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10 The version of this document at lanana.org is 10 The version of this document at lanana.org is no longer maintained.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