linux-kernel - Re: #pragma once?

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Date:	Tue, 7 Jan 2014 10:48:53 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Josh Triplett <josh@...htriplett.org>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Michal Marek <mmarek@...e.cz>,
	Rashika Kheria <rashika.kheria@...il.com>
Subject: Re: #pragma once?

On Tue, Jan 7, 2014 at 6:55 AM, Sam Ravnborg <sam@...nborg.org> wrote:
> On Mon, Jan 06, 2014 at 12:47:07PM -0800, Josh Triplett wrote:
>> [CCing build-system folks and others likely to know about potential
>> issues.]
>>
>> Does anyone have any objection to the use of "#pragma once" instead of
>> the usual #ifndef-#define-...-#endif include guard?  GCC, LLVM/clang,
>> and the latest Sparse all support either method just fine.  (I added
>> support to Sparse myself.)  Both have equivalent performance.  "#pragma
>> once" is simpler, and avoids the possibility of a typo in the defined
>> guard symbol.
> For kernel headers no concern.

Just being cautious:

Do we know the minimum gcc version that supports #pragma once?

Furthermore I found this:
| #pragma once does have one drawback (other than being non-standard) and
| that is if you have the same file in different locations then the
compiler will
| think these are different files.
http://stackoverflow.com/questions/787533/is-pragma-once-a-safe-include-guard

With asm-generic and uapi, do we have multiple header files that
deliberately use the same include guards?
I know we have header files that deliberately don't have include guards
(e.g. asm/unistd.h on some architectures).

> For UAPI headers we should be more carefull - as we do not know which
> compiler it ends up seeing - and what version.

Furthermore some userspace may rely on doing #define XXX to avoid
including a specific kernel header (yes, it's ugly).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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