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macsyscalls

Glossary

XNU

Apple's hybrid kernel — a Mach 3 microkernel core with a 4.4BSD personality bolted on, the kernel at the heart of every Mac and iPhone.

XNU ("X is Not Unix") is the kernel of macOS, iOS, iPadOS, watchOS, tvOS, and visionOS. It descends from two lineages fused at NeXT:

  • Mach 3 from Carnegie Mellon, providing tasks, threads, ports, virtual memory, and IPC.
  • 4.4BSD-Lite, providing the POSIX surface — files, sockets, signals, processes.

The two halves cooperate inside the same address space. The BSD personality calls into Mach for low-level primitives; Mach exposes its services as ports that the BSD code can hold rights to.

The source is open under the Apple Public Source License and lives at apple-oss-distributions/xnu on GitHub, tagged per release. Every syscall in the reference catalogue is enumerated from that source.

XNU's syscall table is defined in bsd/kern/syscalls.master for the BSD side and osfmk/kern/syscall_sw.c for the Mach side. See BSD syscall and Mach trap for the two families.