pidfd_open(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       pidfd_open - obtain a file descriptor that refers to a process

SYNOPSIS         top

       int pidfd_open(pid_t pid, unsigned int flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       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         top

       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         top

       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         top

       pidfd_open() first appeared in Linux 5.3.

CONFORMING TO         top

       pidfd_open() is Linux specific.

NOTES         top

       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):

           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.

       • 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         top

       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);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.11 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/.

Linux                          2021-03-22                  PIDFD_OPEN(2)

Pages that refer to this page: clone(2)pidfd_getfd(2)pidfd_send_signal(2)process_madvise(2)setns(2)syscalls(2)wait(2)sd_event_add_child(3)system_data_types(7)