int numbers_of_characters(String s){
  int count = 0;
  while(s[count] != NULL){
    count++;
  }
  return count;
}

char* returning_characters(String x){
  int count = numbers_of_characters(x);
  char* y = malloc(count);
  strcpy(y, x.c_str());
  return y;
}

void setup() {
  Serial.begin(9600);
  Serial.println("Enter a message: ");
  Serial.println("- REMEMBER: max. of 20 characters only -");

}

void loop() {
  if(Serial.available() > 0){

    String msg = Serial.readStringUntil('\n');
    int x = numbers_of_characters(msg);

    if(x >= 20){
      Serial.println("Your message size exceeds the limit.");
    }
    else {
      Serial.print("Your message: ");
      Serial.println(msg);

      char* message = returning_characters(msg);
      Serial.println(message);
    }
  }

}