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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);
       }

SEE ALSO

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

COLOPHON

       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 https://www.kernel.org/doc/man-pages/.