In Arduino sketches, setup() is the function that runs once at the start of the program, after the board powers up or resets. However, there are specific things you can place before setup() in your code file.

What can go before setup()

• #include statements: To import libraries.

• Global variables and constants: Variables declared here are accessible in both setup() and loop().

• Function declarations/definitions: You can define helper functions before setup() or just declare them and define later.

• Class definitions and object instantiations: Useful for creating objects that need to be accessed globally.

What cannot go before setup()

• Executable code that runs at runtime (except global constructors) — Arduino’s runtime system (crt0) and main() handle initialization before calling setup().

• Hardware-dependent code in global constructors — peripherals may not be ready until after Arduino’s init() runs.

Example: