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3.1.1. Driving Hardware
The kernel is tasked, first and foremost, with controlling the computer’s hardware components. It detects and configures them when the computer powers on, or when a device is inserted or re- moved (for example, a USB device). It also makes them available to higher-level software, through a simplified programming interface, so applications can take advantage of devices without having to address details such as which extension slot an option board is plugged into. The programming interface also provides an abstraction layer; this allows video-conferencing software, for exam- ple, to use a webcam regardless of its maker and model. The software can use the Video for Linux (V4L) interface and the kernel will translate function calls of the interface into actual hardware commands needed by the specific webcam in use.
The kernel exports data about detected hardware through the /proc/ and /sys/ virtual file sys- tems. Applications often access devices by way of files created within /dev/. Specific files rep-
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resent disk drives (for instance, /dev/sda), partitions (/dev/sda1), mice (/dev/input/mouse0), keyboards (/dev/input/event0), sound cards (/dev/snd/*), serial ports (/dev/ttyS*), and other components.
There are two types of device files: block and character. The former has characteristics of a block of data: It has a finite size, and you can access bytes at any position in the block. The latter behaves like a flow of characters. You can read and write characters, but you cannot seek to a given position and change arbitrary bytes. To find out the type of a given device file, inspect the first letter in the output of ls -l. It is either b, for block devices, or c, for character devices:
$ ls -l /dev/sda /dev/ttyS0
brw-rw---- 1 root disk 8, 0 Mar 21 08:44 /dev/sda
crw-rw---- 1 root dialout 4, 64 Mar 30 08:59 /dev/ttyS0
$ ls -l /dev/sda /dev/ttyS0
brw-rw---- 1 root disk 8, 0 Mar 21 08:44 /dev/sda
crw-rw---- 1 root dialout 4, 64 Mar 30 08:59 /dev/ttyS0
As you might expect, disk drives and partitions use block devices, whereas mouse, keyboard, and serial ports use character devices. In both cases, the programming interface includes device- specific commands that can be invoked through the ioctl system call.