From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from caramon.arm.linux.org.uk ([217.147.92.249]:64778 "EHLO caramon.arm.linux.org.uk") by vger.kernel.org with ESMTP id S1750739AbWIMQiX (ORCPT ); Wed, 13 Sep 2006 12:38:23 -0400 Date: Wed, 13 Sep 2006 17:38:06 +0100 From: Russell King Subject: Re: [PATCH 4/6] Implement a general log2 facility in the kernel Message-ID: <20060913163806.GA15563@flint.arm.linux.org.uk> References: <20060913130253.32022.69230.stgit@warthog.cambridge.redhat.com> <20060913130300.32022.69743.stgit@warthog.cambridge.redhat.com> <20060913161734.GE3564@stusta.de> <20060913163136.GA2585@parisc-linux.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20060913163136.GA2585@parisc-linux.org> Sender: linux-arch-owner@vger.kernel.org To: Matthew Wilcox Cc: Adrian Bunk , David Howells , torvalds@osdl.org, akpm@osdl.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org List-ID: On Wed, Sep 13, 2006 at 10:31:36AM -0600, Matthew Wilcox wrote: > On Wed, Sep 13, 2006 at 06:17:34PM +0200, Adrian Bunk wrote: > > On Wed, Sep 13, 2006 at 02:03:00PM +0100, David Howells wrote: > > > From: David Howells > > > > > > This facility provides three entry points: > > > > > > log2() Log base 2 of u32 > > >... > > > > Considering that several arch maintainers have vetoed my patch to revert > > the -ffreestanding removal Andi sneaked in with his usual trick to hide > > generic patches as "x86_64 patch", such a usage of a libc function name > > with a signature different from the one defined in ISO/IEC 9899:1999 is > > a namespace violation that mustn't happen. > > log2 is only defined if math.h gets included. If we're including math.h > at any point in the kernel itself (excluding the bootloader and similar), > we're already screwed six ways from sunday. Adrian's point is that gcc without -ffreestanding may decide to implement log2() itself by issuing the appropriate floating point instructions rather than using a function call into a library to do this operation. Therefore, re-using "log2()" is about as bad as re-using the "strcmp()" name to implement a function which copies strings. And, sure enough, try throwing this at a compiler: int log2(int foo) { return foo; } you get: t.c:2: warning: conflicting types for built-in function 'log2' but not if you use -ffreestanding. Don't re-use C standard library identifiers (or use -ffreestanding.) -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core