CODEWITHSUNDEEP
c++multithreadingwinapi
Martin Juin
Martin Juin

Reputation: 1

SendNotifyMessage do not send correct messages

From one thread I send the message to main thread in window procedure. But it is unsuccessful. When I send messages from the same thread - all is ok

include "stdafx.h"
#include <Windows.h>
#include <atlbase.h>


#define MAX_THREADS 1

HWND m_wnd;

enum
{
    EVENT_CALL = (WM_APP + 0x30),
};


static LRESULT CALLBACK function_call()
{
    //some code
    int test = 0;
    return 0;
}

static LRESULT CALLBACK http_message_proc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
        switch (uMsg)
        {
        case EVENT_CALL:
            function_call();
            return 0;
        }
    return ::DefWindowProc(hWnd, uMsg, wParam, lParam);
}

void CreateNotifyWnd()
{
    WNDCLASSEX   w = { 0 };
    w.cbSize = sizeof(w);
    w.hInstance = (HINSTANCE)&__ImageBase;
    w.lpszClassName = L"uistone_http_event_wnd";
    w.lpfnWndProc = http_message_proc;
    ::RegisterClassEx(&w);
    int error = GetLastError();
    m_wnd = ::CreateWindowEx(0, w.lpszClassName, NULL, 0, 0, 0, 0, 0, HWND_MESSAGE, NULL, w.hInstance, 0);
    error = GetLastError();
}


DWORD WINAPI SendThread(void* request_param)
{
    ::SendNotifyMessage(m_wnd, EVENT_CALL, 11, 12);
    int error = GetLastError();
    return 0;
}


int main()
{
    CreateNotifyWnd();
    HANDLE  hThreadArray[MAX_THREADS];

    hThreadArray[0] = CreateThread(nullptr, 0, SendThread, nullptr, 0, nullptr);
    //::SendNotifyMessage(m_wnd, EVENT_CALL, 11, 12);

    WaitForMultipleObjects(MAX_THREADS, hThreadArray, TRUE, INFINITE);

    return 0;
}

Why I can not catch messages from another thread? Thanks.

Upvotes: 0

Views: 432

Answers (1)

IInspectable
IInspectable

Reputation: 51345

This is documented behavior. This is the relevant part from the SendNotifyMessage documentation:

If the window was created by the calling thread, SendNotifyMessage calls the window procedure for the window and does not return until the window procedure has processed the message. If the window was created by a different thread, SendNotifyMessage passes the message to the window procedure and returns immediately; it does not wait for the window procedure to finish processing the message.

This appears to work when used with a window created on the same thread, because when you call SendNotifyMessage, the function synchronously calls into the window procedure associated with the target window before returning.

If the call crosses threads, on the other hand, you'd have to run a message loop for the - now queued - message to get picked up and passed to the window procedure1). Your application doesn't run a message loop, and it exits before the message ever reaches the target window.

To fix this you'd have to run a message loop. This may or may not be the right approach to your problem. Since we don't know, what problem you are trying to solve, we cannot suggest potentially superior approaches and solutions.

1) See About Messages and Message Queues: Message Routing.

Upvotes: 1

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