What are the linux character devices?

Linux character devices include: 1. The mouse is an external input device for the computer and an indicator for positioning the vertical and horizontal coordinates of the computer display system; 2. The keyboard is an instruction used to operate the computer device. and data input device; 3. Serial port terminal, a terminal device connected using a computer serial port; 4. Control terminal; 5. Console, etc.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

In order to facilitate management, the Linux system divides devices into three basic types:

• Character device

• Block device

• Network devices

They are all displayed in the /dev directory of the file system in the form of a file node (crw--w---- 1 root tty 4, 0 July 11 09:11 tty0 where c represents the character device type).

linux character device

Character device refers to a device that can be read and written directly without buffering, such as mouse, keyboard, and serial port device , modem, etc. The difference from block devices is that the basic unit of character operations is bytes.

Classification of character devices

Character devices mainly include control terminal equipment and serial terminal equipment, such as consoles and keyboards. Based on differences in functions and hardware, character terminal devices are classified as follows:

• Serial port terminal (/dev/ttSn): A terminal device connected using a computer serial port, serial The device data transmission method is 8-bit single-line transmission of the same character. Enter echo 'hello world' > /dev/ttyS0 on the command line to write the input to the corresponding device.

• Pseudo terminal (/dev/ttyp, /dev/ptyp): Corresponding to the fact that there is no real hardware device at the bottom layer, it is used to provide a terminal-style interface for other programs, such as network login to the host. A terminal interface between a network server and a shell program.

• Control terminal (/dev/tty): The main device number is 5. The process control terminal is associated with the process. For example, the login shell process uses the terminal /dev/tty.

• Console (/dev/ttyn,/dev/consol): The monitor for computer input and output. When the console is logged in, tty1 is used, while the ubuntu graphical interface uses tty7 .

• Other types: Current Linux has many other types of device special files for many different devices, such as the /dev/ttyIn device of the ISIDIN device.

The following is a schematic diagram of the structure of the character device

The nature and characteristics of the character device

• Character device is a type of device file system, which is equivalent to the logical device file provided by the underlying hardware to the upper layer. It is like connecting a data port (data register) to a file, and the device driver directly accesses the file. operation, so the read and write operations were directly performed on the port. Also as a file, the character device driver must also implement the basic operations of files such as open(), close(), write(), read(), etc. Of course, terminal redirection operations are also supported.

• Character device file files are read and written in single byte units, and there is no need to set up a hardware buffer. The device is accessed by the operating system as a byte stream. The byte stream is like setting up a transmission pipe between the hardware port and the file system. Bytes are transmitted through the pipe one by one and presented to both readers and writers. This streaming feature is implemented in the driver as a buffer queue. For example: The read-write buffer queue in the console structure

struct tty_struct {
struct termios termios;
int pgrp;
int stopped;
void (*write)(struct tty_struct * tty);
struct tty_queue read_q;               //读队列
struct tty_queue write_q;              //写队列
struct tty_queue secondary;            //tty辅助队列(存放规格化后的字符)
};

• character device is identified by the character device number. The character device number consists of a major device number and a minor device number. For example, the device number of /dev/ttyS0 is (4, 64); the major device number identifies the driver corresponding to the device, and the kernel uses the major device number to match the device and the driver one-to-one. Generally speaking, the minor device number is used by the driver for the code used internally by the driver to differentiate between device details. It is not used by other parts of the kernel.

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