Practical UNIX and Internet Security, 3rd Edition

Device Files

Computer systems usually have peripheral devices attached to them. These devices may be involved with I/O (terminals, printers, modems), they may involve mass storage (disks, tapes), and they may have other specialized functions. The Unix paradigm for devices is to treat each one as a file, some with special characteristics.

Unix devices are represented as inodes, identical to files. The inodes represent either a character device or a block device (described in the sidebar). Each device is also designated by a major device number, indicating the type of device, and a minor device number, indicating which one of many similar devices the inode represents. For instance, the partitions of a physical disk will all have the same major device number, but different minor device numbers. For a serial card, the minor device number may represent which port number is in use. When a program reads from or writes to a device file, the kernel turns the request into an I/O operation with the appropriate device, using the major/minor device numbers as parameters to indicate which device to access.

Block Versus Character Devices

Most devices in Unix are referenced as character devices . These are also known as raw devices because that is what you get—raw access to the device. You must make your read and write calls to the device file in the natural transfer units of the device. Thus, you probably read and write single characters at a time to a terminal device, but you need to read and ...