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10 The LaTeX version of this document is no longer maintained, nor is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