extern "C" {
  
  // the function from the C program we want to call
  extern int answerToLife();
  
  // the function that we want to be visible in the C code
  int halfAnswerToLife() {
    Serial.println("in halfAnswerToLife");
    return 21;
  }
}

void setup() {
  Serial.begin(115200);
  int anwser = answerToLife();
  Serial.print("The answer to life, the universe, and everything is ");
  Serial.println(anwser);
}

void loop() {}