Run make to build the kernel module and the user space code. Then run
sudo insmod testdriver.ko
to load the module (you can use lsmod to check that it's loaded). The module should register a character device (if anything goes wrong, run sudo dmesg to see what happened). Then run
cat /proc/devices | grep testdevice
This will return the device major number next to its name, which should be "testdevice" (Linux character devices have major and minor numbers). Use this major number by running
sudo mknod /dev/mydevice c <major-number> 0
This will make a filesystem node that exposes the character device to user space. Finally, run
sudo ./user
Now the user space code in user.c is communicating with the kernel space code in testdriver.c, and vice versa! When finished, run
sudo rmmod testdriver
sudo rm /dev/mydevice
This removes the kernel module, which unregisters the character device from earlier, and then removes the now-useless filesystem node from the filesystem.