def _mask_to_bytes(self, mask):
        """
        Convert the (type long) mask to a cpu_set_t.
        """
        chunks = []
        shiftmask = (2 ** 64) - 1
        for x in range(16):
            chunks.append(struct.pack('<Q', mask & shiftmask))
            mask >>= 64
        return mitogen.core.b('').join(chunks)