Linux Device Drivers

Classes of Devices and Modules

The Unix way of looking at devices distinguishes between three device types, each devoted to a different task. Linux can load each device type in the form of a module, thus allowing users to experiment with new hardware while still being able to run up-to-date kernel versions and to follow development.

As far as modules are concerned, each module usually implements only one driver, and thus is classifiable, for example, as a char module, or a block module. This division of modules into different types, or classes, is not a rigid one; the programmer can choose to build huge modules implementing different drivers in a single chunk of code. Good programmers, nonetheless, usually create a different module for each new functionality they implement.

Going back to devices, the three flavors are the following:

Character devices

A character (char) device is one that can be accessed like a file, and a char driver is in charge of implementing this behavior. Such a driver usually implements the open, close, read, and write system calls. The console and the parallel ports are examples of char devices, as they are well represented by the stream abstraction. Char devices are accessed by means of filesystem nodes, such as /dev/tty1 and /dev/lp1. The only relevant difference between a char device and a regular file is that you can always step back and forth in the regular file, while most char devices are just a data channel, which you can only access sequentially. ...