How to pass a Numpy array into a cffi function and how to get one back out?

The ctypes attribute of ndarray can interact with the ctypes module, for example, ndarray.ctypes.data is the data address of the array, you can cast it to a float * pointer,
and then pass the pointer to the C function.

import numpy as np
from cffi import FFI

ffi = FFI()
ffi.cdef("void copy(float *in, float *out, int len);")
C = ffi.dlopen("ccode.dll")

a = 42*np.ones(16, dtype=np.float32)
b = np.zeros_like(a)
pa = ffi.cast("float *", a.ctypes.data)
pb = ffi.cast("float *", b.ctypes.data)

C.copy(pa, pb, len(a))
print b

For your question 3:

I think ffi array doesn’t provide numpy the necessary information to access it’s inner buffer. So numpy try to convert it to a float number which failed.

The best solution I can thinks is convert it to list first:

float_in[0:16] = list(arr_in[0:16])