Preprocessor Directives (#define, #include)

The C preprocessor is the first step in the compilation process. It is a text-processing tool that scans the source code for specific directives (lines beginning with `#`) and acts on them before the code is handed over to the actual compiler. Understanding its role is key to managing codebases and using the C standard library effectively. The `#include` directive is perhaps the most frequently used. Its purpose is to insert the entire content of another file into the current source file. You use it in two forms. `#include <filename.h>` is used for system headers (like `<stdio.h>` or `<string.h>`), and it tells the preprocessor to look for the file in the standard system directories. `#include "filename.h"` is used for your own custom header files, and it tells the preprocessor to first look for the file in the current directory. The `#define` directive is used to create macros. In its simplest form, it creates a symbolic constant. The statement `#define PI 3.14159` tells the preprocessor to replace every subsequent occurrence of the identifier `PI` with `3.14159`. This is a direct text replacement, which happens before any C syntax is checked. This is useful for defining constants that are used throughout a program, making the code more readable and easier to modify. If the value of PI needs to be changed, you only have to change it in one place. These two directives form the basis of how C programs are structured and how they interface with libraries.