What is '_KERNEL' in the source ?

[Joseph Koshy]

>           In various kernel source files,i came across '#ifdef _KERNEL'.
> What is '_KERNEL' used for ? In some files _KERNEL is #defined to nothing ??
> Can anybody please explain this ?

It is used to control the visibility of types and prototypes in system headers.
Kernel builds define _KERNEL, but userland compiles usually do not.  Thus a
#include <sys/foo.h> has a different meaning in userland than in the kernel.
See: src/sys/conf/{kern.pre,kmod}.mk

A few userland utilities (e.g., fstat) define _KERNEL before including
headers from <sys/*> because they need more knowledge of kernel
data structures than is the norm for userland.

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