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 nonnegative file descriptor.  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() (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 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_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)