def stop(self):
        """Stops the external measurement program and returns the measurement result,
        if the measurement was running."""
        consumed_energy = collections.defaultdict(dict)
        if not self.is_running():
            return None
        # cpu-energy-meter expects SIGINT to stop and report its result
        self._measurement_process.send_signal(signal.SIGINT)
        (out, err) = self._measurement_process.communicate()
        assert self._measurement_process.returncode is not None
        if self._measurement_process.returncode:
            logging.debug(
                "Energy measurement terminated with return code %s",
                self._measurement_process.returncode)
        self._measurement_process = None
        for line in err.splitlines():
            logging.debug("energy measurement stderr: %s", line)
        for line in out.splitlines():
            line = line.decode('ASCII')
            logging.debug("energy measurement output: %s", line)
            match = re.match(r'cpu(\d+)_([a-z]+)_joules=(\d+\.?\d*)', line)
            if not match:
                continue

            cpu, domain, energy = match.groups()
            cpu = int(cpu)
            energy = Decimal(energy)

            consumed_energy[cpu][domain] = energy
        return consumed_energy
Stops the external measurement program and returns the measurement result,
        if the measurement was running.