06_character_device_driver

Understanding Linux Device Files

Video :

Block and Character Devices

Major & Minor

Where do device files live and why are they “virtual”?

• The /dev directory contains special files called device nodes.

ls /dev
ls /dev | wc -l

• b = block, c = character

ls -lh /dev/mmcblk0 /dev/tty0 /dev/gpiochip0

• minor number

ls -lh /dev/tty{1,2,3,4,5}

• get registerd major numbers

cat /proc/devices

filename doesn’t matter

Character Device: Serial Port example

• Enable Serial port

sudo raspi-config

• install screen

sudo apt install screen -y

• screen serial port (short the UART TX and RX pins)

sudo screen /dev/ttyS0 9600

Whatever we type anyting it will echo back, showing that the serial interface works.
exit form screen: Ctrl + A , Then, press D.

• Create my_serial device node

sudo mknod my_serial c 4 64

• again screen serail bus using new node my_serial:

sudo screen my_serial 9600

This works exact same. like ttyS0

The filename doesn’t matter – only major+minor do.

Block Device: SD Card Memeory block example

• dump the mmcblk0 4kb memory,

sudo hexdump -C /dev/mmcblk0 -n 512 | head

• create our own device node:

sudo mknod my_mblk b 179 0 # 179,0 is the same major and minor number of the mmcblk0

ls -lh my_mblk

• Read 1st 4kb of data of new device node my_mblk

sudo hexdump -C my_mblk -n 512 | head