pidfd_open - Man Page

Standard C library (libc, -lc)

#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

int syscall(SYS_pidfd_open, pid_t pid, unsigned int flags);

Note: glibc provides no wrapper for pidfd_open(), necessitating the use of syscall(2).

The pidfd_open() system call creates a file descriptor that refers to the process whose PID is specified in pid. The file descriptor is returned as the function result; the close-on-exec flag is set on the file descriptor.

The flags argument either has the value 0, or contains the following flag:

PIDFD_NONBLOCK (since Linux 5.10)

Return a nonblocking file descriptor. If the process referred to by the file descriptor has not yet terminated, then an attempt to wait on the file descriptor using waitid(2) will immediately return the error EAGAIN rather than blocking.

On success, pidfd_open() returns a file descriptor (a nonnegative integer). On error, -1 is returned and errno is set to indicate the error.

EINVAL

flags is not valid.

EINVAL

pid is not valid.

EMFILE

The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).

ENFILE

The system-wide limit on the total number of open files has been reached.

ENODEV

The anonymous inode filesystem is not available in this kernel.

ENOMEM

Insufficient kernel memory was available.

ESRCH

The process specified by pid does not exist.

Linux.

Linux 5.3.

The following code sequence can be used to obtain a file descriptor for the child of fork(2):

pid = fork();
if (pid > 0) {     /* If parent */
    pidfd = pidfd_open(pid, 0);
    ...
}

Even if the child has already terminated by the time of the pidfd_open() call, its PID will not have been recycled and the returned file descriptor will refer to the resulting zombie process. Note, however, that this is guaranteed only if the following conditions hold true:

• the disposition of SIGCHLD has not been explicitly set to SIG_IGN (see sigaction(2));
• the SA_NOCLDWAIT flag was not specified while establishing a handler for SIGCHLD or while setting the disposition of that signal to SIG_DFL (see sigaction(2)); and
• the zombie process was not reaped elsewhere in the program (e.g., either by an asynchronously executed signal handler or by wait(2) or similar in another thread).

If any of these conditions does not hold, then the child process (along with a PID file descriptor that refers to it) should instead be created using clone(2) with the CLONE_PIDFD flag.

The program below opens a PID file descriptor for the process whose PID is specified as its command-line argument. It then uses poll(2) to monitor the file descriptor for process exit, as indicated by an EPOLLIN event.

#define _GNU_SOURCE
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>

static int
pidfd_open(pid_t pid, unsigned int flags)
{
    return syscall(SYS_pidfd_open, pid, flags);
}

int
main(int argc, char *argv[])
{
    int            pidfd, ready;
    struct pollfd  pollfd;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <pid>\n", argv[0]);
        exit(EXIT_SUCCESS);
    }

    pidfd = pidfd_open(atoi(argv[1]), 0);
    if (pidfd == -1) {
        perror("pidfd_open");
        exit(EXIT_FAILURE);
    }

    pollfd.fd = pidfd;
    pollfd.events = POLLIN;

    ready = poll(&pollfd, 1, -1);
    if (ready == -1) {
        perror("poll");
        exit(EXIT_FAILURE);
    }

    printf("Events (%#x): POLLIN is %sset\n", pollfd.revents,
           (pollfd.revents & POLLIN) ? "" : "not ");

    close(pidfd);
    exit(EXIT_SUCCESS);
}

clone(2), kill(2), pidfd_getfd(2), pidfd_send_signal(2), poll(2), process_madvise(2), select(2), setns(2), waitid(2), epoll(7)

clone(2), fanotify(7), fanotify_init(2), notcurses_fds(3), org.freedesktop.systemd1(5), pgrep(1), pidfd_getfd(2), pidfd_send_signal(2), process_madvise(2), sd_event_add_child(3), sd_pid_get_owner_uid(3), seccomp_unotify(2), setns(2), syscalls(2), wait(2).