mirror of https://github.com/mkerrisk/man-pages
273 lines
6.7 KiB
Groff
273 lines
6.7 KiB
Groff
.\" Copyright (c) 2019 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH PIDFD_OPEN 2 2020-08-13 "Linux" "Linux Programmer's Manual"
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.SH NAME
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pidfd_open \- obtain a file descriptor that refers to a process
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.PP
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.BI "int pidfd_open(pid_t " pid ", unsigned int " flags );
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.fi
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.SH DESCRIPTION
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The
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.BR pidfd_open ()
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system call creates a file descriptor that refers to
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the process whose PID is specified in
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.IR pid .
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The file descriptor is returned as the function result;
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the close-on-exec flag is set on the file descriptor.
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.PP
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The
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.I flags
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argument is reserved for future use;
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currently, this argument must be specified as 0.
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.SH RETURN VALUE
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On success,
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.BR pidfd_open ()
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returns a file descriptor (a nonnegative integer).
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On error, \-1 is returned and
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.I errno
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is set to indicate the cause of the error.
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.SH ERRORS
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.TP
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.B EINVAL
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.I flags
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is not 0.
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.TP
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.B EINVAL
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.I pid
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is not valid.
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.TP
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.B EMFILE
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The per-process limit on the number of open file descriptors has been reached
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(see the description of
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.BR RLIMIT_NOFILE
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in
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.BR getrlimit (2)).
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.TP
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.B ENFILE
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The system-wide limit on the total number of open files has been reached.
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.TP
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.B ENODEV
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The anonymous inode filesystem is not available in this kernel.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.TP
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.B ESRCH
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The process specified by
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.I pid
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does not exist.
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.SH VERSIONS
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.BR pidfd_open ()
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first appeared in Linux 5.3.
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.SH CONFORMING TO
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.BR pidfd_open ()
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is Linux specific.
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.SH NOTES
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Currently, there is no glibc wrapper for this system call; call it using
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.BR syscall (2).
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.PP
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The following code sequence can be used to obtain a file descriptor
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for the child of
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.BR fork (2):
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.PP
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.in +4n
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.EX
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pid = fork();
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if (pid > 0) { /* If parent */
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pidfd = pidfd_open(pid, 0);
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...
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}
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.EE
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.in
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.PP
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Even if the child has already terminated by the time of the
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.BR pidfd_open ()
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call, its PID will not have been recycled and the returned
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file descriptor will refer to the resulting zombie process.
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Note, however, that this is guaranteed only if the following
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conditions hold true:
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.IP \(bu 2
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the disposition of
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.BR SIGCHLD
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has not been explicitly set to
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.BR SIG_IGN
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(see
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.BR sigaction (2));
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.IP \(bu
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the
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.BR SA_NOCLDWAIT
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flag was not specified while establishing a handler for
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.BR SIGCHLD
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or while setting the disposition of that signal to
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.BR SIG_DFL
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(see
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.BR sigaction (2));
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and
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.IP \(bu
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the zombie process was not reaped elsewhere in the program
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(e.g., either by an asynchronously executed signal handler or by
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.BR wait (2)
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or similar in another thread).
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.PP
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If any of these conditions does not hold,
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then the child process (along with a PID file descriptor that refers to it)
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should instead be created using
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.BR clone (2)
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with the
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.BR CLONE_PIDFD
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flag.
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.\"
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.SS Use cases for PID file descriptors
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A PID file descriptor returned by
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.BR pidfd_open ()
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(or by
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.BR clone (2)
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with the
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.BR CLONE_PID
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flag) can be used for the following purposes:
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.IP \(bu 2
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The
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.BR pidfd_send_signal (2)
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system call can be used to send a signal to the process referred to by
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a PID file descriptor.
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.IP \(bu
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A PID file descriptor can be monitored using
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.BR poll (2),
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.BR select (2),
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and
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.BR epoll (7).
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When the process that it refers to terminates,
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these interfaces indicate the file descriptor as readable.
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Note, however, that in the current implementation,
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nothing can be read from the file descriptor
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.RB ( read (2)
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on the file descriptor fails with the error
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.BR EINVAL ).
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.IP \(bu
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If the PID file descriptor refers to a child of the calling process,
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then it can be waited on using
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.BR waitid (2).
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.IP \(bu
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The
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.BR pidfd_getfd (2)
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system call can be used to obtain a duplicate of a file descriptor
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of another process referred to by a PID file descriptor.
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.IP \(bu
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A PID file descriptor can be used as the argument of
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.BR setns (2)
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in order to move into one or more of the same namespaces as the process
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referred to by the file descriptor.
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.PP
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The
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.BR pidfd_open ()
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system call is the preferred way of obtaining a PID file descriptor
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for an already existing process.
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The alternative is to obtain a file descriptor by opening a
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.I /proc/[pid]
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directory.
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However, the latter technique is possible only if the
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.BR proc (5)
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filesystem is mounted;
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furthermore, the file descriptor obtained in this way is
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.I not
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pollable and can't be waited on with
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.BR waitid (2).
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.SH EXAMPLES
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The program below opens a PID file descriptor for the
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process whose PID is specified as its command-line argument.
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It then uses
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.BR poll (2)
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to monitor the file descriptor for process exit, as indicated by an
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.BR EPOLLIN
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event.
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.\"
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.SS Program source
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\&
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.nf
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#define _GNU_SOURCE
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#include <sys/types.h>
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#include <sys/syscall.h>
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#include <unistd.h>
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#include <poll.h>
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#include <stdlib.h>
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#include <stdio.h>
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#ifndef __NR_pidfd_open
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#define __NR_pidfd_open 434 /* System call # on most architectures */
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#endif
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static int
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pidfd_open(pid_t pid, unsigned int flags)
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{
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return syscall(__NR_pidfd_open, pid, flags);
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}
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int
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main(int argc, char *argv[])
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{
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struct pollfd pollfd;
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int pidfd, ready;
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if (argc != 2) {
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fprintf(stderr, "Usage: %s <pid>\en", argv[0]);
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exit(EXIT_SUCCESS);
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}
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pidfd = pidfd_open(atoi(argv[1]), 0);
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if (pidfd == \-1) {
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perror("pidfd_open");
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exit(EXIT_FAILURE);
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}
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pollfd.fd = pidfd;
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pollfd.events = POLLIN;
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ready = poll(&pollfd, 1, \-1);
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if (ready == \-1) {
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perror("poll");
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exit(EXIT_FAILURE);
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}
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printf("Events (%#x): POLLIN is %sset\en", pollfd.revents,
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(pollfd.revents & POLLIN) ? "" : "not ");
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close(pidfd);
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exit(EXIT_SUCCESS);
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}
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.fi
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.SH SEE ALSO
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.BR clone (2),
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.BR kill (2),
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.BR pidfd_getfd (2),
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.BR pidfd_send_signal (2),
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.BR poll (2),
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.BR select (2),
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.BR setns (2),
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.BR waitid (2),
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.BR epoll (7)
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