Essential Features

Extreme C is a book that will provide you with both the fundamental and the advanced knowledge you need in order to develop and maintain real C applications. Generally, only knowing the syntax of a programming language is not enough to write successful programs with it – and this is of greater importance in C compared to most other languages. So, we're going to cover all of the concepts you need to write great software in C, from simple single process programs to more complex multiple process systems.

This first chapter is primarily concerned with particular features of C that you'll find extremely useful while you're writing C programs. These features are involved in situations you will encounter regularly while writing in C. Although there are a number of great books and tutorials on C programming that explain everything in detail and cover almost all aspects of C syntax, it would be useful to consider some key features here before we go deeper with C.

These features include preprocessor directives, variable pointers, function pointers, and structures. They are common, of course, in today's more modern programming languages, and it's easy to find their counterparts in Java, C#, Python, and so on. For example, references in Java can be considered as similar elements to variable pointers in C. These features and their related concepts are so fundamental, that without them, no piece of software could continue to work, even if it could get executed! Even a simple "hello world" program cannot work without loading a number of shared libraries that require the usage of function pointers!

So, whenever you see something like traffic lights, your car's central computer, the microwave oven in your kitchen, the operating system of your smartphone, or probably any other device that you generally don't think about, they all have pieces of software written in C.

Our lives today have been greatly impacted by the invention of the C programming language, and our world would be very different without C.

This chapter focuses on the essential features and machinery required to write expert C code and contains a handpicked collection of features for us to study in depth. We'll explore the following topics:

• Preprocessor directives, macros, and conditional compilation: Preprocessing is one of those C features that you can't easily find in other programming languages. Preprocessing brings a lot of advantages, and we'll dive into some of its interesting applications, including macros and conditional directives.
• Variable pointers: This section deep-dives into variable pointers and their uses. We'll also find some helpful insights by looking at some of the flaws that could be introduced by misusing variable pointers.
• Functions: This section of the chapter is a deep dive into everything we know about functions, beyond just their syntax. In fact, the syntax is the easy part! In this section, we will look at functions as the building blocks for writing procedural code. This section also talks about the function call mechanism and how a function receives its arguments from the caller function.
• Function pointers: Undoubtedly, function pointers are one of the most important features of C. A function pointer is a pointer that points to an existing function instead of a variable. The ability to store a pointer to an existing logic is profoundly important in algorithm design, and that's why we have a dedicated section on this topic. Function pointers appear in a vast range of applications ranging from loading dynamic libraries to polymorphism, and we'll be seeing plenty more of function pointers across the next couple of chapters.
• Structures: C structures might have a simple syntax and convey a simple idea, but they are the main building blocks for writing well-organized and more object-oriented code. Their importance, together with function pointers, simply cannot be overstated! In the last section of this chapter, we'll revisit all the things that you need to know about structures in C and the tricks around them.

The essential features of C, and their surrounding concepts, play a key role in the Unix ecosystem, and they have led to C being an important and influential technology despite its old age and harsh syntax. We will talk more about the mutual influence of C and Unix upon one another in the upcoming chapters. For now, let's begin this first chapter by talking about preprocessor directives.