Update the mask of signals this signalfd reacts to

        :param signals:
            A replacement set of signal numbers to monitor
        :raises ValueError:
            If :meth:`closed()` is True

def update(self, signals):
        """
        Update the mask of signals this signalfd reacts to

        :param signals:
            A replacement set of signal numbers to monitor
        :raises ValueError:
            If :meth:`closed()` is True
        """
        if self._sfd < 0:
            _err_closed()
        mask = sigset_t()
        sigemptyset(mask)
        if signals is not None:
            for signal in signals:
                sigaddset(mask, signal)
        # flags are ignored when sfd is not -1
        _signalfd(self._sfd, mask, 0)
        self._signals = frozenset(signals)