Pidfd

Pidfds Turn Process Identity Into a Stable Kernel Reference

A supervisor records PID 1842 for a worker, waits for an asynchronous event, then sends a signal. Between those operations the worker can exit, be reaped, and its numeric PID can later identify another process. The number still looks valid, but the identity it denotes has changed. Linux pidfds move this class of process control away from repeated numeric lookup. A PID file descriptor refers to a particular process, giving userspace a kernel-held reference that can be passed to interfaces such as pidfd_send_signal(), polling APIs, and, under additional permission checks, pidfd_getfd().