arduino errors

Canonical explanations and fixes for arduino toolchain error signatures.

Arduino Linker Error: Undefined Reference to 'loop'

The Arduino compiler toolchain requires every sketch to define a loop() function. Omitting this function causes a linker error when the core attempts to resolve it during compilation.

Arduino Linker Error: undefined reference to 'setup'

The Arduino compiler's linker failed to find the mandatory setup() function. Every standard Arduino sketch requires both a setup() and a loop() function to successfully link into an executable.