Special Files

A special file is associated with a particular hardware device or other resource of the computer system. The operating system uses special files, sometimes called device files, to provide file I/O access to specific character and block device drivers.

Special files, at first glance, appear to be just like ordinary files, in that they:
  • Have path names that appear in a directory.
  • Have the same access protection as ordinary files.
  • Can be used in almost every way that ordinary files can be used.
However, there is an important difference between the two. An ordinary file is a logical grouping of data recorded on disk. A special file, on the other hand, corresponds to a device entity. Examples are:
  • An actual device, such as a line printer.
  • A logical subdevice, such as a large section of the disk drive.
  • A pseudo device, such as the physical memory of the computer (/dev/mem) or the null file (/dev/null).

Special files are distinguished from other files by having a file type (c or b, for character or block) stored in the i-nodes to indicate the type of device access provided. The i-node for the special file also contains the device major and minor numbers assigned to the device at device configuration time.

Note: Data corruption, loss of data, or loss of system integrity (a system crash) will occur if devices supporting paging, logical volumes, or mounted file systems are accessed using block special files. Block special files are provided for logical volumes and disk devices on the operating system and are solely for system use in managing file systems, paging devices, and logical volumes. These files should not be used for other purposes.

Several special files are provided with the operating system. By convention, special files are located in the /dev directory.

More information is provided about the following special files:

Item Description
3270cn Provides access to 3270 connection adapters by way of the 3270 connection adapter device driver.
bus Provides access to the hardware bus by way of the machine I/O device driver.
cd Provides access to the cdrom device driver.
console Provides access to the system console.
dials Provides access to the dials.
dump Supports system dump.
entn Provides access to the 3COM Ethernet adapters by way of the Ethernet device handler for this platform.
error Supports error logging.
fd Provides access to the diskette device driver.
fddin Provides access to the FDDI device driver by way of the FDDI device handler.
GIO Provides access to the graphics I/O (GIO) adapter.
ide Provides access to the Integrated Device Electronics (IDE) adapter device driver.
kbd Provides access to the natively attached keyboard.
kmem and mem Provides privileged read and write access to virtual memory.
lft Implements a low-function terminal (LFT) device.
lp Provides access to the line printer device driver.
lpfk Provides access to the lighted program function key (LPFK) array.
lvdd Provides access to the logical volume device driver.
mouse Provides access to the natively attached mouse.
mpqi Provides access to the Multiport Model 2 Adapter (MM2) SDLC device driver.
mpqn Provides access to multiprotocol adapters by way of the Multiprotocol Quad Port (MPQP) device handler.
null Provides access to the null device.
nvram Provides access to platform-specific nonvolatile RAM used for system boot, configuration, and fatal error information.
omd Provides access to the read/write optical device driver.
opn Provides diagnostic interface to the Serial Optical Link device driver.
ops0 Provides access to the Serial Optical Link device driver
pty Provides the pseudo-terminal (pty) device driver.
random Source of secure random output.
rcm Provides application interface to obtain and relinquish status of a graphics process through the Rendering Context Manager (RCM) device driver.
rhdisk Provides raw access to the physical volume (fixed-disk) device driver.
rmt Provides access to the sequential-access bulk-storage medium device driver.
scsi Provides access to the SCSI adapter device driver.
tablet Provides access to the tablet.
tmscsi Provides access to the SCSI Target-mode interface by way of the SCSI tmscsi device driver.
tokn Provides access to the token-ring adapters by way of the token-ring device handler.
trace Supports event tracing.
tty Supports the controlling terminal interface.
urandom Source of secure random output.
vty_server Creates a tty-style connection from the partition on which a virtual terminal server is running to a virtual terminal (not a virtual terminal server) on another partition.
x25sn Provides access to the X.25 Interface Co-Processor/2 adapters.