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[richard offer]
> Or userland libraries/applications that need to bypass libc and make
> direct kernel calls because libc hasn't yet implemented those new
> kernel calls.

Nah, it's still error-prone because it's too hard to guarantee that the
user compiling your program has up-to-date kernel headers in a location
you can find. Too many things can go wrong.

So just '#include <asm/unistd.h>' -- the libc version -- then have your
own header for those few things you consider "too new to be in libc":

/* my_unistd.h */
/* [not sure if all the __{arch}__ defines are right] */
#include <asm/unistd.h> /* from libc, not from kernel */
#ifndef __NR_pivot_root
# ifdef __alpha__
# define __NR_pivot_root 374
# endif
# if defined(__i386__) || defined(__s390__) || defined(__superh__)
# define __NR_pivot_root 217
# endif
# ifdef __mips__
# define __NR_pivot_root (__NR_Linux + 216)
# endif
# ifdef __hppa__
# define __NR_pivot_root (__NR_Linux + 67)
# endif
# ifdef __sparc__
# define __NR_pivot_root 146
# endif
#ifndef __NR_pivot_root
# error Your architecture is not known to support pivot_root(2)
_syscall2(int,pivot_root,char *,new,char *,old)

Yes it's clumsy but it's guaranteed to be where you expect it. (And
it's not nearly as clumsy if you don't feel the need to support all

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