PIDFD_OPEN(2)               Linux Programmer's Manual              PIDFD_OPEN(2)

       pidfd_open - obtain a file descriptor that refers to a process

       #include <sys/types.h>

       int pidfd_open(pid_t pid, unsigned int flags);

       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

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

       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.

       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

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

       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.

       pidfd_open() first appeared in Linux 5.3.

       pidfd_open() is Linux specific.

       Currently, there is no glibc wrapper for this system call; call it using

       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

       • 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

       • 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

       • 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).

       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

   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 */

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

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

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

           pidfd = pidfd_open(atoi(argv[1]), 0);
           if (pidfd == -1) {

           pollfd.fd = pidfd;

           ready = poll(&pollfd, 1, -1);
           if (ready == -1) {

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


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

       This page is part of release 5.10 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

Linux                              2020-08-13                      PIDFD_OPEN(2)