IPC
Pipes, FIFOs, UNIX sockets with fd passing, and shared-memory coordination with futexes and eventfd — pmon grows a log pipeline and a control plane; shmkv goes multi-process.
Pipes, FIFOs, and splice
pmon v4 turns the supervisor into a log pipeline: the pipe as a ring of kernel pages with its capacity read live via F_GETPIPE_SZ, what PIPE_BUF atomicity does and does not promise, FIFO open semantics and the ENXIO trick, SIGPIPE/EPIPE as the writer's contract, and splice moving log bytes into a FIFO without a userspace copy — proven with strace, /proc, and a 5.3 GB/s dd run.
UNIX sockets and fd passing
pmon grows a control plane on a UNIX stream socket: filesystem vs abstract addresses under ss -x, SO_PEERCRED as kernel-vouched local auth, and SCM_RIGHTS sending an open log fd across process boundaries — proven shared at the open-file-description layer with /proc fdinfo on both sides.
Shared-memory coordination: futexes and seqlocks
The coordination ladder from a 1 ms poll loop through POSIX message queues to the futex — the kernel's one real primitive, sleep/wake on a shared word — plus a seqlock for read-mostly snapshots, process-shared atomics in all three languages, and a measured IPC latency matrix, as shmkv v1 gains watchers that never poll.