In the third part of this guide we will explain what system calls actually are and how they work under the Linux kernel. Furthermore, we will also try to write a simple syscall that will greet the user. Do note that this article is part of a series of guides on kernel hacking for Linux, you can find the rest of the guide here: Part one: Introduction to the Linux kernel architecture; Part two: Building a device driver from scratch; Part three(this one): Introduction to syscalls, how to create a new syscall; Be sure to read the previous two before starting with this one.