The C Preprocessor

#include, #define macros and conditional compilation run before the compiler sees your code.

Easy

#define F_CPU 16000000UL
#define SQUARE(x) ((x) * (x))       // parenthesise everything!

#ifdef DEBUG
  #define LOG(msg) printf("%s\n", msg)
#else
  #define LOG(msg)
#endif

#ifndef SENSOR_H                     // include guard
#define SENSOR_H
/* declarations */
#endif

Macros are plain text substitution: SQUARE(i++) increments i twice. Prefer static inline functions and const variables when possible.

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