linux-kernel - Re: using kernel headers in libc headers

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Date:	Mon, 30 Nov 2009 09:55:56 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ulrich Drepper <drepper@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: using kernel headers in libc headers

On 11/30/2009 09:43 AM, Ulrich Drepper wrote:
> On Mon, Nov 30, 2009 at 09:01, H. Peter Anvin <hpa@...or.com> wrote:
>> A better way is to factor out subsets; if <linux/sched.h> has too many
>> things, we can break out the POSIX parts into <linux/sched_posix.h> or
>> (certainly better if we have more than one of these)
>> <linux/sched/posix.h> which can also be included by <linux/sched.h>.
> 
> This is at least as undesirable.
> 
> First, there can be several different of those.  E.g., there are
> different levels of POSIX compliance and the number of growing.  There
> are also conditions like
> 
>   if POSIX version > 2001012 || GNU source
> 
> How do you express this?

Very simple: you factor it into subsets.  The above kind of stuff is
*exactly* why this has no business in the kernel headers -- it exposes
glibc internals way too deeply.

> Second, it makes it hard to impossible for developers to use the
> headers as part of the system documentation.  Many people (me
> included) look at headers and the included comments.  With your scheme
> the set of definitions (e.g., SCHED_* macros) might be spread out over
> several different headers.  Currently they are all nicely group (in
> the kernel and libc headers) and people can see what is available.

That is exactly why I said <linux/sched/foo.h> is preferrable to
<linux/sched_foo.h> -- with more than one subset then it is better to
combine them into a subdirectory so they can be rapidly found.

We already have been through the #ifdef hell once, and we are still
crawling out of it.  It was -- and is -- an utter miserable failure.
Explicitly forcing factoring into subsets and leaving it to the libc
layer to decide what subsets to invoke is the only sane option.  This is
*especially* so when you consider that you have to account for version
skew next time glibc or uclibc or whateverlibc introduces new feature
macros.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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