linux-kernel - Re: #pragma once?

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Message-ID: <20140107105054.GB4891@leaf>
Date:	Tue, 7 Jan 2014 02:50:54 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Sam Ravnborg <sam@...nborg.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 07, 2014 at 10:48:53AM +0100, Geert Uytterhoeven wrote:
> 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?

>From checking the manuals, it goes back to at least 2.95.  Searching
suggests that versions before 3.4 have a few bugs in "#pragma once"
support, but that those bugs only apply to using #pragma once in
combination with precompiled headers, which doesn't apply to the kernel.

- Josh Triplett
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