Question

In my hook procedure (for keyboard) when I click right alt it sends me two codes on press and two on release:

The code:

LRESULT CALLBACK Recorder::keyboardHookProcedure( int code, WPARAM wParam, LPARAM lParam ) {
    if( code < 0 )
        return CallNextHookEx( m_mouseHook, code, wParam, lParam );   

    DWORD _vkCode = reinterpret_cast<KBDLLHOOKSTRUCT*>( lParam )->vkCode;

    qDebug( "vkCode: %d, wparam: %d, lparam: %d", _vkCode, wParam, lParam );

    return CallNextHookEx( 0, code, wParam, lParam );
}

on pressing

vkCode: 162, wparam: 260, lparam: 3398032
vkCode: 165, wparam: 260, lparam: 3398032

on releasing

vkCode: 162, wparam: 257, lparam: 3398032
vkCode: 165, wparam: 257, lparam: 3398032

moreover, notice that when you press the key it sends it as WM_SYSKEYDOWN (260), but when you release it the wParam is WM_KEYUP (257). What the…? Why does it work in such way?

Answer 1

It’s because the right alt-key (ALT-GR) is a shortcut for two keys – ALT and CTRL

You get WM_SYSKEYDOWN but WM_KEYUP because that’s how the messages are defined – if you’re expecting WM_SYSKEYUP, you get that when you release a key that is pressed in conjunction with the ALT key – http://msdn.microsoft.com/en-us/library/windows/desktop/ms646287(v=vs.85).aspx