The Preprocessor

Before your code is compiled, it passes through the preprocessor — a text-substitution step that handles every line starting with #. It doesn't understand C; it just rewrites your source file according to a small set of directives, and hands the result to the compiler.

#include

You've used this since chapter 0x00. It literally pastes the contents of another file in place:

1#include <stdio.h>   // a standard library header, in angle brackets
2#include "helpers.h"  // a header file of your own, in quotes

Angle brackets tell the compiler to look in the system's standard include paths; quotes look in your project first.

#define: constants and macros

1#define MAX_USERS 100
2#define PI 3.14159
3 
4int users[MAX_USERS];
5float area = PI * radius * radius;

Every occurrence of MAX_USERS is replaced with 100 before compilation even begins — the compiler never sees the name MAX_USERS at all. const variables (0x01) are usually preferred for simple constants today, since they have a real type the compiler can check.

Function-like macros

1#define SQUARE(x) ((x) * (x))
2 
3int result = SQUARE(5);       // expands to ((5) * (5))

1#define SQUARE(x) x * x
2 
3SQUARE(2 + 3);  // expands to 2 + 3 * 2 + 3  →  11, not 25

Conditional compilation

#ifdef and related directives include or exclude code before compilation, commonly used for platform-specific code or debug-only logging:

1#define DEBUG
2 
3#ifdef DEBUG
4    printf("Debug: entering main\n");
5#endif

Include guards

Header files use a guard to prevent being pasted into the same file twice — essential once a project has multiple files including the same header, covered next in 0x0F:

1#ifndef HELPERS_H
2#define HELPERS_H
3 
4// declarations go here
5 
6#endif