The Raspberry Pi OS is a Linux distribution optimized for the Raspberry Pi; however, behind the fancy wallpaper is the standard Linux operating system. Chapter 1 , “Getting Started,” introduced the ability to display a string and to exit a program by directly invoking operating system services. In high-level programming languages, runtime libraries include wrappers that facilitate calling the operating system. This creates the impression that these services are integral to the high-level language. In this chapter, an exploration of what these runtime libraries perform behind the scenes to call Linux and the range of available services will be presented.