XNU
| Developer | Apple Inc. |
|---|---|
| Written in | C, C++ |
| OS family | Unix-like |
| Working state | Current |
| Source model | Open-source |
| Initial release | December 1996 |
| Available in | English, others |
| Platforms | IA-32, x86-64, ARM |
| Kernel type | Hybrid |
| License | Apple Public Source License 2.0 |
| Official website | opensource |
XNU is the computer operating system kernel developed at Apple Inc. since December 1996 for use in the macOS operating system and released as free and open-source software as part of the Darwin operating system. It is also used as the kernel for the iOS, tvOS, and watchOS operating systems. XNU is an abbreviation of X is Not Unix.
Originally developed by NeXT for the NeXTSTEP operating system, XNU was a hybrid kernel combining version 2.5 of the Mach kernel developed at Carnegie Mellon University with components from 4.3BSD and an Objective-C API for writing drivers called Driver Kit.
After Apple acquired NeXT, the Mach component was upgraded to 3.0, the BSD components were upgraded with code from the FreeBSD project, and the Driver Kit was replaced with a C++ API for writing drivers called I/O Kit.
XNU is a hybrid kernel, containing features of both monolithic kernels and microkernels, attempting to make the best use of both technologies, such as the message passing ability of microkernels enabling greater modularity and larger portions of the OS to benefit from memory protection, and retaining the speed of monolithic kernels for some critical tasks.
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