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pidfd_open.2: New page documenting pidfd_open(2)
Notes from a conversation on linux-man@ with Christian Brauner: [[ > [*} By the way, going forward, can we call these things > "process FDs", rather than "PID FDs"? The API names are what > they are, an that's okay, but these just as we have socket > FDs that refer to sockets, directory FDs that refer to > directories, and timer FDs that refer to timers, and so on, > these are FDs that refer to *processes*, not "process IDs". > It's a little thing, but I think the naming better, and > it's what I propose to use in the manual pages. The naming was another debate and we ended with this compromise. I would just clarify that a pidfd is a process file descriptor. I wouldn't make too much of a deal of hiding the shortcut "pidfd". People are already using it out there in the wild and it's never proven a good idea to go against accepted practice. ]] Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" 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.\" manual page may be incorrect or out-of-date.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 2019-09-19 "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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.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 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 nonnegative file descriptor.
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On success, \-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 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
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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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.PP
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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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the file descriptor indicates 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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.PP
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See also the discussion of the
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.BR CLONE_PIDFD
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flag in
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.BR clone (2).
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.SH EXAMPLE
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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 monitors the file descriptor for readability
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.RB ( POLLIN )
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using
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.BR poll (2).
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When the process with the specified by PID terminates,
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.BR poll (2)
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returns, and indicates that the file descriptor is readable.
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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/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
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#endif
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static
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int 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 (0x%x): POLLIN is %sset\en", pollfd.revents,
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(pollfd.revents & POLLIN) ? "" : "not ");
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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_send_signal (2),
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.BR poll (2),
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.BR select (2),
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.BR epoll (7)
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