Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932607AbWHAJ7y (ORCPT ); Tue, 1 Aug 2006 05:59:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932608AbWHAJ7y (ORCPT ); Tue, 1 Aug 2006 05:59:54 -0400 Received: from amsfep17-int.chello.nl ([213.46.243.15]:10660 "EHLO amsfep12-int.chello.nl") by vger.kernel.org with ESMTP id S932607AbWHAJ7y (ORCPT ); Tue, 1 Aug 2006 05:59:54 -0400 Subject: Re: do { } while (0) question From: Peter Zijlstra To: Jiri Slaby Cc: Hua Zhong , "'Heiko Carstens'" , "'Andrew Morton'" , linux-kernel@vger.kernel.org, "'Martin Schwidefsky'" In-Reply-To: <44CF22E8.9020307@gmail.com> References: <008e01c6b549$59e52f70$493d010a@nuitysystems.com> <1154425171.32739.2.camel@taijtu> <44CF22E8.9020307@gmail.com> Content-Type: text/plain Date: Tue, 01 Aug 2006 11:59:59 +0200 Message-Id: <1154426399.32739.8.camel@taijtu> Mime-Version: 1.0 X-Mailer: Evolution 2.6.2 (2.6.2-1.fc5.5) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1405 Lines: 44 On Tue, 2006-08-01 at 11:45 +0159, Jiri Slaby wrote: > Peter Zijlstra wrote: > > On Tue, 2006-08-01 at 02:03 -0700, Hua Zhong wrote: > >>> #if KILLER == 1 > >>> #define MACRO > >>> #else > >>> #define MACRO do { } while (0) > >>> #endif > >>> > >>> { > >>> if (some_condition) > >>> MACRO > >>> > >>> if_this_is_not_called_you_loose_your_data(); > >>> } > >>> > >>> How do you want to define KILLER, 0 or 1? I personally choose 0. > >> Really? Does it compile? > > > > No, and that is the whole point. > > > > The empty 'do {} while (0)' makes the missing semicolon a syntax error. > > Bulls^WNope, it was a bad example (we don't want to break the compilation, just > not want to emit a warn or an err). It was a perfectly good example why 'do {} while (0)' is useful. The perhaps mistakenly forgotten ';' after MACRO will not stop your example from compiling if KILLER == 1. Even worse, it will compile and do something totally unexpected. If however you use KILLER != 1, the while(0) will require a ';' and this example will fail to compile. Not compiling when you made a coding error (forgetting ';' is one of the most common) is a great help. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/