Device driver classification

Question

So according to 2 books I have on device drivers(for Linux), notable the O'REILLY version("Linux Device Drivers(3rd edition") there are 3 types of device drivers:

Character drivers that read/write on byte of data at a time.
Block drivers which read and write blocks of data(file storage devices).
Network drivers that send and receive frames(for ethernet and wifi these are 48 bit chunks of data).

So where do video and sound drivers fit in? I ask because AFAIK your screen has a bus that sends frames from the motherboard to the monitor and the image on your screen is a memory map(a square array) that is refreshed at a certain number of frames/sec. The sound driver also sends 16-bit(2 byte) frames to the DAC at a sampling rate of 44.1 KHz. Are these character drivers? I would think they would be in a similar category as network drivers as network interfaces, screens, speakers, microphones, and webcams are all devices that read and write frames(an array of bytes).

user149341 · Accepted Answer

A more accurate way of describing these types of devices would be to say that:

Character devices deal with streams of data. You can write data to them and read data from them, but you can't skip around. Reading data from them may block if nothing is available. Common character devices include terminals, serial ports, and special devices like /dev/null and /dev/random. This need not be one byte at a time, though: many devices return data as larger frames. (For instance, Linux input devices under /dev/input return data in 32-byte frames, each one representing one event.)
Block devices deal with a fixed block of data. You can read and write data to any location in them you want, as well as mapping them to memory using mmap(). They are often used to represent storage devices (like disks), but can also be used to represent other, more unusual things.
Network devices are a special case. They are more or less exclusively used for devices that actually interface with a network (e.g, Ethernet NICs, wireless network hardware, cell modems, etc). They don't show up in /dev at all.

Both character and block devices can implement special operations that don't fit into the normal framework using ioctl() (e.g, changing the baud rate of a serial port, ejecting a CD, etc), so, to some degree, the decision of which one to use can be somewhat arbitrary. Nevertheless, for the types of devices you're describing:

Audio devices are typically represented as character devices.

The older OSS Linux sound system represented a sound device as /dev/dsp: reading data from it would read PCM data representing microphone input, and writing PCM data to it would play that through the sound card. The more modern ALSA sound system uses a more complex set of devices and formats, but the concept is the same.
Video devices are an unusual situation.

Some simple "framebuffer" video drivers are implemented as a block device such as /dev/fb0. This device represents the image on screen — a 1024x768 screen using 32-bit color would be represented by a 3 MB buffer, for instance — and writing to it changes that image. It's most commonly used by mapping it into memory.

Most modern video devices aren't that simple, though. Many of them are implemented partially as a kernel driver and partially as a X11 device driver which may directly map and write to the video hardware. They may sometimes include an extra character or block device (e.g, some Nvidia graphics cards use a character device /dev/nvidia), but the details of what data is read from or written to this device are largely proprietary.

Device driver classification

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