Re: Kernel header policy

[Arjan van de Ven <arjan@infradead.org>]

On Tue, 2005-07-12 at 09:26 -0400, Peter Staubach wrote:
> Horst von Brand wrote:
> 
> >>I am contacting you to express my concern over a growing trend in kernel
> >>development.  I am specifically referring to changes being made to kernel
> >>headers that break compatibility at the userland level, where __KERNEL__
> >>isn't #define'd.
> >>    
> >>
> >
> >The policy with respect to kernel headers is /very/ simple:
> >
> >  T H E Y   A R E   N E V E R   U S E D   F R O M   U S E R L A N D.
> >
> >This general policy makes all your points (trivially) moot.
> >
> 
> I must admit a little confusion here.  Clearly, kernel header files are
> used at the user level.  The kernel and user level applications must share
> definitions for a great many things.

you are incorrect or rather imprecise here. Userspace needs headers
which define the kernel<->Userspace ABI. That is not the same as "the"
kernel headers.

> 
> Perhaps more precisely, the rule is that kernel header files should not be
> #include'd directly from user level applications, but may be #include'd
> indirectly through other header files as appropriate?

actually the rule in linux is that you should use cleaned up ABI
defining headers. There's several sets to chose from even. Generally
those sets have their origins in the kernel but are stripped down to
just the userspace-abi elements. 
(eg no kernel specific things like spinlocks or inlines or ..)