Linux Simple Character Driver

This is a very simple linux character device that has some basic functions that will be explained below. This code has learning purposes. If you are interested in learning how to develop a Linux Device Driver, you can use it as a reference. I tried to write it as simple as possible to make it easy to understand. I hope you can use it to learn something.

How it works

The device has a very simple functionality: Give to user the following answers: FIRST!, SECOND!, THIRD! according to what user has written to it. The device accepts the following inputs: 1, 2 or 3. So, if you write "1" on the device, and read it after that, you will receive the following answer: FIRST!. The device just check the first byte of a value written to it, so if you write 2adfadfadfa on the device, the 2 value will be captured by the device and, after that, if you read the device you will obtain the following string: SECOND!

Loading and unloading the driver

In this repository there is a script called init_char_device. If you execute this device with super user permissions, you will have your module loaded to the kernel and the device node /dev/chardevice0 created on the filesystem. To unload the module, there is another script: release_char_device. Executing this script will unload your driver from the kernel and delete the device nodes created before as well.

Debugging

If you have learning purposed, I advise you to debug the device. The code is full of KERN_DEBUG prints, and debugging you can see these prints and understand better how character driver works. To make this, you have to enable the KERNEL DEBUG LOG LEVEL. This can be done in many ways. One of these ways is executting the following line with super user permission:

$ echo 7 > /proc/sys/kernel/printk

After that, you will be able to see the Driver Debug Messages on the kernel buffer. To read the kernel buffer, one of the options is using unix command dmesg:

$ dmesg

Sample: Using the device

Following are a sequence of steps that I did to init, write, read and debug the device chardevice0 on a linux terminal.

$ cd repository_folder
$ make

Output:
make -C /lib/modules/4.12.14-lp151.28.20-default/build SUBDIRS=/home/repository_folder modules
make[1]: Entering directory '/usr/src/linux-4.12.14-lp151.28.20-obj/x86_64/default'
  CC [M]  /home/repository_folder/char_device.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/repository_folder/char_device.mod.o
  LD [M]  /home/repository_folder/char_device.ko
make[1]: Leaving directory '/usr/src/linux-4.12.14-lp151.28.20-obj/x86_64/default'

$ sudo ./init_char_device
$ dmesg

Output:
[33560.614940] Value assigned to "first": FIRST!
[33560.614941] Value assigned to "second": SECOND!
[33560.614941] Value assigned to "third": THIRD!
[33560.614942] Value assigned to "not defined": NOT DEFINED!
[33560.614942] Value assigned to "user choice": 0

$ echo "1" > /dev/chardevice0
$ head -c 10 /dev/chardevice0 && echo 

Output:
FIRST!

$ echo "2" > /dev/chardevice0
$ head -c 10 /dev/chardevice0 && echo 

Output:
SECOND!

$ echo "3" > /dev/chardevice0
$ head -c 10 /dev/chardevice0 && echo 

Output:
THIRD!

$ echo "anything except 1,2 or 3" > /dev/chardevice0
$ head -c 15 /dev/chardevice0 && echo 

Output:
NOT DEFINED!

$ sudo ./release_char_device

Output:
[34007.162644] Value assigned to "first": FIRST!
[34007.162645] Value assigned to "second": SECOND!
[34007.162646] Value assigned to "third": THIRD!
[34007.162646] Value assigned to "not defined": NOT DEFINED!
[34007.162647] Value assigned to "user choice": 0
[34014.434355] Opening device 0...
[34014.434375] Copying the value 1 from user to kernel...
[34014.434378] Releasing the device 0...
[34019.319451] Opening device 0...
[34019.319456] Copying the value FIRST! from kernel to user...
[34019.319459] Releasing the device 0...
[34023.118632] Opening device 0...
[34023.118644] Copying the value 2 from user to kernel...
[34023.118646] Releasing the device 0...
[34024.423423] Opening device 0...
[34024.423429] Copying the value SECOND! from kernel to user...
[34024.423433] Releasing the device 0...
[34027.850661] Opening device 0...
[34027.850684] Copying the value 3 from user to kernel...
[34027.850686] Releasing the device 0...
[34030.059333] Opening device 0...
[34030.059338] Copying the value THIRD! from kernel to user...
[34030.059341] Releasing the device 0...
[34036.842525] Opening device 0...
[34036.842536] Copying the value a from user to kernel...
[34036.842538] Releasing the device 0...
[34040.815307] Opening device 0...
[34040.815313] Copying the value NOT DEFINED! from kernel to user...
[34040.815316] Releasing the device 0...
[34046.658518] Removing device chardevice0...