From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753585Ab2GNKcq (ORCPT ); Sat, 14 Jul 2012 06:32:46 -0400 Received: from astoria.ccjclearline.com ([64.235.106.9]:59522 "EHLO astoria.ccjclearline.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752172Ab2GNKcp (ORCPT ); Sat, 14 Jul 2012 06:32:45 -0400 Date: Sat, 14 Jul 2012 06:32:40 -0400 (EDT) From: "Robert P. J. Day" X-X-Sender: rpjday@oneiric To: Linux Kernel Mailing List Subject: [PATCH] Documentation/CodingStyle: Explain using GCC's statement expression extension for defining macros Message-ID: User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - astoria.ccjclearline.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - crashcourse.ca X-Source: X-Source-Args: X-Source-Dir: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Add an explanation of how to use GCC's statement expression extension for defining multi-line macros. Signed-off-by: Robert P. J. Day --- diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle index cb9258b..c0eec33 100644 --- a/Documentation/CodingStyle +++ b/Documentation/CodingStyle @@ -600,7 +600,9 @@ may be named in lower case. Generally, inline functions are preferable to macros resembling functions. -Macros with multiple statements should be enclosed in a do - while block: +Macros with multiple statements can be defined in one of two ways. The +original technique encloses the body of the macro in a do - while block, +as in: #define macrofun(a, b, c) \ do { \ @@ -608,6 +610,17 @@ Macros with multiple statements should be enclosed in a do - while block: do_this(b, c); \ } while (0) +An alternative technique is to use GCC's statement expression extension, +explained at http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html, +which allows you to place statements and declarations in a single expression, +as with this example from : + +#define roundup(x, y) \ +({ \ + const typeof(y) __y = y; \ + (((x) + (__y - 1)) / __y) * __y; \ +}) + Things to avoid when using macros: 1) macros that affect control flow: -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================