XNU Alternatives

XNU

XNU kernel is part of the Darwin operating system for use in OS X and iOS operating systems. XNU is an acronym for XNU is Not Unix. XNU is a hybrid kernel combining the Mach kernel developed at Carnegie Mellon University with components from FreeBSD and C++ API for writing drivers called IOKit. XNU runs on I386, X86_64 for both single processor and multi-processor configurations.

XNU Source Tree

config - configurations for exported apis for supported architecture and platform
SETUP - Basic set of tools used for configuring the kernel, versioning and kextsymbol management.
EXTERNAL_HEADERS - Headers sourced from other projects to avoid dependency cycles when building. These headers should be regularly synced when source is updated.
libkern - C++ IOKit library code for handling of drivers and kexts.
libsa - kernel bootstrap code for startup
libsyscall - syscall library interface for userspace programs
libkdd - source for user library for parsing kernel data like kernel chunked data.
makedefs - top level rules and defines for kernel build.
osfmk - Mach kernel based subsystems
pexpert - Platform specific code like interrupt handling, atomics etc.
security - Mandatory Access Check policy interfaces and related implementation.
bsd - BSD subsystems code
tools - A set of utilities for testing, debugging and profiling kernel.