linux-kernel - Re: why so many unexported headers checking __KERNEL_

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Date:	Wed, 26 Mar 2008 13:10:22 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why so many unexported headers checking __KERNEL__?

On Wed, 2008-03-26 at 08:53 -0400, Robert P. J. Day wrote:
> not surprisingly, the only reason i noticed the above was because i
> hacked together a short script that went looking for all of the above
> and i was surprised at the number of files it identified. 

It's not _that_ surprising. Remember, before headers_install people just
used to copy _all_ the headers across, and so the only way to hide stuff
was to wrap entire files in #ifdef __KERNEL__.

> p.s. the other case that could be identified is when a header file has
> its *entire* contents encased in a __KERNEL__ test, (either ifdef or
> ifndef).  AFAICT, unless that kind of test is partitioning *some* of a
> header file content from the remainder, there's little value in a
> __KERNEL__test if the end result is to either:
> 
>   a) leave the file exactly as is, or
>   b) reduce it to empty

Right.

If it's entirely #ifndef __KERNEL__ then it's a userspace header. It
probably doesn't live in the kernel source tree at all.

If it's entirely #ifdef __KERNEL__ then it shouldn't be exported at all
(although when we do that we sometimes have to deal with userspace
programs which include it even though it's empty).

-- 
dwmw2

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