callback.h

        #ifdef CALLBACK_EXPORTS
        #define CALLBACK_API __declspec(dllexport)
        #else
        #define CALLBACK_API __declspec(dllimport)
        #endif

        extern "C" {

        typedef struct myst
        {
            int a;
            int b;
            int c;
        } myst_args;

        typedef void callBackFunc (struct myst);

        CALLBACK_API int __stdcall create_callback(callBackFunc* funcIn);
        CALLBACK_API int __stdcall call_callback();

        }

        class Ccallback {
        public:
            Ccallback(callBackFunc *pFuncIn);
            static void fireCallback();

        private:
            static callBackFunc *pFunc;
        };

callback.cpp

        #include "stdafx.h"
        #include "callback.h"

        // This is the constructor of a class that has been exported.
        // see callback.h for the class definition
        Ccallback::Ccallback(callBackFunc *pFuncIn)
        {
            pFunc = pFuncIn;
            return;
        }

        void 
        Ccallback::fireCallback()
        {
            myst args;
            args.a = 1;
            args.b = 2;
            args.c = 3;
            (*pFunc) (args);
        }

        CALLBACK_API int __stdcall create_callback(callBackFunc* funcIn)
        {
            Ccallback temp(funcIn);
            return 0;
        }

        CALLBACK_API int __stdcall call_callback()
        {
            Ccallback::fireCallback();
            return 0;
        }

        callBackFunc* Ccallback::pFunc = 0;

My Python code is:
        
        import ctypes

        class myst_args(ctypes.Structure):
            _pack_ = 1
            _fields_ = [
                        ('a',    ctypes.c_int), 
                        ('b',    ctypes.c_int),   
                        ('c',    ctypes.c_int)      
                        ]
                        
        callback_t = ctypes.CFUNCTYPE(None, myst_args)
                 
        def onGetEvent(args, **kws):
            print "a =", args.a
            print "b =", args.b
            print "c =", args.c
            
        lib = ctypes.cdll.LoadLibrary("callback.dll")
        _CB_GET = callback_t(onGetEvent)
        lib.create_callback(_CB_GET)
        lib.call_callback()