linux-kernel - Re: #pragma once?

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Message-ID: <52CBD247.8080508@suse.cz>
Date:	Tue, 07 Jan 2014 11:09:11 +0100
From:	Michal Marek <mmarek@...e.cz>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Sam Ravnborg <sam@...nborg.org>,
	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>,
	Rashika Kheria <rashika.kheria@...il.com>
Subject: Re: #pragma once?

On 2014-01-07 10:48, Geert Uytterhoeven wrote:
> Furthermore some userspace may rely on doing #define XXX to avoid
> including a specific kernel header (yes, it's ugly).

This pattern is also sometimes used:
$ head -6 include/linux/spinlock_up.h
#ifndef __LINUX_SPINLOCK_UP_H
#define __LINUX_SPINLOCK_UP_H

#ifndef __LINUX_SPINLOCK_H
# error "please don't include this file directly"
#endif

And there is nothing ugly about it. So #pragma once is probably a good
idea for most headers that are not exposed to userspace. But making it a
requirement in scripts/checkpatch.pl or Documentation/CodingStyle means
that it will become hard to defend the few legitimate uses of ifndef
guards against people who have a printed copy of checkpatch.pl under
their pillow.

Michal
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