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Getting the main window of a thread (VC++)

Posted on 1998-07-08
Last Modified: 2013-12-03
I've launched a process using CreateProcess (so I have the associated PROCESS_INFORMATION and STARTUPINFO stuff).  How do I get the process's main window?

Question by:SeanPS
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Accepted Solution

alexo earned 100 total points
ID: 1409653
There is no one API to do it (because a process might not have a main window or have more than one).

Instead, you need to enumerate the top level windows using EnumWindows() and check which one corresponds with you process by comparing its GetWindowThreadProcessId() with your process ID.
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Expert Comment

ID: 1409654
A minor correction.  GetWindowThreadProcessId() returns the thread ID and sets the second argument to the process ID.  The PROCESS_INFORMATION structure will contain both.

Something like (modulo bugs and missing parts):

    HWND global_hwnd = NULL;

    int WINAPI WinMain(...)
        CreateProcess(..., &pi);
        WaitForInputIdle(pi.hProcess, INFINITE);
        EnumWindows(EnumWindowsProc, (LPARAM)pi.dwProcessId);

        if (global_hwnd != NULL)
            // Do whatever...

        // ...

    BOOL CALLBACK EnumWindowsProc(HWND hwnd, LPARAM lParam)
        DWORD pid;
        GetWindowThreadProcessId(hwnd, &pid);
        if (pid == (DWORD)lParam)
            global_hwnd = hwnd; // Found the window
            return FALSE;
        else return TRUE;


Author Comment

ID: 1409655
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LVL 11

Expert Comment

ID: 1409656
Have fun!

Author Comment

ID: 1409657
based on your original ideas, it seems that EnumThreadWindows() is a slightly better alternative.  i was hoping that i could easily create a CWinThread object from the stuff i have, but that doesn't seem to be the case.

thanks for your help
LVL 11

Expert Comment

ID: 1409658
>> it seems that EnumThreadWindows() is a slightly better alternative.
It is, provided that only one thread in the process is creating windows.

>> i was hoping that i could easily create a CWinThread object from the stuff i have
Check the MFC sources.  You'll see that CWinThread is a complicated beast.

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