pidfd_open

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

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  re-
       turned  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 nonnegative file descriptor.  On er-
       ror, -1 is returned and errno is set to indicate the cause of  the  er-
       ror.

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  getr-
              limit(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  us-
       ing 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, how-
       ever,  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() (of 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  in-
          terfaces  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_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).

EXAMPLE
       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 (0x%x): POLLIN is %sset\n", pollfd.revents,
                   (pollfd.revents & POLLIN) ? "" : "not ");

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       clone(2), kill(2), pidfd_send_signal(2), poll(2), select(2), waitid(2),
       epoll(7)

COLOPHON
       This page is part of release 5.05 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                             2019-11-19                     PIDFD_OPEN(2)