Introduction to Linux kernel Character Device Drivers

Introduction

Continuing the FLUSP kernel course (MAC0470/5856), I followed the Introduction to Linux kernel Character Device Drivers tutorial.

After the simple “hello world” modules, this was my first real driver. The goal was to create a basic character device (simple_char) that shows up in /dev/ and lets user-space programs read and write data.

What I Did

I added drivers/misc/simple_char.c with the usual file_operations (open, read, write, release) and a small kernel buffer. Used alloc_chrdev_region() + cdev for dynamic major number registration — no more fighting for static majors.

Quick additions to Kconfig and Makefile, built as module, copied to the VM.

Testing

Inside the VM:

modprobe simple_char
dmesg | grep simple_char
sudo mknod /dev/simple_char c 237 0

Then two tiny C programs: one to write a string, another to read it back. It worked!

Challenges

• Had to check /proc/devices every time to get the right major.

What I Learned

• Character devices are the classic bridge between kernel and user space via the filesystem.
• file_operations is where the magic happens.
• Always protect data movement between kernel and user space.