Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762638AbYJJVWZ (ORCPT ); Fri, 10 Oct 2008 17:22:25 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759310AbYJJVWR (ORCPT ); Fri, 10 Oct 2008 17:22:17 -0400 Received: from terminus.zytor.com ([198.137.202.10]:51214 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758815AbYJJVWR (ORCPT ); Fri, 10 Oct 2008 17:22:17 -0400 Message-ID: <48EFC775.9090900@zytor.com> Date: Fri, 10 Oct 2008 14:21:57 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: Linus Torvalds CC: Vegard Nossum , Ingo Molnar , Linux Kernel Mailing List , Andrew Morton , Thomas Gleixner Subject: Re: [git pull] x86 updates for v2.6.28, phase #1 References: <20081009234705.GA20540@elte.hu> <20081010152631.GB26231@elte.hu> <20081010155830.GA4767@elte.hu> <19f34abd0810101339x66afc6d3mb6efa1a52e186816@mail.gmail.com> <48EFBE65.60505@zytor.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2216 Lines: 59 Linus Torvalds wrote: > > Yeah. If I recall the namespace rules correctly, you have to have a > underscore followed by another underscore or an upper-case letter. Those > cannot even be used for local variables by a conforming program. > > Single-underscore + lower-case letter is only reserved as an external > identifier. I _think_. Strictly speaking, they are "reserved for identifiers with file scope in both identifier and tag space" (POSIX.1-2004, xsi ยง 2.2.2, lines 718-). > So a conforming program could still do > > int main(int argc, char **argv) > { > int _asm_x86_types_h = 1; > return _asm_x86_types_h; > } > > and I _think_ it should be ok. Correct. > So a system macro definition that could mess with something (admittedly > crazy as that) has to be either double underscores or underscore + > uppercase, so as long as the prefix is "_ASM" (or _LINUX), we're ok. > > Double underscores in the _middle_ of the identifiers are a non-issue, and > are just ugly. Double underscores when used in external identifiers (not an issue here) can conflict with some C++ mangling schemes, in particular the old Cfront scheme (which had to be compatible with a standard C compiler.) gcc seems to use another scheme, but it doesn't either appear to include any non-C characters, which means it's at least theoretically possible to step on its toes. >> That being said, I don't personally like the double underscores. > > I agree, inside the identifier in particular they do seem pointless. At > the beginning, they are useful for things like __i386__ or __x86, where > you need the double underscore because 'i' is lower-case. > > Or maybe I mis-remember the name space rules. No, you're right on track. There is an additional rule, which is _de_facto_ since it is out of scope for the standards: identifiers of the form __foo__ (double underscores at the start *and* end) are often considered reserved for the *compiler*. -hpa -- 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/