Python ctypes how to read array from a void pointer return

Question

So I am trying to read an unsigned short array which is returned by a void pointer in the c library I am using. The function definition in the header is like this:

void* Foo(int a, int b)

This function end up returning an a pointer to an array of type unsigned short.

In python I have been trying to return the array using ctypes without success. Here is what I have:

import ctypes
import numpy as np

libc = ctypes.cdll.LoadLibrary("Library.dll")

Vec=np.zeros((1000,),dtype=np.uint16)

c_ushort_p = ctypes.POINTER(ctypes.c_ushort)
Vec_p=confVec.ctypes.data_as(c_ushort_p)

libc.Foo.restype = ctypes.c_void_p
Vec_p=libc.Foo(1,1)

print Vec_p

This returns "None".

If I try:

...
libc.Foo.restype = ctypes.c_void_p
Vec_p=libc.Foo(1,1)

print Vec_p[0]

I get TypeError: 'NoneType' object has no attribute 'getitem'.

I also tried this:

...
libc.Foo.restype = ctypes.c_void_p
Vec_p=ctypes.cast(libc.Foo(1,1),c_ushort_p)

print Vec_p

Which returns , where print Vec_p[0] gives "ValueError: NULL pointer access"

Can anyone give me any assistance?

Answer 1

The DLL is returning a null pointer. These are represented as None in ctypes.

I don't know why the function behaves that way but the output you see makes it quite clear that your function returns null.