Is SetWindowsHookEx reentrant (with MessageBox????) ??? or is it a

=?Utf-8?B?Y294?= <>
Mon, 7 Jul 2008 07:04:05 -0700
I am using Windows Hook (type = WH_GETMESSAGE) and it seems that it is
reentrant. I'd like to check, because I don't see this kind of information at
the documentation.

I make a test application that just call SetWindowsHookEx(..);

On my WindowProc(..) I call 3 times the PostMessage(..) function with wParam
= 1, 2 and 3.

For my surprise, If I call MessageBox(..) inside my HookProc function, its
works like a reentrant function. All the calls stops at MessageBox
isntruction pointer and it processos first the last PostMessage sent
(wparam=3), after that wparam=2 and the last one wParam=1. Is it really a
message queue or a message stack??

My codes segments:

installing the hook:

    g_hHook = SetWindowsHookEx( WH_GETMESSAGE, MyGetProc, hInstance,
GetCurrentThreadId( ) );

the window proc:

    case WM_COMMAND:
        wmId = LOWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
            case IDM_MYTEST:
                PostMessage( hWnd, WM_MYTEST, 1, 0 );
                PostMessage( hWnd, WM_MYTEST, 2, 0 );
                PostMessage( hWnd, WM_MYTEST, 3, 0 );

the HookProc:

LRESULT CALLBACK MyGetProc( int code, WPARAM wParam, LPARAM lParam )
    MSG *msg = (MSG *)lParam;
    TCHAR szLog[512];
    if( code == HC_ACTION )
        if( msg->message == WM_MYTEST )
            _sntprintf_s( szLog, sizeof(szLog)/sizeof(TCHAR), _TRUNCATE, _T(
"WM_MYTEST: %d" ), msg->wParam );
            MessageBox( msg->hwnd, szLog, _T("test"), MB_OK );
            MessageBeep( MB_ICONEXCLAMATION );
    return CallNextHookEx( g_hHook, code, wParam, lParam );

The output:
When the Menu is selected and the 3 PostMessages called, I receive 3
messageboxes with ids 3, 2, 1. I expected receive the 3 MessageBoxes, but ids
1, 2, 3

When I execute the code in debug mode, the code is breaked at MessageBox
call and the instruction pointer goes to the begin of MyGetProc(..). On the
last MessageBox(..) it continues and goes on, after that it go on with wParam
=2, and the last one...

Anybody here can explain me why this kind of behaviour?? If I do not call
MessageBox(..) the HookProc works as expected. It will call the HookProc in
the correct order and waits the end of one call to start anothers.

If I change the code to this:
LRESULT CALLBACK MyGetProc( int code, WPARAM wParam, LPARAM lParam )
    MSG *msg = (MSG *)lParam;
    TCHAR szLog[512];
    if( code == HC_ACTION )
        if( msg->message == WM_MYTEST )
            MessageBeep( MB_ICONEXCLAMATION );
    return CallNextHookEx( g_hHook, code, wParam, lParam );

and checked out how it work in debugger..

Any explanation will be excelent.

Sorry about my english mistakes.

Regards, Guilherme

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