IPC between a C DLL and Python application to process data

Question

I would like to send over a message structure from a DLL-callback function to a python application so I can log the messages.

For this I would like to use ZeroMQ. Sadly I am unable to get the messages to python using the example provided by ZeroMQ.

DLL:

// dllmain.cpp : Defines the entry point for the DLL application.
#include "pch.h"
#include <windows.h>
#include <stdio.h>
#include <string.h>
#include <zmq.h>
HHOOK tHook;
HMODULE hinstDLL;
void* requester;
void* context;
LRESULT CALLBACK meconnect(int code, WPARAM wParam, LPARAM lParam) {
    if (code == HC_ACTION) {
        LPMSG data = (LPMSG)lParam;
        UINT message = data->message;
        switch (message)
        {
        case WM_POINTERUPDATE:
            if (!IS_POINTER_INCONTACT_WPARAM(wParam))
                break;
        case WM_POINTERDOWN:
        case WM_POINTERUP:
            POINTER_INFO pointerInfo = {};
            GetPointerInfo(GET_POINTERID_WPARAM(wParam), &pointerInfo);        
            int request_nbr;
            for (request_nbr = 0; request_nbr != 10; request_nbr++) {
                char buffer[10];
                printf("Sending Hello %d…\n", request_nbr);
                zmq_send(requester, data, 5, 0);
                zmq_recv(requester, buffer, 10, 0);
                printf("Received World %d\n", request_nbr);
            }        
        }
    }
    return(CallNextHookEx(tHook, code, wParam, lParam));
}
extern "C" __declspec(dllexport) BOOL ConnectServer() {
    printf("Connecting to hello world server…\n");
    static void* context = zmq_ctx_new();
    static void* requester = zmq_socket(context, ZMQ_REQ);
    zmq_connect(requester, "tcp://127.0.0.1:5555");
    printf("connected");
    return TRUE;
}
extern "C" __declspec(dllexport) BOOL DisconnectServer() {
    zmq_close(requester);
    zmq_ctx_destroy(context);
    return TRUE;
}
extern "C" __declspec(dllexport) BOOL SetHook()
{
    tHook = SetWindowsHookEx(WH_GETMESSAGE, meconnect, hinstDLL, 0);

    if (tHook == NULL)
        return FALSE;
    else
        return TRUE;
}
extern "C" __declspec(dllexport) BOOL UnHook()
{
    return UnhookWindowsHookEx(tHook);
}


BOOL APIENTRY DllMain(HMODULE hModule,
    DWORD  ul_reason_for_call,
    LPVOID lpReserved
)
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
        hinstDLL = hModule;
        break;
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

Python:

context = zmq.Context()
socket = context.socket(zmq.REP)
socket.bind("tcp://127.0.0.1:5555")

def message_msg_loop():
    while True:
        #  Wait for next request from client
        message = socket.recv()
        print("Received request: %s" % message)

        #  Do some 'work'
        time.sleep(1)

        #  Send reply back to client
        socket.send(b"World")

def pointer_msg_loop():
    global lib
    lib = cdll.LoadLibrary(r'C:\Users\Braun\Documents\BA_Thesis\ba-oliver-braun-logging-tool-code\MessagesDll\x64\Release\HOOKDLL.dll')
    print(lib)
    res = lib.ConnectServer()
    res = lib.SetHook()
    pythoncom.PumpMessages()
    res = lib.UnHook()

Basically my plan was to detect a certain event through windows messages and pass the message structure from the DLL-callback over to the server in Python, so I can handle the data there and put them into a log file. It does not seem to work though.

score 2 · Answer 1 · answered Feb 19 '20 at 14:48

_{In case one has never worked with ZeroMQ,
one may here enjoy to first look at "ZeroMQ Principles in less than Five Seconds"
before diving into further details}

Simplicity helps us start,
_{rather than remain headbanging into Complexity First}

Best avoid all the complexities :
- set .setsockopt( zmq.LINGER, 0 ) # ALWAYS, never know what version will try to join the Club
- prototype with PUSH/PULL(it a) meets the spec.+b) does not block in a mutual deadlock as all REQ/REP do ) - never share a socket ( yes, the requester ought be a private, non-shared instance )
- always read-in and assert-eval the return-codes from the ZeroMQ API calls ( detect many issues on-spot )

Can you POSACK / prove the both of the module-level declarations

...
void* requester;
void* context;
LRESULT CALLBACK meconnect(...) {...}
...

actually work as expected, or does the ConnectServer(){...}'s internal, in-scope declarations mask both of these globals ?

extern "C" __declspec(dllexport) BOOL ConnectServer() {
    printf("Connecting to hello world server…\n");
    static void* context = zmq_ctx_new();                   // shadows out void* context
    static void* requester = zmq_socket(context, ZMQ_REQ); //  shadows out void* requester
    zmq_connect(requester, "tcp://127.0.0.1:5555");
    printf("connected");
    return TRUE;
}

Hey thanks for helping me out. I experimented abit and found out that pasting all the zmq related code into callback lets me receive messages in python. So the problem is definitely the globals. — Oliver Braun, Feb 19 '20 at 14:57
Great to hear that 've solved your problem + do not forget the other things, most importantly the explicit LINGER. — user3666197, Feb 19 '20 at 14:59
I still sadly have no idea how to split up the code between the function so the globals are working. Also i have no idea how to set up the above LINGER and what it means. — Oliver Braun, Feb 19 '20 at 15:04
Received request: b'\xd2\x03\x0b\x00\x00' This is what i get in my python program — Oliver Braun, Feb 19 '20 at 15:10

IPC between a C DLL and Python application to process data

1 Answers1

Simplicity helps us start,rather than remain headbanging into Complexity First

Simplicity helps us start,
_{rather than remain headbanging into Complexity First}