Python ctypes TypeError for defined function

Question

I have a known function type definition of wintypes.HANDLE, wintypes.LPVOID with a return value of wintypes.DWORD.

Using ctypes I've defined the type and function and tried to make the call with a handle and lpvoid reference:

ftype = CFUNCTYPE(wintypes.HANDLE, wintypes.LPVOID, wintypes.DWORD)
function = ftype(address)

base = wintypes.LPVOID(0x0)
ptr = function(GetCurrentProcess(), byref(base))

However, when executing I receive an error: ctypes.ArgumentError: argument 2: <class 'TypeError'>: wrong type

Can someone please help me identify the problem?

What type should I be passing, if not the defined lpvoid?

Well, a byref(base) would be void **. Wouldn't you just pass base? — Tim Roberts
– Tim Roberts, Commented Aug 1, 2021 at 3:16
Tried that too, same error message argument 2: <class 'TypeError'>: wrong type — user1220022
– user1220022, Commented Aug 1, 2021 at 4:03
No, the FIRST argument is the return type. Still, I admit i was wrong. — Tim Roberts
– Tim Roberts, Commented Aug 1, 2021 at 5:06

Mark Tolonen · Accepted Answer · 2021-08-01 18:56:47Z

Here's a minimal example. I created a DLL with the function signature described and extracted the address to demo the Python code.

#include <windows.h>
#include <stdio.h>

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

API DWORD function(HANDLE h, LPVOID pv) {
    printf("h=%p pv=%p\n", h, pv);
    return 1;
}

from ctypes import *
from ctypes import wintypes

dll = CDLL('./test')
address = addressof(dll.function)

# return value is the FIRST parameter to CFUNCTYPE
ftype = CFUNCTYPE(wintypes.DWORD, wintypes.HANDLE, wintypes.LPVOID)

# ftype(address) doesn't expect an unwrapped C address, but a Python function,
# so I used from_address() instead.
function = ftype.from_address(address)

ret = function(0x123,None) # None can be used for a null pointer
print(ret)

Output:

h=0000000000000123 pv=0000000000000000
1

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