testfifo — Linux Kernel Character Device

A loadable kernel module implementing a FIFO character device (/dev/testfifo), built as a structured exercise in kernel module development.

What it does

Userspace programs can open /dev/testfifo and write data in, then read it back out in order — a classic first-in, first-out pipe backed by a kernel buffer.

Features

• Character device registration — cdev + alloc_chrdev_region, with automatic /dev node creation via class_create/device_create
• Circular buffer — fixed 4096-byte kernel buffer allocated with kmalloc, managed with head/tail/count pointers
• Read/write file operations — copy_from_user / copy_to_user for safe crossing of the kernel/userspace boundary
• Mutex synchronisation — mutex_lock_interruptible protects shared buffer state against concurrent access
• ioctl interface — query buffer size, query bytes available, flush the buffer
• Userspace test program — a C program in userspace/ that exercises all functionality and edge cases end to end

Building, Usage and Testing

1. Build

From the main directory:

make
cd userspace && make

2. Test the Kernel Module

Start the virtual kernel

vng --run

Insert the character device

sudo insmod testfifo.ko

Observe kernel logs

sudo dmesg | tail -5

Check that the device file exists

ls -la /dev/testfifo

Write to the device and read it back

sudo chmod 666 /dev/testfifo
echo "hello" > /dev/testfifo
cat /dev/testfifo

Deregister and remove the module

sudo rmmod testfifo

Confirm removal in logs and device listing

sudo dmesg | tail -5
ls -la /dev/testfifo

3. Run the Userspace Test Program

vng --run
sudo insmod testfifo.ko
sudo userspace/fifo_userspace_test

Notes

Some comments in the code are intentionally verbose — they reflect design decisions and learning notes rather than production documentation style.