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and ``/dev`` directory nodes for the Linux operating 8 system. 9 10 The version of this document at lanana.org is no longer maintained.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