How to read output of a dll with custom structure using ctypes?

Question

I am working with a DLL called CL3_IF.dll that is described in its documentation as:

Format: CL3IF_VERSION_INFO CL3IF_GetVersion(void)

Parameters: —

Return value: Returns the version of this DLL.

typedef struct {
 INT majorNumber;
 INT minorNumber;
 INT revisionNumber;
 INT buildNumber;
} CL3IF_VERSION_INFO;

Explanation: This function gets the DLL version.

I have used the following python code:

import ctypes
CLP=ctypes.WinDLL('CL3_IF.dll')
class CLIF_VERSION_INFO(ctypes.Structure):
    _fields_ = [("majorNumber", ctypes.c_int),
                ("minorNumber", ctypes.c_int),
                ("revisionNumber", ctypes.c_int),
                ("buildNumber", ctypes.c_int)]
CLP.CL3IF_GetVersion(ctypes.c_void_p, CLIF_VERSION_INFO)

When I run the above code I get the following error:

ArgumentError: argument 1: <class 'TypeError'>: Don't know how to convert parameter 1

I want to be able to get the version info in a usable form, and many other functions of the dll give their outputs in complex structures.

The most complex function I will be using will be this

If anybody knows how I would go about reading the output of this, I would greatly appreciate it.

Answer 1

The returned structure is a return value, not an argument, and void means no parameters, whereas c_void_p means a void* parameter. Make sure to set .argtypes and .restype correctly.

Also note that WinDLL is for 32-bit __stdcall APIs. Use CDLL for __cdecl (C calling convention) APIs. On 64-bit Python both will work, because there is only one calling convention, but for portable code use the correct one.

Full example below:

test.cpp

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

typedef int INT;

typedef struct {
    INT majorNumber;
    INT minorNumber;
    INT revisionNumber;
    INT buildNumber;
} CL3IF_VERSION_INFO;

extern "C"
{

API CL3IF_VERSION_INFO CL3IF_GetVersion(void) {
    CL3IF_VERSION_INFO info = {1,2,3,4};
    return info;
}

}

test.py

import ctypes

class CLIF_VERSION_INFO(ctypes.Structure):

    _fields_ = [("majorNumber", ctypes.c_int),
                ("minorNumber", ctypes.c_int),
                ("revisionNumber", ctypes.c_int),
                ("buildNumber", ctypes.c_int)]

    def __repr__(self):
        return f'CLIF_VERSION_INFO(major={self.majorNumber},minor={self.minorNumber},rev={self.revisionNumber},build={self.buildNumber})'

dll = ctypes.CDLL('./test')
dll.CL3IF_GetVersion.argtypes = ()
dll.CL3IF_GetVersion.restype = CLIF_VERSION_INFO

print(dll.CL3IF_GetVersion())

Output:

CLIF_VERSION_INFO(major=1,minor=2,rev=3,build=4)