Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965127AbXAJVts (ORCPT ); Wed, 10 Jan 2007 16:49:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965128AbXAJVts (ORCPT ); Wed, 10 Jan 2007 16:49:48 -0500 Received: from smtp.ono.com ([62.42.230.12]:56422 "EHLO resmaa02.ono.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S965127AbXAJVtr (ORCPT ); Wed, 10 Jan 2007 16:49:47 -0500 Date: Wed, 10 Jan 2007 22:49:22 +0100 From: "J.A. =?UTF-8?B?TWFnYWxsw7Nu?=" To: "linux-os \(Dick Johnson\)" Cc: "Robert P. J. Day" , "Stefan Richter" , "Linux kernel mailing list" Subject: Re: macros: "do-while" versus "({ })" and a compile-time error Message-ID: <20070110224922.2de6a641@werewolf-wl> In-Reply-To: References: <45A3D1DF.4020205@s5r6.in-berlin.de> X-Mailer: Claws Mail 2.7.0cvs4 (GTK+ 2.10.7; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3497 Lines: 101 On Wed, 10 Jan 2007 07:16:55 -0500, "linux-os \(Dick Johnson\)" wrote: > > On Wed, 10 Jan 2007, Robert P. J. Day wrote: > > > On Tue, 9 Jan 2007, linux-os (Dick Johnson) wrote: > > > >> > >> On Tue, 9 Jan 2007, Stefan Richter wrote: > >> > >>> Robert P. J. Day wrote: > >>>> just to stir the pot a bit regarding the discussion of the two > >>>> different ways to define macros, > >>> > >>> You mean function-like macros, right? > >>> > >>>> i've just noticed that the "({ })" > >>>> notation is not universally acceptable. i've seen examples where > >>>> using that notation causes gcc to produce: > >>>> > >>>> error: braced-group within expression allowed only inside a function > >>> > >>> And function calls and macros which expand to "do { expr; } while (0)" > >>> won't work anywhere outside of functions either. > >>> > >>>> i wasn't aware that there were limits on this notation. can someone > >>>> clarify this? under what circumstances *can't* you use that notation? > >>>> thanks. > >>> > >>> The limitations are certainly highly compiler-specific. > >> > >> I don't think so. You certainly couldn't write working 'C' code like > >> this: > >> > >> do { a = 1; } while(0); > >> > >> This _needs_ to be inside a function. In fact any runtime operations > >> need to be inside functions. It's only in assembly that you could > >> 'roll your own' code like: > >> > >> main: > >> ret 0 > >> > >> > >> Most of these errors come about as a result of changes where a macro > >> used to define a constant. Later on, it was no longer a constant in > >> code that didn't actually get compiled during the testing. > > > > just FYI, the reason i brought this up in the first place is that i > > noticed that the ALIGN() macro in kernel.h didn't verify that the > > alignment value was a power of 2, so i thought -- hmmm, i wonder if > > there are any invocations where that's not true, so i (temporarily) > > rewrote ALIGN to incorporate that check, and the build blew up > > including include/net/neighbour.h, which contains the out-of-function > > declaration: > > > > struct neighbour > > { > > ... > > unsigned char ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))]; > > ... > > > > so it's not a big deal, it was just me goofing around and breaking > > things. > > > > rday > > > Hmmm, in that case you would be trying to put code inside a structure! > Neat --if you could do it! > The ({ }) is a block expression, ie, it allows declaring variables and executing code. Its a gcc extension trying to resemble what other languages like ML have: ML: f = let y = x*x in 2*y + sin(y) end GNU C: f = ({ int y = x*x; 2*y + sin(y); }) So you can put it on every place you could also put a { } block or declare a variable. {} is a compund command and ({ }) is a compund expression (or block expression, do not know which is the good name in engelish). -- J.A. Magallon \ Software is like sex: \ It's better when it's free Mandriva Linux release 2007.1 (Cooker) for i586 Linux 2.6.19-jam03 (gcc 4.1.2 20061110 (prerelease) (4.1.2-0.20061110.1mdv2007.1)) #1 SMP PREEMPT - 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/