Chapter 3. 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.

Attention: 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.

Related Information

File Formats Overview defines and describes file formats.

Header Files Overview describes header files.