 2012 at 06:20:09PM -0500, Daniel Santos wrote: > >> Negative sized arrays wont create a compile-time error in some cases > >> starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced > >> the error function attribute that will. This patch modifies > >> BUILD_BUG_ON to behave like BUILD_BUG already does, using the error > >> function attribute so that you don't have to build the entire kernel to > >> discover that you have a problem, and then enjoy trying to track it down > >> from a link-time error. > > > > Rather than doing both, and potentially producing two errors for the > > same issue, how about using __compiletime_error only, and only using the > > negative-sized array when __compiletime_error has no useful definition? > > > > For instance, in compiler.h, when defining __compiletime_error as an > > empty macro in the fallback case, you could define a > > __compiletime_error_fallback() macro that declares a negative-sized > > array; you could then define __compiletime_error_fallback() as an empty > > macro when it doesn't exist. > It may also make sense to define > #define BUILD_BUG() BUILD_BUG_ON(1) > > I haven't examined this really closely yet and my eyes are getting a > little bleary :) > > > Really, I would have liked to be able to set the error message as > well, but we would have to have the macro generate a unique function > name for each time its expanded to make that work. Example: > > #define BUILD_BUG_ON(cond) BUILD_BUG_ON_MSG(cond, #cond) > #define BUILD_BUG_ON_MSG(cond, msg) \ > do { \ > extern void __build_bug_on_failed ## something_unique(void)\ > __compiletime_error("BUILD_BUG_ON failed: " msg); \ > __compiletime_error_fallback(cond); \ > if (cond) \ > __build_bug_on_failed(); \ > } while(0) > > I just don't know any tricks to generate unique pre-processor > value automatically. Assuming you don't call BUILD_BUG_ON_MSG more than once per line: /tmp$ cat test.c #define BUILD_BUG_ON_MSG_INTERNAL2(cond, msg, line) \ do { \ extern void __build_bug_on_failed_ ## line (void) __attribute__((error(msg))); \ if (cond) \ __build_bug_on_failed_ ## line(); \ } while (0) #define BUILD_BUG_ON_MSG_INTERNAL(cond, msg, line) BUILD_BUG_ON_MSG_INTERNAL2(cond, msg, line) #define BUILD_BUG_ON_MSG(cond, msg) BUILD_BUG_ON_MSG_INTERNAL(cond, msg, __LINE__) void f(void) { BUILD_BUG_ON_MSG(0, "test 1"); BUILD_BUG_ON_MSG(1, "test 2"); BUILD_BUG_ON_MSG(0, "test 3"); BUILD_BUG_ON_MSG(1, "test 4"); } /tmp$ gcc -c test.c test.c: In function ‘f’: test.c:14:119: error: call to ‘__build_bug_on_failed_14’ declared with attribute error: test 2 test.c:16:119: error: call to ‘__build_bug_on_failed_16’ declared with attribute error: test 4 - Josh Triplett