syscall stealing

Hello ,
I'm new into kernel module programming and to improve my knowledges i would like to create a simple kernel module. My module should create a proc entry and increment a number everytime a key is pressed.
To do that i have to "hijack" the "getkeycode" function but i don't now how to do that with 2.6 kernel series.
Is anyone can help me telling me where to find documentation or an existing module witch use such a technique