Understanding the Linux Kernel, 3rd Edition

Device Files

As mentioned in Chapter 1, Unix-like operating systems are based on the notion of a file, which is just an information container structured as a sequence of bytes. According to this approach, I/O devices are treated as special files called device files ; thus, the same system calls used to interact with regular files on disk can be used to directly interact with I/O devices. For example, the same write( ) system call may be used to write data into a regular file or to send it to a printer by writing to the /dev/lp0 device file.

According to the characteristics of the underlying device drivers, device files can be of two types: block or character. The difference between the two classes of hardware devices is not so clear-cut. At least we can assume the following:

• The data of a block device can be addressed randomly, and the time needed to transfer a data block is small and roughly the same, at least from the point of view of the human user. Typical examples of block devices are hard disks, floppy disks , CD-ROM drives, and DVD players.

• The data of a character device either cannot be addressed randomly (consider, for instance, a sound card), or they can be addressed randomly, but the time required to access a random datum largely depends on its position inside the device (consider, for instance, a magnetic tape driver).

Network cards are a notable exception to this schema, because they are hardware devices that are not directly associated with device files.

Device files ...