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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.