1.. _admin_devices: 2 3Linux allocated devices (4.x+ version) 4====================================== 5 6This list is the Linux Device List, the official registry of allocated 7device numbers and ``/dev`` directory nodes for the Linux operating 8system. 9 10The version of this document at lanana.org is no longer maintained. This 11version in the mainline Linux kernel is the master document. Updates 12shall be sent as patches to the kernel maintainers (see the 13:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` document). 14Specifically explore the sections titled "CHAR and MISC DRIVERS", and 15"BLOCK LAYER" in the MAINTAINERS file to find the right maintainers 16to involve for character and block devices. 17 18This document is included by reference into the Filesystem Hierarchy 19Standard (FHS). The FHS is available from https://www.pathname.com/fhs/. 20 21Allocations marked (68k/Amiga) apply to Linux/68k on the Amiga 22platform only. Allocations marked (68k/Atari) apply to Linux/68k on 23the Atari platform only. 24 25This document is in the public domain. The authors requests, however, 26that semantically altered versions are not distributed without 27permission of the authors, assuming the authors can be contacted without 28an 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 62Additional ``/dev/`` directory entries 63-------------------------------------- 64 65This section details additional entries that should or may exist in 66the /dev directory. It is preferred that symbolic links use the same 67form (absolute or relative) as is indicated here. Links are 68classified as "hard" or "symbolic" depending on the preferred type of 69link; if possible, the indicated type of link should be used. 70 71Compulsory links 72++++++++++++++++ 73 74These 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 85Note: ``/dev/X0R`` is <letter X>-<digit 0>-<letter R>. 86 87Recommended links 88+++++++++++++++++ 89 90It 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=============== =============== =============== =============================== 101 102Suggested earlier ``/dev/scd?`` alternative names for ``/dev/sr?`` 103CD-ROM and other optical drives (using SCSI commands) were removed 104in ``udev`` version 174 that was released in 2011. 105 106Locally defined links 107+++++++++++++++++++++ 108 109The following links may be established locally to conform to the 110configuration of the system. This is merely a tabulation of existing 111practice, and does not constitute a recommendation. However, if they 112exist, they should have the following uses. 113 114=============== =============== =============== =============================== 115/dev/mouse mouse port symbolic Current mouse device 116/dev/tape tape device symbolic Current tape device 117/dev/cdrom CD-ROM device symbolic Current CD-ROM device 118/dev/scanner scanner symbolic Current scanner device 119/dev/modem modem port symbolic Current dialout device 120/dev/root root device symbolic Current root filesystem 121/dev/swap swap device symbolic Current swap device 122=============== =============== =============== =============================== 123 124``/dev/modem`` should not be used for a modem which supports dialin as 125well as dialout, as it tends to cause lock file problems. If it 126exists, ``/dev/modem`` should point to the appropriate primary TTY device 127(the use of the alternate callout devices is deprecated). 128 129For SCSI devices, ``/dev/tape`` and ``/dev/cdrom`` should point to the 130*cooked* devices (``/dev/st*`` and ``/dev/sr*``, respectively), whereas 131``/dev/scanner`` should point to the appropriate generic 132SCSI device (``/dev/sg*``). 133 134``/dev/mouse`` may point to a primary serial TTY device, a hardware mouse 135device, or a socket for a mouse driver program (e.g. ``/dev/gpmdata``). 136 137Sockets and pipes 138+++++++++++++++++ 139 140Non-transient sockets and named pipes may exist in /dev. Common entries are: 141 142=============== =============== =============================================== 143/dev/printer socket lpd local socket 144/dev/log socket syslog local socket 145/dev/gpmdata socket gpm mouse multiplexer 146=============== =============== =============================================== 147 148Mount points 149++++++++++++ 150 151The following names are reserved for mounting special filesystems 152under /dev. These special filesystems provide kernel interfaces that 153cannot be provided with standard device nodes. 154 155=============== =============== =============================================== 156/dev/pts devpts PTY slave filesystem 157/dev/shm tmpfs POSIX shared memory maintenance access 158=============== =============== =============================================== 159 160Terminal devices 161---------------- 162 163Terminal, or TTY devices are a special class of character devices. A 164terminal device is any device that could act as a controlling terminal 165for a session; this includes virtual consoles, serial ports, and 166pseudoterminals (PTYs). 167 168All terminal devices share a common set of capabilities known as line 169disciplines; these include the common terminal line discipline as well 170as SLIP and PPP modes. 171 172All terminal devices are named similarly; this section explains the 173naming and use of the various types of TTYs. Note that the naming 174conventions include several historical warts; some of these are 175Linux-specific, some were inherited from other systems, and some 176reflect Linux outgrowing a borrowed convention. 177 178A hash mark (``#``) in a device name is used here to indicate a decimal 179number without leading zeroes. 180 181Virtual consoles and the console device 182+++++++++++++++++++++++++++++++++++++++ 183 184Virtual consoles are full-screen terminal displays on the system video 185monitor. Virtual consoles are named ``/dev/tty#``, with numbering 186starting at ``/dev/tty1``; ``/dev/tty0`` is the current virtual console. 187``/dev/tty0`` is the device that should be used to access the system video 188card on those architectures for which the frame buffer devices 189(``/dev/fb*``) are not applicable. Do not use ``/dev/console`` 190for this purpose. 191 192The console device, ``/dev/console``, is the device to which system 193messages should be sent, and on which logins should be permitted in 194single-user mode. Starting with Linux 2.1.71, ``/dev/console`` is managed 195by the kernel; for previous versions it should be a symbolic link to 196either ``/dev/tty0``, a specific virtual console such as ``/dev/tty1``, or to 197a serial port primary (``tty*``, not ``cu*``) device, depending on the 198configuration of the system. 199 200Serial ports 201++++++++++++ 202 203Serial ports are RS-232 serial ports and any device which simulates 204one, either in hardware (such as internal modems) or in software (such 205as the ISDN driver.) Under Linux, each serial ports has two device 206names, the primary or callin device and the alternate or callout one. 207Each kind of device is indicated by a different letter. For any 208letter X, the names of the devices are ``/dev/ttyX#`` and ``/dev/cux#``, 209respectively; for historical reasons, ``/dev/ttyS#`` and ``/dev/ttyC#`` 210correspond to ``/dev/cua#`` and ``/dev/cub#``. In the future, it should be 211expected that multiple letters will be used; all letters will be upper 212case for the "tty" device (e.g. ``/dev/ttyDP#``) and lower case for the 213"cu" device (e.g. ``/dev/cudp#``). 214 215The names ``/dev/ttyQ#`` and ``/dev/cuq#`` are reserved for local use. 216 217The alternate devices provide for kernel-based exclusion and somewhat 218different defaults than the primary devices. Their main purpose is to 219allow the use of serial ports with programs with no inherent or broken 220support for serial ports. Their use is deprecated, and they may be 221removed from a future version of Linux. 222 223Arbitration of serial ports is provided by the use of lock files with 224the names ``/var/lock/LCK..ttyX#``. The contents of the lock file should 225be the PID of the locking process as an ASCII number. 226 227It is common practice to install links such as /dev/modem 228which point to serial ports. In order to ensure proper locking in the 229presence of these links, it is recommended that software chase 230symlinks and lock all possible names; additionally, it is recommended 231that a lock file be installed with the corresponding alternate 232device. In order to avoid deadlocks, it is recommended that the locks 233are acquired in the following order, and released in the reverse: 234 235 1. The symbolic link name, if any (``/var/lock/LCK..modem``) 236 2. The "tty" name (``/var/lock/LCK..ttyS2``) 237 3. The alternate device name (``/var/lock/LCK..cua2``) 238 239In the case of nested symbolic links, the lock files should be 240installed in the order the symlinks are resolved. 241 242Under no circumstances should an application hold a lock while waiting 243for another to be released. In addition, applications which attempt 244to create lock files for the corresponding alternate device names 245should take into account the possibility of being used on a non-serial 246port TTY, for which no alternate device would exist. 247 248Pseudoterminals (PTYs) 249++++++++++++++++++++++ 250 251Pseudoterminals, or PTYs, are used to create login sessions or provide 252other capabilities requiring a TTY line discipline (including SLIP or 253PPP capability) to arbitrary data-generation processes. Each PTY has 254a master side, named ``/dev/pty[p-za-e][0-9a-f]``, and a slave side, named 255``/dev/tty[p-za-e][0-9a-f]``. The kernel arbitrates the use of PTYs by 256allowing each master side to be opened only once. 257 258Once the master side has been opened, the corresponding slave device 259can be used in the same manner as any TTY device. The master and 260slave devices are connected by the kernel, generating the equivalent 261of a bidirectional pipe with TTY capabilities. 262 263Recent versions of the Linux kernels and GNU libc contain support for 264the System V/Unix98 naming scheme for PTYs, which assigns a common 265device, ``/dev/ptmx``, to all the masters (opening it will automatically 266give you a previously unassigned PTY) and a subdirectory, ``/dev/pts``, 267for the slaves; the slaves are named with decimal integers (``/dev/pts/#`` 268in our notation). This removes the problem of exhausting the 269namespace and enables the kernel to automatically create the device 270nodes for the slaves on demand using the "devpts" filesystem. 271