Learn about kernel command using Linux system calls - explore the SCI and add your own calls.
A system call is an interface between a user-space application and a service that the kernel provides. Because the service is provided in the kernel, a direct call cannot be performed; instead, you must use a process of crossing the user-space/kernel boundary. The way you do this differs based on the particular architecture. For this reason, I'll stick to the most common architecture, i386.
System calls are an efficient way of traversing between user-space and the kernel to request services in the kernel-space. But they are also tightly controlled, and it's much easier simply to add a new /proc file system entry to provide the user/kernel interactions. When speed is important, however, system calls are an ideal way to squeeze the greatest performance out of your application.
Linux' system calls -- we use them every day. But do you know how a system call is performed from user-space to the kernel -- Explore the Linux system call interface (SCI), learn how to add new system calls (and alternatives for doing so), and discover utilities related to the SCI.)