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In Unix-like
operating systems An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also inc ...
, a device file or special file is an interface to a
device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...
that appears in a
file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.


Overview

Device files usually provide simple interfaces to standard devices (such as printers and serial ports), but can also be used to access specific unique resources on those devices, such as
disk partitions Disk partitioning or disk slicing is the creation of one or more regions on Computer data storage#Secondary storage, secondary storage, so that each region can be managed separately. These regions are called partitions. It is typically the first ...
. Additionally, device files are useful for accessing system resources that have no connection with any actual device, such as
data sink In computing, a sink, or data sink generally refers to the destination of data flow. The word sink has multiple uses in computing. In software engineering, an event sink is a class or function that receives events from another object or function, ...
s and random number generators. There are two general kinds of device files in Unix-like operating systems, known as ''character special files'' and ''block special files''. The difference between them lies in how much data is read and written by the operating system and hardware. These together can be called device special files in contrast to named pipes, which are not connected to a device but are not ordinary files either. MS-DOS borrowed the concept of special files from Unix but renamed them ''devices''. Because early versions of MS-DOS did not support a
directory Directory may refer to: * Directory (computing), or folder, a file system structure in which to store computer files * Directory (OpenVMS command) * Directory service, a software application for organizing information about a computer network's u ...
hierarchy, devices were distinguished from regular files by making their names reserved words, for example: the infamous CON. These were chosen for a degree of compatibility with
CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
and are still present in modern Windows for backwards compatibility. In some Unix-like systems, most device files are managed as part of a virtual file system traditionally mounted at /dev, possibly associated with a controlling daemon, which monitors hardware addition and removal at run time, making corresponding changes to the device file system if that's not automatically done by the kernel, and possibly invoking scripts in system or user space to handle special device needs. The
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
, DragonFly BSD and
Darwin Darwin may refer to: Common meanings * Charles Darwin (1809–1882), English naturalist and writer, best known as the originator of the theory of biological evolution by natural selection * Darwin, Northern Territory, a territorial capital city i ...
have a dedicated file system ''devfs''; device nodes are managed automatically by this file system, in
kernel space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...
. Linux used to have a similar ''devfs'' implementation, but it was abandoned later, and then removed since version 2.6.17; Linux now primarily uses a user space implementation known as '' udev'', but there are many variants. In Unix systems which support chroot process isolation, such as Solaris Containers, typically each chroot environment needs its own /dev; these mount points will be visible on the host OS at various nodes in the global file system tree. By restricting the device nodes populated into chroot instances of /dev, hardware isolation can be enforced by the chroot environment (a program can not meddle with hardware that it can neither see nor name—an even stronger form of
access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of ''accessing'' may mean consuming ...
than Unix file system permissions). MS-DOS managed hardware device contention (see TSR) by making each device file exclusive open. An application attempting to access a device already in use would discover itself unable to open the device file node. A variety of
device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...
semantics are implemented in Unix and Linux concerning
concurrent access In information technology and computer science, especially in the fields of computer programming, operating systems, multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, while g ...
.


Unix and Unix-like systems

Device nodes correspond to resources that an operating system's kernel has already allocated. Unix identifies those resources by a ''major number'' and a ''minor number'', both stored as part of the structure of a node. The assignment of these numbers occurs uniquely in different operating systems and on different
computer platform A computing platform or digital platform is an environment in which a piece of software is executed. It may be the hardware or the operating system (OS), even a web browser and associated application programming interfaces, or other underlying s ...
s. Generally, the major number identifies the device driver and the minor number identifies a particular device (possibly out of many) that the driver controls: in this case, the system may pass the minor number to a driver. However, in the presence of
dynamic number allocation Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyn ...
, this may not be the case (e.g. on
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
5 and up). As with other special file types, the computer system accesses device nodes using standard system calls and treats them like regular computer files. Two standard types of device files exist; unfortunately their names are rather counter-intuitive for historical reasons, and explanations of the difference between the two are often incorrect as a result.


Character devices

''Character special files'' or ''character devices'' provide unbuffered, direct access to the hardware device. They do not necessarily allow programs to read or write single characters at a time; that is up to the device in question. The character device for a hard disk, for example, will normally require that all reads and writes be aligned to block boundaries and most certainly will not allow reading a single byte. Character devices are sometimes known as ''raw devices'' to avoid the confusion surrounding the fact that a character device for a piece of block-based hardware will typically require programs to read and write aligned blocks.


Block devices

''Block special files'' or ''block devices'' provide buffered access to hardware devices, and provide some abstraction from their specifics. Unlike character devices, block devices will always allow the programmer to read or write a block of any size (including single characters/bytes) and any alignment. The downside is that because block devices are buffered, the programmer does not know how long it will take before written data is passed from the kernel's buffers to the actual device, or indeed in what order two separate writes will arrive at the physical device. Additionally, if the same hardware exposes both character and block devices, there is a risk of data corruption due to clients using the character device being unaware of changes made in the buffers of the block device. Most systems create both block and character devices to represent hardware like hard disks. FreeBSD and Linux notably do not; the former has removed support for block devices, while the latter creates only block devices. In Linux, to get a character device for a disk, one must use the "raw" driver, though one can get the same effect as opening a character device by opening the block device with the Linux-specific flag.


Pseudo-devices

Device nodes on Unix-like systems do not necessarily have to correspond to
physical device A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by th ...
s. Nodes that lack this correspondence form the group of ''pseudo-devices''. They provide various functions handled by the operating system. Some of the most commonly used (character-based) pseudo-devices include: * accepts and discards all input written to it; provides an end-of-file indication when read from. * accepts and discards all input written to it; produces a continuous stream of null characters (zero-value bytes) as output when read from. * produces a continuous stream of null characters (zero-value bytes) as output when read from, and generates an ("disk full") error when attempting to write to it. * produces bytes generated by the kernel's cryptographically secure pseudorandom number generator. Its exact behavior varies by implementation, and sometimes variants such as or are also provided. Additionally, BSD-specific pseudo-devices with an interface may also include: * allows userland processes to control PF through an interface. * provides access to devices otherwise not found as nodes, used by to implement RAID management in
OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project em ...
and
NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...
. * used by NetBSD's envsys framework for hardware monitoring, accessed in the userland through by the utility. *


Node creation

Nodes are created by the system call. The command-line program for creating nodes is also called . Nodes can be moved or deleted by the usual filesystem system calls (, ) and
commands Command may refer to: Computing * Command (computing), a statement in a computer language * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on Apple Macintosh computer keyboards * ...
(, ). Some Unix versions include a script named ''makedev'' or ''MAKEDEV'' to create all necessary devices in the directory . It only makes sense on systems whose devices are statically assigned major numbers (e.g., by means of hardcoding it in their kernel module). While some other Unix systems such as
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
, used kernel-based device node management via devfs only, and not supporting manual node creation. system call and command exist to keep compatibility with POSIX, but manually created device nodes outside devfs will not function at all.


Naming conventions

The following prefixes are used for the names of some devices in the hierarchy, to identify the type of device: * : line printers (compare lp) * : pseudo-terminals (virtual terminals) * :
terminal Terminal may refer to: Computing Hardware * Terminal (electronics), a device for joining electrical circuits together * Terminal (telecommunication), a device communicating over a line * Computer terminal, a set of primary input and output devic ...
s Some additional prefixes have come into common use in some operating systems: * : frame buffer * : (platform)
floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
s, though this same abbreviation is also commonly used to refer to
file descriptor In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket. File descriptors typically have ...
* : ("classic") IDE driver (previously used for ATA hard disk drive, ATAPI
optical disc drive In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only r ...
s, etc.) ** : the master device on the first ATA channel (usually identified by major number 3 and minor number 0) ** : the slave device on the first ATA channel ** : the master device on the second ATA channel ** : the slave device on the second ATA channel * , :
parallel port In computing, a parallel port is a type of interface found on early computers (personal and otherwise) for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once ( parallel ...
s * :
Main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...
(character device) * NVMe driver: ** : first registered device's device controller (character device) ** : first registered device's first namespace (block device) ** : first registered device's first namespace's first partition (block device) * MMC driver: ** : storage driver for MMC media ( SD cards, eMMC chips on laptops, etc.) *** : first registered device *** : first registered device's first partition *
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
driver, also used by
libATA libATA is a library used inside the Linux kernel to support ATA host controllers and devices. libATA provides an ATA driver API, class transports for ATA and ATAPI devices, and SCSI / ATA Translation for ATA devices according to the T10 SAT spe ...
(modern PATA/ SATA driver), USB, IEEE 1394, etc.: ** : mass-storage driver (block device) *** : first registered device *** , etc.: second, third, etc. registered devices ** : Enclosure driver ** : generic SCSI layer ** : "ROM" driver (data-oriented optical disc drives; scd is just a secondary alias) ** :
magnetic tape Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use magne ...
driver * :
terminal Terminal may refer to: Computing Hardware * Terminal (electronics), a device for joining electrical circuits together * Terminal (telecommunication), a device communicating over a line * Computer terminal, a set of primary input and output devic ...
s ** : (platform) serial port driver ** : USB serial converters, modems, etc. The canonical list of the prefixes used in Linux can be found in the Linux Device List, the official registry of allocated device numbers and directory nodes for the Linux operating system. For most devices, this prefix is followed by a number uniquely identifying the particular device. For hard drives, a letter is used to identify devices and is followed by a number to identify
partitions Partition may refer to: Computing Hardware * Disk partitioning, the division of a hard disk drive * Memory partition, a subdivision of a computer's memory, usually for use by a single job Software * Partition (database), the division of a ...
. Thus a file system may "know" an area on a disk as , for example, or "see" a networked terminal session as associated with . On disks using the typical PC
master boot record A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...
, the device numbers of primary and the optional extended partition are numbered 1 through 4, while the indexes of any logical partitions are 5 and onwards, regardless of the layout of the former partitions (their parent extended partition does not need to be the fourth partition on the disk, nor do all four primary partitions have to exist). Device names are usually not portable between different Unix-like system variants, for example, on some
BSD The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...
systems, the IDE devices are named , , etc.


devfs

devfs is a specific implementation of a device file system on Unix-like operating systems, used for presenting device files. The underlying mechanism of implementation may vary, depending on the OS. Maintaining these special files on a physically-implemented file system such as a hard drive is inconvenient, and as it needs kernel assistance anyway, the idea arose of a special-purpose logical file system that is not physically stored. Defining when devices are ready to appear is not trivial. The devfs approach is for the device driver to request creation and deletion of devfs entries related to the devices it enables and disables.


PC DOS, TOS, OS/2, and Windows

A device file is a reserved keyword used in PC DOS, TOS, OS/2, and Windows systems to allow access to certain ports and devices. MS-DOS borrowed the concept of special files from Unix but renamed them ''devices''. Because early versions of MS-DOS did not support a
directory Directory may refer to: * Directory (computing), or folder, a file system structure in which to store computer files * Directory (OpenVMS command) * Directory service, a software application for organizing information about a computer network's u ...
hierarchy, devices were distinguished from regular files by making their names reserved words. This means that certain file names were reserved for devices, and should not be used to name new files or directories. The reserved names themselves were chosen to be compatible with "special files" handling of PIP command in
CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
. There were two kinds of devices in DOS: Block Devices (used for disk drives) and Character Devices (generally all other devices, including COM and PRN devices). DOS uses device files for accessing printers and ports. Most versions of Windows also contain this support, which can cause confusion when trying to make files and folders of certain names, as they cannot have these names. Versions 2.x of MS-DOS provide the
AVAILDEV CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
CONFIG.SYS CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
parameter that, if set to FALSE, makes these special names only active if prefixed with \DEV\, thus allowing ordinary files to be created with these names.
GEMDOS GEM (for Graphics Environment Manager) is an operating environment released by Digital Research (DRI) in 1985 for use with the DOS operating system on Intel 8088 and Motorola 68000 microprocessors. GEM is known primarily as the graphical user i ...
, the DOS-like part of
Atari TOS TOS (The Operating System) is the operating system of the Atari ST range of computers. This range includes the 520ST and 1040ST, their STF/M/FM and STE variants and the Mega ST/STE. Later, 32-bit machines ( TT, Falcon030) were developed using a ...
, supported similar device names to DOS, but unlike DOS it required a trailing ":" character (on DOS, this is optional) to identify them as devices as opposed to normal filenames (thus "CON:" would work on both DOS and TOS, but "CON" would name an ordinary file on TOS but the console device on DOS). In MiNT and
MagiC Magic or Magick most commonly refers to: * Magic (supernatural), beliefs and actions employed to influence supernatural beings and forces * Ceremonial magic, encompasses a wide variety of rituals of magic * Magical thinking, the belief that unrela ...
, a special UNIX-like unified filesystem view accessed via the "U:" drive letter also placed device files in "U:\DEV". Using shell redirection and pipes, data can be sent to or received from a device. For example, typing the following will send the file c:\data.txt to the printer:
TYPE c:\data.txt > PRN
PIPE, MAILSLOT, and MUP are other standard Windows devices.


IOCS

The 8-bit operating system of
Sharp Sharp or SHARP may refer to: Acronyms * SHARP (helmet ratings) (Safety Helmet Assessment and Rating Programme), a British motorcycle helmet safety rating scheme * Self Help Addiction Recovery Program, a charitable organisation founded in 199 ...
pocket computers like the PC-E500, PC-E500S etc. consists of a
BASIC BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
interpreter, a DOS 2-like File Control System (FCS) implementing a rudimentary
12-bit FAT File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
-like filesystem, and a BIOS-like
Input/Output Control System Input/Output Control System (IOCS) is any of several packages on early IBM entry-level and mainframe computers that provided low level access to records on peripheral equipment. IOCS provides functionality similar to 1960s packages from other ve ...
(IOCS) implementing a number of standard character and block device drivers as well as special file devices including STDO:/SCRN: (display), STDI:/KYBD: (keyboard), COM: (serial I/O), STDL:/PRN: (printer), CAS: (cassette tape), E:/F:/G: (memory file), S1:/S2:/S3: (memory card), X:/Y: (floppy), SYSTM: (system), and NIL: (function).


Implementations


See also

*
devfsd devfsd is a device manager for the Linux kernel. Primarily, it creates device nodes in the directory when kernel drivers make the underlying hardware accessible. The nodes exist in a virtual device file system named devfs. In systems that suppor ...
*
sysfs sysfs is a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. In addition ...
* Block size * Blocking * Buffer *
File system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
* Hardware abstraction * Storage area network * User space * Unix file types * udev


References


Further reading

* * * * * * * * * * * {{DEFAULTSORT:Device File System Interfaces of the Linux kernel Pseudo file systems supported by the Linux kernel Special-purpose file systems Unix file system technology