linux-kernel - Re: #pragma once?

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Message-ID: <20140107103806.GA4891@leaf>
Date:	Tue, 7 Jan 2014 02:38:06 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Michal Marek <mmarek@...e.cz>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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>,
	Rashika Kheria <rashika.kheria@...il.com>
Subject: Re: #pragma once?

On Tue, Jan 07, 2014 at 11:09:11AM +0100, Michal Marek wrote:
> 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.

That's debatable, but it's certainly reasonable to try to enforce
non-inclusion of "internal" headers directly.  However, for headers not
exposed to userspace, it'd be easy to write that as:

include/linux/spinlock.h:
#pragma once
#define LINUX_SPINLOCK_H_INCLUDED

include/linux/spinlock_up.h:
#pragma once
#ifndef LINUX_SPINLOCK_H_INCLUDED
#error "Only include this file via spinlock.h, never directly"
#endif

> 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.

Any mention in CodingStyle or check in checkpatch would need to cover
the two exceptions: uapi, and headers that are intentionally parsed
multiple times for preprocessor magic (TRACE_HEADER_MULTI_READ).

- Josh Triplett
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