Special file

In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file. There are also special files in MS-DOS, OS/2, and Microsoft Windows. They allow software to interact with a device driver using standard input/output system calls, which simplifies many tasks and unifies user-space I/O mechanisms.

Device files often provide simple interfaces to peripheral devices such as printers and serial ports, but they can also be used to access specific resources on those devices, such as disk partitions. Additionally, device files are useful for accessing system resources that have no connection with any actual device such as data sinks 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 data written to them and read from them is processed 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 hierarchy, devices were distinguished from regular files by making their names reserved words, chosen for a degree of compatibility with CP/M.

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