pidfd_open man page

pidfd_open ā€” obtain a file descriptor that refers to a process

Synopsis

#include <sys/types.h>

int pidfd_open(pid_t pid, unsigned int flags);

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 nonnegative file descriptor. On error, -1 is returned and errno is set to indicate the cause of the error.

Errors

EINVAL

flags is not 0.

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.

Versions

pidfd_open() first appeared in Linux 5.3.

Conforming to

pidfd_open() is Linux specific.

Notes

Currently, there is no glibc 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):

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:

If any of these conditions does not hold, then the child process should instead be created using clone(2) with the CLONE_PIDFD flag.

The pidfd_send_signal(2) system call can be used to send a signal to the process referred to by a PID file descriptor.

A PID file descriptor can be monitored using poll(2), select(2), and epoll(7). When the process that it refers to terminates, these interfaces indicate the file descriptor as readable. Note, however, that in the current implementation, nothing can be read from the file descriptor (read(2) on the file descriptor fails with the error EINVAL).

The pidfd_open() system call is the preferred way of obtaining a PID file descriptor. The alternative is to obtain a file descriptor by opening a /proc/[pid] directory. However, the latter technique is possible only if the proc(5) file system is mounted; furthermore, the file descriptor obtained in this way is not pollable.

See also the discussion of the CLONE_PIDFD flag in clone(2).

Example

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.

Program source

#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 (0x%x): POLLIN is %sset\n", pollfd.revents,
            (pollfd.revents & POLLIN) ? "" : "not ");

    exit(EXIT_SUCCESS);
}

See Also

clone(2), kill(2), pidfd_send_signal(2), poll(2), select(2), epoll(7)

Colophon

This page is part of release 5.03 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Referenced By

clone(2), pidfd_send_signal(2), syscalls(2).

2019-09-19 Linux Programmer's Manual