Convert a ctypes pointer array to a numpy array.

def ctypes2numpy(cptr, length, dtype):
    """Convert a ctypes pointer array to a numpy array.
    """
    if not isinstance(cptr, ctypes.POINTER(ctypes.c_float)):
        raise RuntimeError('expected float pointer')
    res = np.zeros(length, dtype=dtype)
    if not ctypes.memmove(res.ctypes.data, cptr, length * res.strides[0]):
        raise RuntimeError('memmove failed')
    return res