Convert a ctypes pointer array to a numpy array.

def ctypes2numpy(cptr, length, dtype):
    """Convert a ctypes pointer array to a numpy array.
    """
    NUMPY_TO_CTYPES_MAPPING = {
        np.float32: ctypes.c_float,
        np.uint32: ctypes.c_uint,
    }
    if dtype not in NUMPY_TO_CTYPES_MAPPING:
        raise RuntimeError('Supported types: {}'.format(NUMPY_TO_CTYPES_MAPPING.keys()))
    ctype = NUMPY_TO_CTYPES_MAPPING[dtype]
    if not isinstance(cptr, ctypes.POINTER(ctype)):
        raise RuntimeError('expected {} pointer'.format(ctype))
    res = np.zeros(length, dtype=dtype)
    if not ctypes.memmove(res.ctypes.data, cptr, length * res.strides[0]):
        raise RuntimeError('memmove failed')
    return res