XNU

XNU
The XNU kernel
DeveloperApple Inc.
Written inC, C++
OS familyUnix-like (before 2007), Unix[1]
Working stateCurrent
Source modelOpen-source
Initial releaseDecember 1996 (1996-12)
Repository
PlatformsCurrent: x86-64, 64-bit ARM, 32-bit ARM (32-bit ARM support is closed-source)
Historical: PowerPC (32-bit and 64-bit), IA-32
Kernel typeHybrid
LicenseApple Public Source License 2.0
Preceded byMach, Nukernel
Official websitegithub.com/apple-oss-distributions/xnu

XNU ("X is Not Unix") is the computer operating system (OS) kernel developed at Apple Inc. since December 1996 for use in the Mac OS X (now macOS) operating system and released as free and open-source software as part of the Darwin OS, which, in addition to being the basis for macOS, is also the basis for Apple TV Software, iOS, iPadOS, watchOS, visionOS, and tvOS.[2]

XNU was originally developed by NeXT for the NeXTSTEP operating system. It was a hybrid kernel derived from version 2.5 of the Mach kernel developed at Carnegie Mellon University, which incorporated the bulk of the 4.3BSD kernel modified to run atop Mach primitives, along with an application programming interface (API) in Objective-C for writing drivers named DriverKit.[3]

After Apple acquired NeXT, the kernel was updated with code derived from OSFMK 7.3 from OSF,[4] and the FreeBSD project, and the DriverKit was replaced with new API on a restricted subset of C++[5] (based on Embedded C++)[6] named IOKit.

By keeping the BSD kernel into the third part of XNU,[7] XNU became UNIX-based when macOS achieved UNIX certification under the Single UNIX Specification (SUS) by The Open Group.[8] Despite this, Apple retained the original 'XNU' name, which stands for 'X is Not Unix,' a relic from its NeXTSTEP origins before macOS was UNIX-certified. This has led to confusion, as the name suggests that XNU is separate from UNIX, even though macOS, as a whole, is officially recognized as a UNIX operating system.[9]

  1. ^ https://fahrplan.events.ccc.de/congress/2007/Fahrplan/attachments/986_inside_the_mac_osx_kernel.pdf
  2. ^ "Porting UNIX/Linux Applications to Mac OS X: Glossary". Apple Computer. 2005. Retrieved June 7, 2017.
  3. ^ "XNU GitHub source code README". GitHub. December 16, 2021.
  4. ^ Magee, Jim. WWDC 2000 Session 106 – Mac OS X: Kernel. 14 minutes in. Archived from the original on December 21, 2021.
  5. ^ "The libkern C++ Runtime". IOKit Device Driver Design Guidelines.
  6. ^ Amit Singh. "What is Mac OS X?". Archived from the original on April 19, 2019. Retrieved March 4, 2015.
  7. ^ "How Mac OS X Works". HowStuffWorks. August 8, 2011. Retrieved March 12, 2025.
  8. ^ McKay, Dave (October 21, 2019). "Is macOS UNIX? (and What Does That Mean?)". How-To Geek. Retrieved March 12, 2025.
  9. ^ "Apple macOS 15 Sequoia is officially UNIX". Archived from the original on January 24, 2025. Retrieved March 12, 2025.

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