How to cast a ctypes pointer to an instance of a Python class

Sossisos Published at Dev

Sossisos

Say you have the following C code:

typedef void (*PythonCallbackFunc)(void* userData);

void cb(PythonCallbackFunc pcf, void* userData)
{
    pcf(userData);
}

and the following Python 3 code:

import ctypes

class PythonClass():
    def foo():
        print("bar")

CALLBACK_TYPE = ctypes.CFUNCTYPE(None, ctypes.c_void_p)

def callback(userData):
    instanceOfPythonClass = ???(userData) # <-- this part right here
    instanceOfPythonClass.foo()

lib = ctypes.cdll.LoadLibrary("path/to/lib.dll")

pc = PythonClass()

lib.cb(ctypes.byref(pc), CALLBACK_TYPE(callback))

Where "path/to/lib.dll" is a compiled binary of the C code up top.

How would one go about casting the userData parameter in "callback" back to an instance of PythonClass, so one could call the function "foo()"?

CristiFati

Based on [Python 3.Docs]: ctypes - A foreign function library for Python, I did some changes to your code in order to make it work.

dll.c:

#include <stdio.h>

#if defined(_WIN32)
#  define DLL_EXPORT __declspec(dllexport)
#else
#  define DLL_EXPORT
#endif

#define C_TAG "From C"
#define PRINT_MSG_0() printf("%s - [%s] (%d) - [%s]\n", C_TAG, __FILE__, __LINE__, __FUNCTION__)


typedef void (*PythonCallbackFuncPtr)(void *userData);


DLL_EXPORT void callPython(PythonCallbackFuncPtr callbackFunc, void *userData)
{
    PRINT_MSG_0();
    callbackFunc(userData);
}

code.py:

import sys
from ctypes import CDLL, CFUNCTYPE, \
    py_object


CallbackFuncType = CFUNCTYPE(None, py_object)

dll_dll = CDLL("./dll.dll")
call_python_func = dll_dll.callPython
call_python_func.argtypes = [CallbackFuncType, py_object]


class PythonClass():
    def foo(self):
        print("Dummy Python method")


def callback(userData):
    print("From Python: {:s}".format(callback.__name__))
    userData.foo()


def main():
    instance = PythonClass()
    call_python_func(CallbackFuncType(callback), instance)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Notes:

When dealing with Python types, use ctypes.py_object (which is a wrapper over PyObject) rather than ctypes.c_void_p
Always define argtypes (and restype) for C functions that you call from Python (e.g. call_python_func (which wraps callPython))
PythonClass.foo() was missing the 1^st (self) argument and thus being just a function defined inside PythonClass instead of a method
Did other non critical changes (mostly renames)

Output:

(py35x64_test) e:\Work\Dev\StackOverflow\q052053434>"c:\Install\x86\Microsoft\Visual Studio Community\2015\vc\vcvarsall.bat" x64

(py35x64_test) e:\Work\Dev\StackOverflow\q052053434>dir /b
code.py
dll.c

(py35x64_test) e:\Work\Dev\StackOverflow\q052053434>cl /nologo /DDLL dll.c  /link /DLL /OUT:dll.dll
dll.c
   Creating library dll.lib and object dll.exp

(py35x64_test) e:\Work\Dev\StackOverflow\q052053434>dir /b
code.py
dll.c
dll.dll
dll.exp
dll.lib
dll.obj

(py35x64_test) e:\Work\Dev\StackOverflow\q052053434>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code.py
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32

From C - [dll.c] (18) - [callPython]
From Python: callback
Dummy Python method

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