Orange Pi5 kernel

^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   1) .. _admin_devices:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   2) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   3) Linux allocated devices (4.x+ version)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   4) ======================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   5) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   6) This list is the Linux Device List, the official registry of allocated
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   7) device numbers and ``/dev`` directory nodes for the Linux operating
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   8) system.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300   9) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  10) The LaTeX version of this document is no longer maintained, nor is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  11) the document that used to reside at lanana.org.  This version in the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  12) mainline Linux kernel is the master document.  Updates shall be sent
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  13) as patches to the kernel maintainers (see the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  14) :ref:`Documentation/process/submitting-patches.rst <submittingpatches>` document).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  15) Specifically explore the sections titled "CHAR and MISC DRIVERS", and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  16) "BLOCK LAYER" in the MAINTAINERS file to find the right maintainers
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  17) to involve for character and block devices.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  18) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  19) This document is included by reference into the Filesystem Hierarchy
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  20) Standard (FHS).	 The FHS is available from https://www.pathname.com/fhs/.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  21) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  22) Allocations marked (68k/Amiga) apply to Linux/68k on the Amiga
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  23) platform only.	Allocations marked (68k/Atari) apply to Linux/68k on
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  24) the Atari platform only.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  25) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  26) This document is in the public domain.	The authors requests, however,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  27) that semantically altered versions are not distributed without
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  28) permission of the authors, assuming the authors can be contacted without
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  29) an unreasonable effort.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  30) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  31) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  32) .. attention::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  33) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  34)   DEVICE DRIVERS AUTHORS PLEASE READ THIS
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  35) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  36)   Linux now has extensive support for dynamic allocation of device numbering
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  37)   and can use ``sysfs`` and ``udev`` (``systemd``) to handle the naming needs.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  38)   There are still some exceptions in the serial and boot device area. Before
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  39)   asking   for a device number make sure you actually need one.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  40) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  41)   To have a major number allocated, or a minor number in situations
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  42)   where that applies (e.g. busmice), please submit a patch and send to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  43)   the authors as indicated above.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  44) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  45)   Keep the description of the device *in the same format
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  46)   as this list*. The reason for this is that it is the only way we have
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  47)   found to ensure we have all the requisite information to publish your
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  48)   device and avoid conflicts.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  49) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  50)   Finally, sometimes we have to play "namespace police."  Please don't be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  51)   offended.  We often get submissions for ``/dev`` names that would be bound
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  52)   to cause conflicts down the road.  We are trying to avoid getting in a
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  53)   situation where we would have to suffer an incompatible forward
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  54)   change.  Therefore, please consult with us **before** you make your
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  55)   device names and numbers in any way public, at least to the point
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  56)   where it would be at all difficult to get them changed.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  57) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  58)   Your cooperation is appreciated.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  59) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  60) .. include:: devices.txt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  61)    :literal:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  62) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  63) Additional ``/dev/`` directory entries
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  64) --------------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  65) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  66) This section details additional entries that should or may exist in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  67) the /dev directory.  It is preferred that symbolic links use the same
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  68) form (absolute or relative) as is indicated here.  Links are
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  69) classified as "hard" or "symbolic" depending on the preferred type of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  70) link; if possible, the indicated type of link should be used.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  71) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  72) Compulsory links
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  73) ++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  74) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  75) These links should exist on all systems:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  76) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  77) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  78) /dev/fd		/proc/self/fd	symbolic	File descriptors
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  79) /dev/stdin	fd/0		symbolic	stdin file descriptor
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  80) /dev/stdout	fd/1		symbolic	stdout file descriptor
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  81) /dev/stderr	fd/2		symbolic	stderr file descriptor
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  82) /dev/nfsd	socksys		symbolic	Required by iBCS-2
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  83) /dev/X0R	null		symbolic	Required by iBCS-2
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  84) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  85) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  86) Note: ``/dev/X0R`` is <letter X>-<digit 0>-<letter R>.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  87) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  88) Recommended links
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  89) +++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  90) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  91) It is recommended that these links exist on all systems:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  92) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  93) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  94) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  95) /dev/core	/proc/kcore	symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  96) /dev/ramdisk	ram0		symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  97) /dev/ftape	qft0		symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  98) /dev/bttv0	video0		symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  99) /dev/radio	radio0		symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) /dev/i2o*	/dev/i2o/*	symbolic	Backward compatibility
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101) /dev/scd?	sr?		hard		Alternate SCSI CD-ROM name
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) Locally defined links
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105) +++++++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) The following links may be established locally to conform to the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) configuration of the system.  This is merely a tabulation of existing
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) practice, and does not constitute a recommendation.  However, if they
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) exist, they should have the following uses.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) /dev/mouse	mouse port	symbolic	Current mouse device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) /dev/tape	tape device	symbolic	Current tape device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) /dev/cdrom	CD-ROM device	symbolic	Current CD-ROM device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) /dev/cdwriter	CD-writer	symbolic	Current CD-writer device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) /dev/scanner	scanner		symbolic	Current scanner device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) /dev/modem	modem port	symbolic	Current dialout device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) /dev/root	root device	symbolic	Current root filesystem
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) /dev/swap	swap device	symbolic	Current swap device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121) =============== =============== =============== ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) ``/dev/modem`` should not be used for a modem which supports dialin as
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) well as dialout, as it tends to cause lock file problems.  If it
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) exists, ``/dev/modem`` should point to the appropriate primary TTY device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) (the use of the alternate callout devices is deprecated).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128) For SCSI devices, ``/dev/tape`` and ``/dev/cdrom`` should point to the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129) *cooked* devices (``/dev/st*`` and ``/dev/sr*``, respectively), whereas
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 130) ``/dev/cdwriter`` and /dev/scanner should point to the appropriate generic
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 131) SCSI devices (/dev/sg*).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 132) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 133) ``/dev/mouse`` may point to a primary serial TTY device, a hardware mouse
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 134) device, or a socket for a mouse driver program (e.g. ``/dev/gpmdata``).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 135) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 136) Sockets and pipes
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 137) +++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 138) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 139) Non-transient sockets and named pipes may exist in /dev.  Common entries are:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 140) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 141) =============== =============== ===============================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 142) /dev/printer	socket		lpd local socket
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 143) /dev/log	socket		syslog local socket
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 144) /dev/gpmdata	socket		gpm mouse multiplexer
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 145) =============== =============== ===============================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 146) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 147) Mount points
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 148) ++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 149) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 150) The following names are reserved for mounting special filesystems
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 151) under /dev.  These special filesystems provide kernel interfaces that
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 152) cannot be provided with standard device nodes.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 153) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 154) =============== =============== ===============================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 155) /dev/pts	devpts		PTY slave filesystem
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 156) /dev/shm	tmpfs		POSIX shared memory maintenance access
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 157) =============== =============== ===============================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 158) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 159) Terminal devices
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 160) ----------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 161) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 162) Terminal, or TTY devices are a special class of character devices.  A
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 163) terminal device is any device that could act as a controlling terminal
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 164) for a session; this includes virtual consoles, serial ports, and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 165) pseudoterminals (PTYs).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 166) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 167) All terminal devices share a common set of capabilities known as line
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 168) disciplines; these include the common terminal line discipline as well
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 169) as SLIP and PPP modes.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 170) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 171) All terminal devices are named similarly; this section explains the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 172) naming and use of the various types of TTYs.  Note that the naming
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 173) conventions include several historical warts; some of these are
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 174) Linux-specific, some were inherited from other systems, and some
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 175) reflect Linux outgrowing a borrowed convention.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 176) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 177) A hash mark (``#``) in a device name is used here to indicate a decimal
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 178) number without leading zeroes.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 179) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 180) Virtual consoles and the console device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 181) +++++++++++++++++++++++++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 182) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 183) Virtual consoles are full-screen terminal displays on the system video
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 184) monitor.  Virtual consoles are named ``/dev/tty#``, with numbering
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 185) starting at ``/dev/tty1``; ``/dev/tty0`` is the current virtual console.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 186) ``/dev/tty0`` is the device that should be used to access the system video
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 187) card on those architectures for which the frame buffer devices
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 188) (``/dev/fb*``) are not applicable. Do not use ``/dev/console``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 189) for this purpose.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 190) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 191) The console device, ``/dev/console``, is the device to which system
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 192) messages should be sent, and on which logins should be permitted in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 193) single-user mode.  Starting with Linux 2.1.71, ``/dev/console`` is managed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 194) by the kernel; for previous versions it should be a symbolic link to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 195) either ``/dev/tty0``, a specific virtual console such as ``/dev/tty1``, or to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 196) a serial port primary (``tty*``, not ``cu*``) device, depending on the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 197) configuration of the system.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 198) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 199) Serial ports
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 200) ++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 201) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 202) Serial ports are RS-232 serial ports and any device which simulates
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 203) one, either in hardware (such as internal modems) or in software (such
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 204) as the ISDN driver.)  Under Linux, each serial ports has two device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 205) names, the primary or callin device and the alternate or callout one.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 206) Each kind of device is indicated by a different letter.	 For any
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 207) letter X, the names of the devices are ``/dev/ttyX#`` and ``/dev/cux#``,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 208) respectively; for historical reasons, ``/dev/ttyS#`` and ``/dev/ttyC#``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 209) correspond to ``/dev/cua#`` and ``/dev/cub#``. In the future, it should be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 210) expected that multiple letters will be used; all letters will be upper
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 211) case for the "tty" device (e.g. ``/dev/ttyDP#``) and lower case for the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 212) "cu" device (e.g. ``/dev/cudp#``).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 213) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 214) The names ``/dev/ttyQ#`` and ``/dev/cuq#`` are reserved for local use.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 215) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 216) The alternate devices provide for kernel-based exclusion and somewhat
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 217) different defaults than the primary devices.  Their main purpose is to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 218) allow the use of serial ports with programs with no inherent or broken
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 219) support for serial ports.  Their use is deprecated, and they may be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 220) removed from a future version of Linux.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 221) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 222) Arbitration of serial ports is provided by the use of lock files with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 223) the names ``/var/lock/LCK..ttyX#``. The contents of the lock file should
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 224) be the PID of the locking process as an ASCII number.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 225) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 226) It is common practice to install links such as /dev/modem
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 227) which point to serial ports.  In order to ensure proper locking in the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 228) presence of these links, it is recommended that software chase
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 229) symlinks and lock all possible names; additionally, it is recommended
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 230) that a lock file be installed with the corresponding alternate
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 231) device.	 In order to avoid deadlocks, it is recommended that the locks
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 232) are acquired in the following order, and released in the reverse:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 233) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 234) 	1. The symbolic link name, if any (``/var/lock/LCK..modem``)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 235) 	2. The "tty" name (``/var/lock/LCK..ttyS2``)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 236) 	3. The alternate device name (``/var/lock/LCK..cua2``)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 237) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 238) In the case of nested symbolic links, the lock files should be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 239) installed in the order the symlinks are resolved.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 240) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 241) Under no circumstances should an application hold a lock while waiting
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 242) for another to be released.  In addition, applications which attempt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 243) to create lock files for the corresponding alternate device names
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 244) should take into account the possibility of being used on a non-serial
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 245) port TTY, for which no alternate device would exist.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 246) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 247) Pseudoterminals (PTYs)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 248) ++++++++++++++++++++++
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 249) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 250) Pseudoterminals, or PTYs, are used to create login sessions or provide
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 251) other capabilities requiring a TTY line discipline (including SLIP or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 252) PPP capability) to arbitrary data-generation processes.	 Each PTY has
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 253) a master side, named ``/dev/pty[p-za-e][0-9a-f]``, and a slave side, named
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 254) ``/dev/tty[p-za-e][0-9a-f]``.  The kernel arbitrates the use of PTYs by
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 255) allowing each master side to be opened only once.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 256) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 257) Once the master side has been opened, the corresponding slave device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 258) can be used in the same manner as any TTY device.  The master and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 259) slave devices are connected by the kernel, generating the equivalent
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 260) of a bidirectional pipe with TTY capabilities.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 261) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 262) Recent versions of the Linux kernels and GNU libc contain support for
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 263) the System V/Unix98 naming scheme for PTYs, which assigns a common
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 264) device, ``/dev/ptmx``, to all the masters (opening it will automatically
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 265) give you a previously unassigned PTY) and a subdirectory, ``/dev/pts``,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 266) for the slaves; the slaves are named with decimal integers (``/dev/pts/#``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 267) in our notation).  This removes the problem of exhausting the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 268) namespace and enables the kernel to automatically create the device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 269) nodes for the slaves on demand using the "devpts" filesystem.