pidfd_open(2)

NAME
pidfd_open - obtain a file descriptor that refers to a task

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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).

DESCRIPTION
The pidfd_open() system call creates a file descriptor that refers to the task referenced by 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 flags:

PIDFD_NONBLOCK (since Linux 5.10)
    Return a nonblocking file descriptor. If the task 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.

PIDFD_THREAD (since Linux 6.9)
    Create a file descriptor that refers to a specific thread, rather than a process (thread-group leader). If this flag is not set, pid must refer to a process.

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

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.

STANDARDS
Linux.

HISTORY
Linux 5.3.

NOTES
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 task that it refers to terminates and becomes a zombie, these interfaces indicate the file descriptor as readable (EPOLLIN). When the task is reaped, these interfaces produce a hangup event (EPOLLHUP).

    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 polling behavior depends on whether PIDFD_THREAD flag was used when obtaining the file descriptor:

    •   With PIDFD_THREAD, the file descriptor becomes readable when the task exits and becomes a zombie, even if the thread-group is not empty.

    •   Without PIDFD_THREAD, the file descriptor becomes readable only when the last thread in the thread group exits.

•   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.

•   A PID file descriptor can be used as the argument of process_madvise(2) in order to provide advice on the memory usage patterns of 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 <poll.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/syscall.h>
    #include <sys/types.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);
    }

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)