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Preprocessor

The preprocessor comes before the compiler.

It's job is to prepare the source code for compilation.

Anything that begins with a # is for the preprocessor, such as includes

#include <stdio.h>

#include "multiply.h"

Which tells the compiler to compile these files together as one

The preprocessor can also be used for macros

#define SQUARE(x) multiply(x,x)

Macros, when called aren't like typical functions, instead the preprocessor replaces the macro call with the code itself.

SQUARE(10) would be replaced with multiply(10,10)

They can generate allsorts of code, such as CONSTANTS

#if LEVEL > 0
	/* conditionally do this code */
#endif