PIDFD_OPEN

Section: Linux Programmer's Manual (2)
Updated: 2020-08-13
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NAME

pidfd_open - 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 file descriptor (a nonnegative integer). 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:

•: 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.

Use cases for PID file descriptors

A PID file descriptor returned by pidfd_open() (or by clone(2) with the CLONE_PID flag) can be used for the following purposes:

•: 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).
•: If the PID file descriptor refers to a child of the calling process, then it can be waited on using waitid(2).
•: The pidfd_getfd(2) system call can be used to obtain a duplicate of a file descriptor of another process referred to by a PID file descriptor.
•: A PID file descriptor can be used as the argument of setns(2) in order to move into one or more of the same namespaces as the process referred to by the file descriptor.

The pidfd_open() system call is the preferred way of obtaining a PID file descriptor for an already existing process. 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) filesystem is mounted; furthermore, the file descriptor obtained in this way is not pollable and can't be waited on with waitid(2).

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.

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

close(pidfd);
exit(EXIT_SUCCESS); }

COLOPHON

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