Device Files

One of the kernel important role is to allow applications to access hardware devices
In the Linux kernel, most devices are presented to user space applications through two different abstractions
- Character device
  - Originally, an infinite stream of bytes, with no beginning, no end, no size. The pure example: a serial port.
  - Used for serial ports, terminals, but also sound cards, video acquisition devices, frame buffers
  - Most of the devices that are not block devices are represented as character devices by the Linux kernel
- Block device
  - A device composed of fixed-sized blocks, that can be read and written to store data
  - Used for hard disks, USB keys, SD cards, etc.
Internally, the kernel identifies each device by a triplet of information
- Type (character or block)
- Major (typically the category of device)
- Minor (typically the identifier of the device)

Devices: everything is a file

A very important Unix design decision was to represent most of the ``system objects'' as files
It allows applications to manipulate all “system objects” with the normal file API (open, read, write, close, etc.)
So, devices had to be represented as files to the applications
This is done through a special artifact called a device file
It is a special type of file, that associates a file name visible to user space applications to the triplet (type, major, minor) that the kernel understands
All device files are by convention stored in the /dev directory

Before Linux 2.6.32, on basic Linux systems, the device files had to be created manually using the mknod command
- mknod /dev/ [c|b] major minor
- Needed root privileges
- Coherency between device files and devices handled by the kernel was left to the system developer
The devtmpfs virtual filesystem can be mounted on /dev and contains all the devices known to the kernel. The CONFIG_DEVTMPFS_MOUNT kernel configuration option makes the kernel mount it automatically at boot time, except when booting on an initramfs.