Event multiplexing on process identifiers (PIDs)

wait4pid.py

#!/usr/bin/env python3
"""This script demonstrates event multiplexing on process identifiers (PIDs).
More specifically, we issue file descriptor (FD) from PID and. Then we wait
events on the descriptor. Note that polling such events is feasible because of
obtaining FD from PID with pidfd_open() system call. This feature was
introduced in 5.3 (Sep 2019). A usage example is below.

    $ ./wait4pid.py 1337 &
    $ kill -9 1337
    process with pid 1337 exited
"""

from ctypes import CDLL
from select import POLLIN, poll
from sys import argv


def pidfd_open(pid, flags=None):
    """Function pidfd_open wraps making system call since there is no binding
    in libc.
    """
    return CDLL(None).syscall(434, pid, flags or 0)


def watch(pid, timeout=None):
    fd = pidfd_open(pid)

    poller = poll()
    poller.register(fd, POLLIN)

    for fd, ev in poller.poll(timeout):
        print('process with pid', pid, 'exited')
        poller.unregister(fd)


def main():
    try:
        pid = int(argv[1])
        timeout = float(argv[2]) if len(argv) >= 3 else None
    except Exception:
        print('usage:', argv[0], '<pid:int> [timeout:float]')
        return

    watch(pid, timeout)


if __name__ == '__main__':
    main()