269 lines
12 KiB
ReStructuredText
269 lines
12 KiB
ReStructuredText
.. _admin_devices:
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Linux allocated devices (4.x+ version)
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======================================
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This list is the Linux Device List, the official registry of allocated
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device numbers and ``/dev`` directory nodes for the Linux operating
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system.
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The version of this document at lanana.org is no longer maintained. This
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version in the mainline Linux kernel is the master document. Updates
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shall be sent as patches to the kernel maintainers (see the
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:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` document).
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Specifically explore the sections titled "CHAR and MISC DRIVERS", and
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"BLOCK LAYER" in the MAINTAINERS file to find the right maintainers
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to involve for character and block devices.
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This document is included by reference into the Filesystem Hierarchy
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Standard (FHS). The FHS is available from https://www.pathname.com/fhs/.
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Allocations marked (68k/Amiga) apply to Linux/68k on the Amiga
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platform only. Allocations marked (68k/Atari) apply to Linux/68k on
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the Atari platform only.
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This document is in the public domain. The authors requests, however,
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that semantically altered versions are not distributed without
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permission of the authors, assuming the authors can be contacted without
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an unreasonable effort.
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.. attention::
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DEVICE DRIVERS AUTHORS PLEASE READ THIS
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Linux now has extensive support for dynamic allocation of device numbering
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and can use ``sysfs`` and ``udev`` (``systemd``) to handle the naming needs.
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There are still some exceptions in the serial and boot device area. Before
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asking for a device number make sure you actually need one.
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To have a major number allocated, or a minor number in situations
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where that applies (e.g. busmice), please submit a patch and send to
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the authors as indicated above.
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Keep the description of the device *in the same format
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as this list*. The reason for this is that it is the only way we have
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found to ensure we have all the requisite information to publish your
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device and avoid conflicts.
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Finally, sometimes we have to play "namespace police." Please don't be
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offended. We often get submissions for ``/dev`` names that would be bound
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to cause conflicts down the road. We are trying to avoid getting in a
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situation where we would have to suffer an incompatible forward
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change. Therefore, please consult with us **before** you make your
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device names and numbers in any way public, at least to the point
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where it would be at all difficult to get them changed.
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Your cooperation is appreciated.
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.. include:: devices.txt
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:literal:
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Additional ``/dev/`` directory entries
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--------------------------------------
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This section details additional entries that should or may exist in
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the /dev directory. It is preferred that symbolic links use the same
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form (absolute or relative) as is indicated here. Links are
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classified as "hard" or "symbolic" depending on the preferred type of
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link; if possible, the indicated type of link should be used.
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Compulsory links
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++++++++++++++++
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These links should exist on all systems:
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=============== =============== =============== ===============================
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/dev/fd /proc/self/fd symbolic File descriptors
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/dev/stdin fd/0 symbolic stdin file descriptor
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/dev/stdout fd/1 symbolic stdout file descriptor
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/dev/stderr fd/2 symbolic stderr file descriptor
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/dev/nfsd socksys symbolic Required by iBCS-2
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/dev/X0R null symbolic Required by iBCS-2
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=============== =============== =============== ===============================
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Note: ``/dev/X0R`` is <letter X>-<digit 0>-<letter R>.
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Recommended links
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+++++++++++++++++
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It is recommended that these links exist on all systems:
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=============== =============== =============== ===============================
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/dev/core /proc/kcore symbolic Backward compatibility
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/dev/ramdisk ram0 symbolic Backward compatibility
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/dev/ftape qft0 symbolic Backward compatibility
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/dev/bttv0 video0 symbolic Backward compatibility
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/dev/radio radio0 symbolic Backward compatibility
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/dev/i2o* /dev/i2o/* symbolic Backward compatibility
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/dev/scd? sr? hard Alternate SCSI CD-ROM name
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=============== =============== =============== ===============================
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Locally defined links
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+++++++++++++++++++++
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The following links may be established locally to conform to the
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configuration of the system. This is merely a tabulation of existing
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practice, and does not constitute a recommendation. However, if they
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exist, they should have the following uses.
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=============== =============== =============== ===============================
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/dev/mouse mouse port symbolic Current mouse device
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/dev/tape tape device symbolic Current tape device
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/dev/cdrom CD-ROM device symbolic Current CD-ROM device
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/dev/cdwriter CD-writer symbolic Current CD-writer device
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/dev/scanner scanner symbolic Current scanner device
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/dev/modem modem port symbolic Current dialout device
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/dev/root root device symbolic Current root filesystem
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/dev/swap swap device symbolic Current swap device
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=============== =============== =============== ===============================
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``/dev/modem`` should not be used for a modem which supports dialin as
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well as dialout, as it tends to cause lock file problems. If it
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exists, ``/dev/modem`` should point to the appropriate primary TTY device
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(the use of the alternate callout devices is deprecated).
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For SCSI devices, ``/dev/tape`` and ``/dev/cdrom`` should point to the
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*cooked* devices (``/dev/st*`` and ``/dev/sr*``, respectively), whereas
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``/dev/cdwriter`` and /dev/scanner should point to the appropriate generic
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SCSI devices (/dev/sg*).
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``/dev/mouse`` may point to a primary serial TTY device, a hardware mouse
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device, or a socket for a mouse driver program (e.g. ``/dev/gpmdata``).
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Sockets and pipes
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+++++++++++++++++
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Non-transient sockets and named pipes may exist in /dev. Common entries are:
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=============== =============== ===============================================
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/dev/printer socket lpd local socket
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/dev/log socket syslog local socket
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/dev/gpmdata socket gpm mouse multiplexer
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=============== =============== ===============================================
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Mount points
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++++++++++++
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The following names are reserved for mounting special filesystems
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under /dev. These special filesystems provide kernel interfaces that
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cannot be provided with standard device nodes.
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=============== =============== ===============================================
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/dev/pts devpts PTY slave filesystem
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/dev/shm tmpfs POSIX shared memory maintenance access
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=============== =============== ===============================================
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Terminal devices
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----------------
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Terminal, or TTY devices are a special class of character devices. A
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terminal device is any device that could act as a controlling terminal
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for a session; this includes virtual consoles, serial ports, and
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pseudoterminals (PTYs).
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All terminal devices share a common set of capabilities known as line
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disciplines; these include the common terminal line discipline as well
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as SLIP and PPP modes.
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All terminal devices are named similarly; this section explains the
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naming and use of the various types of TTYs. Note that the naming
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conventions include several historical warts; some of these are
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Linux-specific, some were inherited from other systems, and some
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reflect Linux outgrowing a borrowed convention.
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A hash mark (``#``) in a device name is used here to indicate a decimal
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number without leading zeroes.
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Virtual consoles and the console device
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+++++++++++++++++++++++++++++++++++++++
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Virtual consoles are full-screen terminal displays on the system video
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monitor. Virtual consoles are named ``/dev/tty#``, with numbering
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starting at ``/dev/tty1``; ``/dev/tty0`` is the current virtual console.
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``/dev/tty0`` is the device that should be used to access the system video
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card on those architectures for which the frame buffer devices
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(``/dev/fb*``) are not applicable. Do not use ``/dev/console``
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for this purpose.
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The console device, ``/dev/console``, is the device to which system
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messages should be sent, and on which logins should be permitted in
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single-user mode. Starting with Linux 2.1.71, ``/dev/console`` is managed
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by the kernel; for previous versions it should be a symbolic link to
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either ``/dev/tty0``, a specific virtual console such as ``/dev/tty1``, or to
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a serial port primary (``tty*``, not ``cu*``) device, depending on the
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configuration of the system.
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Serial ports
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++++++++++++
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Serial ports are RS-232 serial ports and any device which simulates
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one, either in hardware (such as internal modems) or in software (such
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as the ISDN driver.) Under Linux, each serial ports has two device
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names, the primary or callin device and the alternate or callout one.
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Each kind of device is indicated by a different letter. For any
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letter X, the names of the devices are ``/dev/ttyX#`` and ``/dev/cux#``,
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respectively; for historical reasons, ``/dev/ttyS#`` and ``/dev/ttyC#``
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correspond to ``/dev/cua#`` and ``/dev/cub#``. In the future, it should be
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expected that multiple letters will be used; all letters will be upper
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case for the "tty" device (e.g. ``/dev/ttyDP#``) and lower case for the
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"cu" device (e.g. ``/dev/cudp#``).
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The names ``/dev/ttyQ#`` and ``/dev/cuq#`` are reserved for local use.
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The alternate devices provide for kernel-based exclusion and somewhat
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different defaults than the primary devices. Their main purpose is to
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allow the use of serial ports with programs with no inherent or broken
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support for serial ports. Their use is deprecated, and they may be
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removed from a future version of Linux.
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Arbitration of serial ports is provided by the use of lock files with
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the names ``/var/lock/LCK..ttyX#``. The contents of the lock file should
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be the PID of the locking process as an ASCII number.
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It is common practice to install links such as /dev/modem
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which point to serial ports. In order to ensure proper locking in the
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presence of these links, it is recommended that software chase
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symlinks and lock all possible names; additionally, it is recommended
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that a lock file be installed with the corresponding alternate
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device. In order to avoid deadlocks, it is recommended that the locks
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are acquired in the following order, and released in the reverse:
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1. The symbolic link name, if any (``/var/lock/LCK..modem``)
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2. The "tty" name (``/var/lock/LCK..ttyS2``)
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3. The alternate device name (``/var/lock/LCK..cua2``)
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In the case of nested symbolic links, the lock files should be
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installed in the order the symlinks are resolved.
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Under no circumstances should an application hold a lock while waiting
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for another to be released. In addition, applications which attempt
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to create lock files for the corresponding alternate device names
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should take into account the possibility of being used on a non-serial
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port TTY, for which no alternate device would exist.
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Pseudoterminals (PTYs)
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++++++++++++++++++++++
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Pseudoterminals, or PTYs, are used to create login sessions or provide
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other capabilities requiring a TTY line discipline (including SLIP or
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PPP capability) to arbitrary data-generation processes. Each PTY has
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a master side, named ``/dev/pty[p-za-e][0-9a-f]``, and a slave side, named
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``/dev/tty[p-za-e][0-9a-f]``. The kernel arbitrates the use of PTYs by
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allowing each master side to be opened only once.
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Once the master side has been opened, the corresponding slave device
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can be used in the same manner as any TTY device. The master and
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slave devices are connected by the kernel, generating the equivalent
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of a bidirectional pipe with TTY capabilities.
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Recent versions of the Linux kernels and GNU libc contain support for
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the System V/Unix98 naming scheme for PTYs, which assigns a common
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device, ``/dev/ptmx``, to all the masters (opening it will automatically
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give you a previously unassigned PTY) and a subdirectory, ``/dev/pts``,
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for the slaves; the slaves are named with decimal integers (``/dev/pts/#``
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in our notation). This removes the problem of exhausting the
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namespace and enables the kernel to automatically create the device
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nodes for the slaves on demand using the "devpts" filesystem.
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