The Preprocessor

The Preprocessor

Before compiling, a preprocessor scans the file. It handles # directives.

Includes

#include <stdio.h>   // standard library
#include "myfile.h"  // your own header

Macros (defines)

#define PI 3.14159

Everywhere PI appears, the text 3.14159 replaces it before compile.

Macro with an argument

#define MAX(a, b) ((a) > (b) ? (a) : (b))

Why macros?

Useful for constants and quick helpers, but macros are error-prone. Prefer const when possible.

TL;DR

• Runs before compilation.
• Handles #include and #define.
• Macros are simple text swaps.
• Prefer constants over overkill macros.