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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-24 04:52:22 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-24 04:52:22 +0000
commit3d08cd331c1adcf0d917392f7e527b3f00511748 (patch)
tree312f0d1e1632f48862f044b8bb87e602dcffb5f9 /man5/proc_pid_fd.5
parentAdding debian version 6.7-2. (diff)
downloadmanpages-3d08cd331c1adcf0d917392f7e527b3f00511748.tar.xz
manpages-3d08cd331c1adcf0d917392f7e527b3f00511748.zip
Merging upstream version 6.8.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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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 2023-08-15 "Linux man-pages 6.7"
-.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)