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+.\" Copyright (C) 1994, 1995, Daniel Quinlan <quinlan@yggdrasil.com>
+.\" Copyright (C) 2002-2008, 2017, Michael Kerrisk <mtk.manpages@gmail.com>
+.\" Copyright (C) 2023, Alejandro Colomar <alx@kernel.org>
+.\"
+.\" SPDX-License-Identifier: GPL-3.0-or-later
+.\"
+.TH proc_pid_fd 5 (date) "Linux man-pages (unreleased)"
+.SH NAME
+/proc/pid/fd/ \- file descriptors
+.SH DESCRIPTION
+.TP
+.IR /proc/ pid /fd/
+This is a subdirectory containing one entry for each file which the
+process has open, named by its file descriptor, and which is a
+symbolic link to the actual file.
+Thus, 0 is standard input, 1 standard output, 2 standard error, and so on.
+.IP
+For file descriptors for pipes and sockets,
+the entries will be symbolic links whose content is the
+file type with the inode.
+A
+.BR readlink (2)
+call on this file returns a string in the format:
+.IP
+.in +4n
+.EX
+type:[inode]
+.EE
+.in
+.IP
+For example,
+.I socket:[2248868]
+will be a socket and its inode is 2248868.
+For sockets, that inode can be used to find more information
+in one of the files under
+.IR /proc/net/ .
+.IP
+For file descriptors that have no corresponding inode
+(e.g., file descriptors produced by
+.BR bpf (2),
+.BR epoll_create (2),
+.BR eventfd (2),
+.BR inotify_init (2),
+.BR perf_event_open (2),
+.BR signalfd (2),
+.BR timerfd_create (2),
+and
+.BR userfaultfd (2)),
+the entry will be a symbolic link with contents of the form
+.IP
+.in +4n
+.EX
+.RI anon_inode: file-type
+.EE
+.in
+.IP
+In many cases (but not all), the
+.I file-type
+is surrounded by square brackets.
+.IP
+For example, an epoll file descriptor will have a symbolic link
+whose content is the string
+.IR "anon_inode:[eventpoll]" .
+.IP
+.\"The following was still true as at kernel 2.6.13
+In a multithreaded process, the contents of this directory
+are not available if the main thread has already terminated
+(typically by calling
+.BR pthread_exit (3)).
+.IP
+Programs that take a filename as a command-line argument,
+but don't take input from standard input if no argument is supplied,
+and programs that write to a file named as a command-line argument,
+but don't send their output to standard output
+if no argument is supplied, can nevertheless be made to use
+standard input or standard output by using
+.IR /proc/ pid /fd
+files as command-line arguments.
+For example, assuming that
+.I \-i
+is the flag designating an input file and
+.I \-o
+is the flag designating an output file:
+.IP
+.in +4n
+.EX
+.RB "$" " foobar \-i /proc/self/fd/0 \-o /proc/self/fd/1 ..."
+.EE
+.in
+.IP
+and you have a working filter.
+.\" The following is not true in my tests (MTK):
+.\" Note that this will not work for
+.\" programs that seek on their files, as the files in the fd directory
+.\" are not seekable.
+.IP
+.I /proc/self/fd/N
+is approximately the same as
+.I /dev/fd/N
+in some UNIX and UNIX-like systems.
+Most Linux MAKEDEV scripts symbolically link
+.I /dev/fd
+to
+.IR /proc/self/fd ,
+in fact.
+.IP
+Most systems provide symbolic links
+.IR /dev/stdin ,
+.IR /dev/stdout ,
+and
+.IR /dev/stderr ,
+which respectively link to the files
+.IR 0 ,
+.IR 1 ,
+and
+.I 2
+in
+.IR /proc/self/fd .
+Thus the example command above could be written as:
+.IP
+.in +4n
+.EX
+.RB "$" " foobar \-i /dev/stdin \-o /dev/stdout ..."
+.EE
+.in
+.IP
+Permission to dereference or read
+.RB ( readlink (2))
+the symbolic links in this directory is governed by a ptrace access mode
+.B PTRACE_MODE_READ_FSCREDS
+check; see
+.BR ptrace (2).
+.IP
+Note that for file descriptors referring to inodes
+(pipes and sockets, see above),
+those inodes still have permission bits and ownership information
+distinct from those of the
+.IR /proc/ pid /fd
+entry,
+and that the owner may differ from the user and group IDs of the process.
+An unprivileged process may lack permissions to open them, as in this example:
+.IP
+.in +4n
+.EX
+.RB "$" " echo test | sudo \-u nobody cat"
+test
+.RB "$" " echo test | sudo \-u nobody cat /proc/self/fd/0"
+cat: /proc/self/fd/0: Permission denied
+.EE
+.in
+.IP
+File descriptor 0 refers to the pipe created by the shell
+and owned by that shell's user, which is not
+.IR nobody ,
+so
+.B cat
+does not have permission
+to create a new file descriptor to read from that inode,
+even though it can still read from its existing file descriptor 0.
+.SH SEE ALSO
+.BR proc (5)