using kernel headers in libc headers

[Ulrich Drepper <drepper@gmail.com>]

The kernel headers nowadays are cleaned up to separate in-kernel
definitions from those that are exported.  That's of course good but
it's unfortunately not sufficient to use (most of) the headers in
glibc.

Take <linux/sched.h> for instance.  The <sched.h> header is defined in
POSIX and therefore the specify which symbols can be defined in which
configuration.  The <linux/sched.h> defines all kinds of symbols,
though, unconditionally.  If you compare it with the glibc header
you'd see that all the CLONE_* symbols and some of the SCHED_* symbols
are only defined in certain configurations (non-POSIX configurations).

It's likely in everybody's interest to get the kernel headers used.
But for this mode #ifdefs are needed.  I'm willing to do the work.
Easy enough to do when I'm going through the headers one-by-one to
enable them in glibc.  But I don't want to start this work unless it's
going to be accepted.  The symbols would be something like

  __USE_MISC

or

  __USE_GNU

These are not macros which the user must define.  The user sets macros
like _GNU_SOURCE etc which then set all the appropriate __USE_* macros
(see "info libc 'Feature Test Macros'" on machines with glibc
installed).  These names are somewhat glibc-specific but other libcs
can (or already have) adapt them.

So, what do you say?