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What is 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.

How to build XNU

Building DEVELOPMENT kernel

The xnu make system can build kernel based on KERNEL_CONFIGS & ARCH_CONFIGS variables as arguments. Here is the syntax:

make SDKROOT=<sdkroot> ARCH_CONFIGS=<arch> KERNEL_CONFIGS=<variant>


  • <sdkroot>: path to MacOS SDK on disk. (defaults to /)
  • <variant>: can be debug, development, release, profile and configures compilation flags and asserts throughout kernel code.
  • <arch> : can be valid arch to build for. (E.g. i386 or X86_64)

To build a kernel for the same architecture as running OS, just type

$ make
$ make SDKROOT=macosx.internal

Additionally, there is support for configuring architectures through ARCH_CONFIGS and kernel configurations with KERNEL_CONFIGS.



  • By default, architecture is set to the build machine architecture, and the default kernel config is set to build for DEVELOPMENT.

This will also create a bootable image, kernel.[config], and a kernel binary with symbols, kernel.[config].unstripped.

  • To build with RELEASE kernel configuration


Building FAT kernel binary

Define architectures in your environment or when running a make command.

$ make ARCH_CONFIGS="I386 X86_64" exporthdrs all

Other makefile options

  • $ make MAKEJOBS=-j8 # this will use 8 processes during the build. The default is 2x the number of active CPUS.
  • $ make -j8 # the standard command-line option is also accepted
  • $ make -w # trace recursive make invocations. Useful in combination with VERBOSE=YES
  • $ make BUILD_LTO=0 # build without LLVM Link Time Optimization
  • $ make REMOTEBUILD=user@remotehost # perform build on remote host
  • $ make BUILD_JSON_COMPILATION_DATABASE=1 # Build Clang JSON Compilation Database

The XNU build system can optionally output color-formatted build output. To enable this, you can either set the XNU_LOGCOLORS environment variable to y, or you can pass LOGCOLORS=y to the make command.

Debug information formats

By default, a DWARF debug information repository is created during the install phase; this is a "bundle" named kernel.development.<variant>.dSYM To select the older STABS debug information format (where debug information is embedded in the kernel.development.unstripped image), set the BUILD_STABS environment variable.

$ export BUILD_STABS=1
$ make

Building KernelCaches

To test the xnu kernel, you need to build a kernelcache that links the kexts and kernel together into a single bootable image. To build a kernelcache you can use the following mechanisms:

  • Using automatic kernelcache generation with kextd. The kextd daemon keeps watching for changing in /System/Library/Extensions directory. So you can setup new kernel as

    $ cp BUILD/obj/DEVELOPMENT/X86_64/kernel.development /System/Library/Kernels/
    $ touch /System/Library/Extensions
    $ ps -e | grep kextd
  • Manually invoking kextcache to build new kernelcache.

    $ kextcache -q -z -a x86_64 -l -n -c /var/tmp/kernelcache.test -K /var/tmp/kernel.test /System/Library/Extensions

Running KernelCache on Target machine

The development kernel and iBoot supports configuring boot arguments so that we can safely boot into test kernel and, if things go wrong, safely fall back to previously used kernelcache. Following are the steps to get such a setup:

  1. Create kernel cache using the kextcache command as /kernelcache.test

  2. Copy exiting boot configurations to alternate file

    $ cp /Library/Preferences/SystemConfiguration/ /next_boot.plist
  3. Update the kernelcache and boot-args for your setup

    $ plutil -insert "Kernel Cache" -string "kernelcache.test" /next_boot.plist
    $ plutil -replace "Kernel Flags" -string "debug=0x144 -v kernelsuffix=test " /next_boot.plist
  4. Copy the new config to /Library/Preferences/SystemConfiguration/

    $ cp /next_boot.plist /Library/Preferences/SystemConfiguration/boot.plist
  5. Bless the volume with new configs.

    $ sudo -n bless  --mount / --setBoot --nextonly --options "config=boot"

    The --nextonly flag specifies that use the boot.plist configs only for one boot. So if the kernel panic's you can easily power reboot and recover back to original kernel.

Creating tags and cscope

Set up your build environment and from the top directory, run:

$ make tags     # this will build ctags and etags on a case-sensitive volume, only ctags on case-insensitive
$ make TAGS     # this will build etags
$ make cscope   # this will build cscope database

Coding styles (Reindenting files)

Source files can be reindented using clang-format setup in .clang-format. XNU follows a variant of WebKit style for source code formatting. Please refer to format styles at WebKit website. Further options about style options is available at clang docs

Note: clang-format binary may not be part of base installation. It can be compiled from llvm clang sources and is reachable in $PATH.

From the top directory, run:

$ make reindent # reindent all source files using clang format.

How to install a new header file from XNU

To install IOKit headers, see additional comments in iokit/IOKit/Makefile.

XNU installs header files at the following locations -

a. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers
b. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders
c. $(DSTROOT)/usr/include/
d. $(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders

Kernel.framework is used by kernel extensions.
The System.framework and /usr/include are used by user level applications.
The header files in framework's PrivateHeaders are only available for ** Apple Internal Development **.

The directory containing the header file should have a Makefile that creates the list of files that should be installed at different locations. If you are adding first header file in a directory, you will need to create Makefile similar to xnu/bsd/sys/Makefile.

Add your header file to the correct file list depending on where you want to install it. The default locations where the header files are installed from each file list are -

a. `DATAFILES` : To make header file available in user level -

b. `PRIVATE_DATAFILES` : To make header file available to Apple internal in
   user level -

c. `KERNELFILES` : To make header file available in kernel level -

d. `PRIVATE_KERNELFILES` : To make header file available to Apple internal
   for kernel extensions -

The Makefile combines the file lists mentioned above into different install lists which are used by build system to install the header files.

If the install list that you are interested does not exist, create it by adding the appropriate file lists. The default install lists, its member file lists and their default location are described below -

a. `INSTALL_MI_LIST` : Installs header file to a location that is available to everyone in user level.
    Locations -
   Definition -

b.  `INSTALL_MI_LCL_LIST` : Installs header file to a location that is available
   for Apple internal in user level.
   Locations -
   Definition -

c. `INSTALL_KF_MI_LIST` : Installs header file to location that is available
   to everyone for kernel extensions.
   Locations -
   Definition -

d. `INSTALL_KF_MI_LCL_LIST` : Installs header file to location that is
   available for Apple internal for kernel extensions.
   Locations -
   Definition -

e. `EXPORT_MI_LIST` : Exports header file to all of xnu (bsd/, osfmk/, etc.)
   for compilation only. Does not install anything into the SDK.
   Definition -

If you want to install the header file in a sub-directory of the paths described in (1), specify the directory name using two variables INSTALL_MI_DIR and EXPORT_MI_DIR as follows -

INSTALL_MI_DIR = dirname
EXPORT_MI_DIR = dirname

A single header file can exist at different locations using the steps mentioned above. However it might not be desirable to make all the code in the header file available at all the locations. For example, you want to export a function only to kernel level but not user level.

You can use C language's pre-processor directive (#ifdef, #endif, #ifndef) to control the text generated before a header file is installed. The kernel only includes the code if the conditional macro is TRUE and strips out code for FALSE conditions from the header file.

Some pre-defined macros and their descriptions are -

a. `PRIVATE` : If true, code is available to all of the xnu kernel and is
   not available in kernel extensions and user level header files.  The
   header files installed in all the paths described above in (1) will not
   have code enclosed within this macro.

b. `KERNEL_PRIVATE` : If true, code is available to all of the xnu kernel and Apple
    internal kernel extensions.

c. `BSD_KERNEL_PRIVATE` : If true, code is available to the xnu/bsd part of
   the kernel and is not available to rest of the kernel, kernel extensions
   and user level header files.  The header files installed in all the
   paths described above in (1) will not have code enclosed within this macro.

d. `KERNEL` :  If true, code is available only in kernel and kernel
   extensions and is not available in user level header files.  Only the
   header files installed in following paths will have the code -


   you should check [Testing the kernel][] for details.

How to add a new syscall

Testing the kernel

XNU kernel has multiple mechanisms for testing.

  • Assertions - The DEVELOPMENT and DEBUG kernel configs are compiled with assertions enabled. This allows developers to easily test invariants and conditions.

  • XNU Power On Self Tests (XNUPOST): The XNUPOST config allows for building the kernel with basic set of test functions that are run before first user space process is launched. Since XNU is hybrid between MACH and BSD, we have two locations where tests can be added.

    xnu/osfmk/tests/     # For testing mach based kernel structures and apis.
    bsd/tests/           # For testing BSD interfaces.

    Please follow the documentation at osfmk/tests/

  • User level tests: The tools/tests/ directory holds all the tests that verify syscalls and other features of the xnu kernel. The make target xnu_tests can be used to build all the tests supported.

    $ make RC_ProjectName=xnu_tests SDKROOT=/path/to/SDK

    These tests are individual programs that can be run from Terminal and report tests status by means of std posix exit codes (0 -> success) and/or stdout. Please read detailed documentation in tools/tests/unit_tests/

Kernel data descriptors

XNU uses different data formats for passing data in its api. The most standard way is using syscall arguments. But for complex data it often relies of sending memory saved by C structs. This packaged data transport mechanism is fragile and leads to broken interfaces between user space programs and kernel apis. libkdd directory holds user space library that can parse custom data provided by the same version of kernel. The kernel chunked data format is described in detail at libkdd/

Debugging the kernel

The xnu kernel supports debugging with a remote kernel debugging protocol (kdp). Please refer documentation at [technical note] TN2063 By default the kernel is setup to reboot on a panic. To debug a live kernel, the kdp server is setup to listen for UDP connections over ethernet. For machines without ethernet port, this behavior can be altered with use of kernel boot-args. Following are some common options.

  • debug=0x144 - setups debug variables to start kdp debugserver on panic
  • -v - print kernel logs on screen. By default XNU only shows grey screen with boot art.
  • kdp_match_name=en1 - Override default port selection for kdp. Supported for ethernet, thunderbolt and serial debugging.

To debug a panic'ed kernel, use llvm debugger (lldb) along with unstripped symbol rich kernel binary.

sh$ lldb kernel.development.unstripped

And then you can connect to panic'ed machine with kdp_remote [ip addr] or gdb_remote [hostip : port] commands.

Each kernel is packaged with kernel specific debug scripts as part of the build process. For security reasons these special commands and scripts do not get loaded automatically when lldb is connected to machine. Please add the following setting to your ~/.lldbinit if you wish to always load these macros.

settings set target.load-script-from-symbol-file true

The tools/lldbmacros directory contains the source for each of these commands. Please follow the for detailed explanation of commands and their usage.


The Darwin Kernel (mirror)







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