linux-kernel - Re: [git pull] x86 updates for v2.6.28, phase #1

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Date:	Fri, 10 Oct 2008 14:21:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [git pull] x86 updates for v2.6.28, phase #1

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 § 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
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