Can the HWND from CreateWindow/CreateDialog be GetMessage'd from another thread?

Question

Using the Win32 APIs, is it possible to create a Window or Dialog in one thread then collect events for it from another thread?

Are HWNDs tied to threads?

Trying the contrived example below I never see GetMessage() fire.

HWND g_hWnd;

DWORD WINAPI myThreadProc(LPVOID lpParam)
{
    while(GetMessage(&msg, hWnd, 0, 0) > 0)
    {
       ...
    }

}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) 
{
    hWnd = CreateDialog(hInstance, MAKEINTRESOURCE(IDD_MYDIALOG), 0, myDlgProc);
    CreateThread(NULL, 0 myThreadProc, NULL, 0, NULL);
    ...
}

But here, I do.

HWND g_hWnd;
HINSTANCE g_hInstance;

DWORD WINAPI myThreadProc(LPVOID lpParam)
{
    hWnd = CreateDialog(hInstance, MAKEINTRESOURCE(IDD_MYDIALOG), 0, myDlgProc);

    while(GetMessage(&msg, hWnd, 0, 0) > 0)
    {
       ...
    }

}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) 
{
    g_hInstance = hInstance;
    CreateThread(NULL, 0 myThreadProc, NULL, 0, NULL);
    ...
}

Can somebody explain what I'm seeing?

score 5 · Accepted Answer · answered Mar 06 '09 at 00:12

No.

GetMessage returns messages on the current thread's input queue. The HWND parameter is a filter, so that GetMessage only returns messages in the current thread's input queue intended for that window.

Windows have thread affinity - messages intended for a window get handled on the thread that created and therefore owns the window.

score 2 · Answer 2 · answered Mar 06 '09 at 00:11

2

From the MSDN:

The GetMessage function retrieves a message from the calling thread's message queue

So no, what you describe is not directly possible.

answered Mar 06 '09 at 00:11

newgre

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score 2 · Answer 3 · edited Jun 20 '20 at 09:12

2

In your first example the Dialog and GetMessage are in separate threads. And the documentation says:

The GetMessage function retrieves a message from the calling thread's message queue.

The second example works since the calling thread (for GetMessage) also owns the Dialog.

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 06 '09 at 00:12

dirkgently

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score 2 · Answer 4 · answered Mar 07 '09 at 10:20

2

Use AttachThreadInput.

answered Mar 07 '09 at 10:20

Valentin Galea

1,084
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Not possible on Windows CE/Mobile. – Johann Gerell Mar 09 '09 at 09:42
1

Recommending use of a dangerous API, without even mentioning the consequences, is grossly negligent. **-1**. – IInspectable Apr 30 '17 at 19:26
This links together the GetMessage calls for all those threads. They all have to be calling it at the same time, before a message will go to any of them. – user253751 Dec 06 '22 at 04:54

score 0 · Answer 5 · answered Mar 06 '09 at 00:19

In your example programm finish after create window.

But anyway in win32 all threads have own message queue.

And all message queues get messages for windows created in this thread.

see:

http://msdn.microsoft.com/en-us/library/ms644928(VS.85).aspx (Using Messages and Message Queues)

http://msdn.microsoft.com/en-us/library/ms644936(VS.85).aspx (GetMessage Function)

score 0 · Answer 6 · answered Mar 06 '09 at 05:09

You can of course change the window procedure that handles messages for any window. Check the SetWindowLong function - http://msdn.microsoft.com/en-us/library/ms633591(VS.85).aspx - there are some rules as to what address space the new proc is. I suggest using a dll. Another way is to sub class the window message queue.

score -3 · Answer 7 · edited Jan 07 '19 at 14:53

-3

Of course you can !

Just use remote code injection ! (very classic !)

edited Jan 07 '19 at 14:53

Samuel

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answered Mar 07 '09 at 10:12

2

HA! If you define "possible" to mean, "by doing something else entirely", then yes, everything is possible. – Shog9 Dec 26 '09 at 19:28

Can the HWND from CreateWindow/CreateDialog be GetMessage'd from another thread?

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