W’s of character drivers
We already know what drivers are, and why we need them. What is so special about character drivers? If we write drivers for byte-oriented operations (or, in C lingo, character-oriented operations), then we refer to them as character drivers. Since the majority of devices are byte-oriented, the majority of device drivers are character device drivers.
Take, for example, serial drivers, audio drivers, video drivers, camera drivers, and basic I/O drivers. In fact, all device drivers that are neither storage nor network device drivers are some type of a character driver. Let’s look into the commonalities of these character drivers, and how Shweta wrote one of them.
The complete connection
As shown in Figure 1, for any user-space application to operate on a byte-oriented device (in hardware space), it should use the corresponding character device driver (in kernel space). Character driver usage is done through the corresponding character device file(s), linked to it through the virtual file system (VFS). What this means is that an application does the usual file operations on the character device file. Those operations are translated to the corresponding functions in the linked character device driver by the VFS. Those functions then do the final low-level access to the actual device to achieve the desired results.
Note that though the application does the usual file operations, their outcome may not be the usual ones. Rather, they would be as driven by the corresponding functions in the device driver. For example, a write followed by a read may not fetch what has just been written to the character device file, unlike for regular files. Remember that this is the usual expected behaviour for device files. Let’s take an audio device file as an example. What we write into it is the audio data we want to play back, say through a speaker. However, the read would get us audio data that we are recording, say through a microphone. The recorded data need not be the played-back data.
In this complete connection from the application to the device, there are four major entities involved:
- Application
- Character device file
- Character device driver
- Character device
The interesting thing is that all of these can exist independently on a system, without the other being present. The mere existence of these on a system doesn’t mean they are linked to form the complete connection. Rather, they need to be explicitly connected. An application gets connected to a device file by invoking the open system call on the device file.
Device file(s) are linked to the device driver by specific registrations done by the driver. The driver is linked to a device by its device-specific low-level operations. Thus we form the complete connection. With this, note that the character device file is not the actual device, but just a place-holder for the actual device.
Major and minor numbers
The connection between the application and the device file is based on the name of the device file. However, the connection between the device file and the device driver is based on the number of the device file, not the name. This allows a user-space application to have any name for the device file, and enables the kernel-space to have a trivial index-based linkage between the device file and the device driver. This device file number is more commonly referred to as the <major, minor> pair, or the major and minor numbers of the device file.
Earlier (till kernel 2.4), one major number was for one driver, and the minor number used to represent the sub-functionalities of the driver. With kernel 2.6, this distinction is no longer mandatory; there could be multiple drivers under the same major number, but obviously, with different minor number ranges.
However, this is more common with the non-reserved major numbers, and standard major numbers are typically preserved for single drivers. For example, 4 for serial interfaces, 13 for mice, 14 for audio devices, and so on. The following command would list the various character device files on your system:
