DESCRIPTION

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 is reserved for future use; currently, this argument must be specified as 0.

RETURN VALUE

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

ERRORS

VERSIONS

pidfd_open() first appeared in Linux 5.3.

CONFORMING TO

pidfd_open() is Linux specific.

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

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

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:

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.

EXAMPLES

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

#ifndef __NR_pidfd_open
#define __NR_pidfd_open 434   /* System call # on most architectures */
#endif

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

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

    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);
}

SEE ALSO

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