This chapter does not require you to install any special software if you're on Linux.

If you are a Windows user, you can install the Windows Subsystem for Linux (WSL). Follow these steps in order to install WSL:

1. Open PowerShell as administrator and run the following: 

Enable-WindowsOptionalFeature -Online -FeatureName Microsoft-Windows-Subsystem-Linux

2. Restart your computer when prompted.
3. Install your favorite Linux distribution from the Microsoft Store.

Over the years, the IT landscape has shifted dramatically. Multicore CPUs that challenge the Von Neumann machine, the internet, and distributed systems are just some of the changes that occurred in the last 30 years. So, where does system programming stand in this landscape?

Let's start with the standard textbook definition first.

System programming (or systems programming) is the activity of programming computer system software. The primary distinguishing characteristic of system programming when compared to application programming is that application programming aims to produce software that provides services directly to the user (for example, a word...

APIs are series subroutine definitions, communication protocols, and tools for building software. The most important aspects of an API are the functionalities that it offers, combined with its documentation, which facilitates the user in the usage and implementation of the software itself in another software. An API can be the interface that allows an application software to use a system software.

An API usually has a specific release policy that is meant to be used by a specific group of recipients. This can be the following:

• Private and for internal use only
• Partner and usable by determined groups only—this may include companies that want to integrate the service with theirs
• Public and available for every user

The protection ring, also referred to as hierarchical protection domains, is the mechanism used to protect a system against failure. Its name is derived from the hierarchical structure of its levels of permission, represented by concentric rings, with privilege decreasing when moving to the outside rings. Between each ring there are special gates that allow the outer ring to access the inner ring resources in a restricted manner.

The number and order of rings depend on the CPU architecture. They are usually numbered with decreasing privilege, making ring 0 the most privileged one. This is true for i386 and x64 architecture that use four rings (from ring...

System calls are the way operating systems provide access to the resources for the applications. It is an API implemented by the kernel for accessing the hardware safely.

There are some categories that we can use to split the numerous functions offered by the operating system. These include the control of the running applications and their flow, the filesystem access, and the network.

This type of services includes load, which adds a program to memory and prepares for its execution before passing control...

In order to ensure consistency between operating systems, IEEE formalized some standards for operating systems. These are described in the following sections.

Portable Operating System Interface (POSIX) for Unix represents a series of standards for operating system interfaces. The first version dates back to 1988 and covers a series of topics like filenames, shells, and regular expressions.

There are many features defined by POSIX, and they are organized in four different standards, each one focusing on a different aspect of the Unix compliance. They are all named POSIX followed by a number.

In this chapter, we saw what system programming means—writing system software that has some strict requirements, such as being tied to the hardware, using a low-level language, and working in a resource-constrained environment. Its practices can be really useful when building distributed systems that normally require optimizing resource usage. We discussed APIs, definitions that allows software to be used by other software, and listed the different types—the ones in the operating system, libraries and frameworks, and remote and web APIs.

We analyzed how, in operating systems, the access to resources is arranged in hierarchical levels called protection rings that prevent uncontrolled usage in order to improve security and avoid failures from the applications. The Linux model simplifies this hierarchy to just two levels called user and kernel...

1. What is the difference between application and system programming?
2. What is an API? Why are APIs so important?
3. Could you explain how protection rings work?
4. Can you make some examples of what cannot be done in user space?
5. What's a system call?
6. Which calls are used in Unix to manage a process?
7. Why is POSIX useful?
8. Is Windows POSIX compliant?