Headers

When compiling, the headers are processed in the following order:

=== kernel ===

If you include say, sys/vnode.h

It reads the headers in the following order, the header generally has a #include_next to load the next header with the same name.

[1]   zfs/include/sys/vnode.h
[2]   spl/include/sys/vnode.h
[3]   /System/Library/Frameworks/Kernel.framework/Headers/sys/vnode.h

=== userland ===

[1]   zfs/include/sys/vnode.h
[2]   zfs/lib/libspl/include/sys/vnode.h
[3]   /usr/include/sys/vnode.h

So, in the case of "zfs_context.h"

[1]   zfs/include/sys/zfs_context.h             Used by both kernel and userland. Defines for both go here.
[K]   spl/include/sys/zfs_context.h             Used only by kernel
[U]   zfs/lib/libspl/include/sys/zfs_context.h  Used only by userland