Calling the C++ function in python using ctypes

Question

I have C++ code which has a function which takes, three arguments

Pointer argument
LPWSTR argument
(LPWSTR variable)reference as argument Below is the C++ Code syntax

HRESULT WINAPIV GIDItem (Address* pa, LPWSTR cmdvalue, LPWSTR* string_value)

I am able to load the dll in Ctypes

    import ctypes
    uDl = ctypes.CDLL(r"test1")
    uDl_function=uDl.CAIA  # we are creating function pointer
    pa=uDl_function('something')
    uD = ctypes.CDLL(r"test.dll")
    uD_function =uD.GIDItem # we are creating function pointer
    string_value= ctypes.c_wchar_p()
    cmdvalue=ctypes.c_wchar_p("CM")
    dI=uD_function(ctypes.POINTER(pa),cmdvalue,ctypes.byref(string_value))

I am getting below error,

dI=uD_function(ctypes.POINTER(pa),cmdvalue,ctypes.byref(string_value))
TypeError: must be a ctypes type

I was just looking some article about that DLL, in C++ the Dll been called like below

fpGIDItem (pA, L"CMD", &cD)

When you look to the above code "CMD" is the cmdvalue and string value is sent with &cD

Please help me over this

I can't tell where pa is declared. Am I missing something? — gnodab
– gnodab, Commented Apr 23, 2020 at 18:03
You probably need to include a minimal working example. In the meantime see if this helps. — gnodab
– gnodab, Commented Apr 23, 2020 at 18:19
I don't have c++ dll code, i just know the input type, function name, and return — Sreevathsabr
– Sreevathsabr, Commented Apr 23, 2020 at 18:21
POINTER is a type and you need an instance of a type. Since the first parameter is Address* is can probably be just pa if that is what is returned from the first function. byref(string_value) is actually passing a wchar_t** which isn't what you want either. Just pass string_value. Update your question with the full C prototypes of both functions if you want a better answer. — Mark Tolonen
– Mark Tolonen, Commented Apr 25, 2020 at 0:19

Mark Tolonen · Accepted Answer · 2020-04-25 17:36:58Z

2

ctypes.POINTER(pa) is a type instead of an instance which is why it is giving the error you see.

If you create a working example of the use in C, it would be easier to identify how to write it in Python, but I'll take a stab at it.

Set .argtypes and restype on the function correctly so ctypes will type check for you.

Something like the following should work.

from ctypes import *

dll = CDLL(r'test1')
dll.CAIA.argtypes = c_wchar_p,
dll.CAIA.restype = c_void_p     # generic void* should work.

dll2 = ctypes.CDLL(r'test.dll')
dll2.GIDItem.argtypes = c_void_p,c_wchar_p,POINTER(c_wchar_p)
dll2.GIDItem.restype = c_long

pa = dll.CAIA('something')
string_value = c_wchar_p()
result = dll2.GIDItem(pa,'CM',byref(string_value))

answered Apr 25, 2020 at 17:36

Mark Tolonen

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1 Comment

Sreevathsabr Over a year ago

Thanks, a lot Mark! It worked well and fine.. It's getting the expected result. But I have the only thing if you how to enable the logs so that that I can log any error when we have any error when we called function from ctypes.

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