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Subsystem · The POSIX personality

BSD

Processes, files, signals, sockets, system calls — the FreeBSD-derived layer that makes XNU look like a Unix.

apple-oss-distributions/xnubsd/kern/init_main.cStart reading the kernel source here.View on GitHub(line —)
BSD process and file descriptor tableAnatomy of struct proc: pid, ucred, pointer to the Mach task, and a file-descriptor table whose entries dispatch through fileops to vnodes, sockets, or kqueues.struct procbsd/sys/proc_internal.hp_pidpid_tp_pptrparentucreduid · gid · entitlementstask →pointer to Mach taskfdt →file-descriptor tablebsd_info ↔ taskMach taskosfmk/kern/task.hfdt — array of fileproc *stdinvnodestdoutvnodestderrvnodefd 3socketfd 4kqueueEach slot's fileglob.fg_ops table dispatches read / write / closeto the right type — vnode for files, socket, kqueue.read(fd, …) →
BSD: structure at a glance.

Articles in this series

The BSD personality: how XNU pretends to be Unix
Processes, file descriptors, signals, sockets — the FreeBSD-derived layer that sits on top of Mach and makes macOS pass POSIX.
bsdposixkernelxnu
Signals in XNU: where POSIX semantics meet Mach reality
POSIX says signals are per-process. Mach says everything is a thread. Here's how XNU bridges the two — pending masks, delivery threads, the AST mechanism, and exception ports.
bsdsignalskernelxnu
How fork(), exec(), and posix_spawn work on XNU
The Unix-est of Unix calls, implemented on a Mach kernel. Why fork is awkward on macOS, what exec actually replaces, and why posix_spawn is now the preferred way to start a process.
bsdprocessforkexec
How macOS boots: iBoot to launchd
The full chain from power-on to your login window. Boot ROM, iBoot, kernelcache, kernel_init, bsdinit_task, launchd — what each stage does and how control transfers.
bsdbootlaunchdiboot
The XNU network stack: mbufs, TCP, and Skywalk
From the classic 4.4 BSD TCP/IP stack to Apple's modern Skywalk replacement — how packets traverse XNU's networking code, and why Apple is moving the data plane out of the BSD layer.
bsdnetworkingtcpskywalk
What happens when you launch an app on macOS
From double-click to first window: LaunchServices, launchd, posix_spawn, AMFI, dyld, the shared cache, sandbox profile installation, the runloop. Six subsystems in three seconds.
bsdlaunchddyldamfi
dyld in depth: chained fixups, prebuilt loaders, and dlopen internals
Inside dyld — Loader data structures, the modern chained-fixups format, PrebuiltLoaders in the shared cache, and what dlopen actually does at runtime.
bsddyldloadermach-o
libdispatch internals: how GCD actually dispatches your blocks
Dispatch queues, the workqueue thread pool, voucher adoption, async vs sync — what happens between your dispatch_async call and your block running on a worker thread.
bsdlibdispatchgcdconcurrency
TCP internals on XNU: the state machine, congestion control, and the receive path
The 4.4 BSD TCP stack as it actually runs on macOS — state transitions, send/receive locking, Apple's congestion-control variants, and the kqueue path back to your app.
bsdnetworkingtcpkernel