pidfd_prepare
pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret_file);
retval = pidfd_prepare(pid, flags | PIDFD_STALE, &pidfile);
pidfd = pidfd_prepare(pid, flags, &pidfd_file);
pidfd = pidfd_prepare(scm->pid, PIDFD_STALE, &pidfd_file);
pidfd = pidfd_prepare(peer_pid, flags, &pidfd_file);