Serial.readBytes() reads characters from the serial port into a buffer. The function terminates if the determined length has been read, or it times out (see Serial.setTimeout()).
Serial.readBytes() returns the number of characters placed in the buffer. A 0 means no valid data was found.
Serial.readBytes() inherits from the Stream utility class.
buffer: the buffer to store the bytes in (char or byte)
length : the number of bytes to read (int)