Could not send backspace key using ::SendInput() to wordpad application

Question

I have used sendinput() function and windows keyboard hooks to develop a custom keyboard for indian languages. Project is in google code here: http://code.google.com/p/ekalappai

The keyboad hook and sendinput functions are placed in a win32 dll. And they are called from a Qt exe. Our application works fine for most keys and applications. I find the following issue:

I could not send Backspace key to few applications like Wordpad/Openoffice/MsOffice. I find same issue with Arrowkeys and delete keys.

Here is my code:

extern "C" __declspec(dllexport) void GenerateKey(int vk , bool bExtended)
{
    //update previous characters
    previous_2_character = previous_1_character;
    previous_1_character = vk;

    KEYBDINPUT kb={0};
    INPUT Input={0};

    //keydown
    kb.wVk    =  0;
    kb.wScan = vk;/*enter unicode here*/;
    kb.dwFlags = KEYEVENTF_UNICODE; // KEYEVENTF_UNICODE=4
    Input.type = INPUT_KEYBOARD;
    Input.ki = kb;

    ::SendInput(1,&Input,sizeof(Input));

    //keyup
    kb.wVk    =  0;
    kb.wScan = vk;/*enter unicode here*/;
    kb.dwFlags = KEYEVENTF_UNICODE|KEYEVENTF_KEYUP; //KEYEVENTF_UNICODE=4
    Input.type = INPUT_KEYBOARD;
    Input.ki = kb;

    ::SendInput(1,&Input,sizeof(Input));
}

Full dll code is here: http://code.google.com/p/ekalappai/source/browse/trunk/ekhook/ekhook/dllmain.cpp

Calling code:

generatekey = (GenerateKey) myLib->resolve( "GenerateKey" );

generatekey(44,FALSE); //comma - THis works in wordpad/MsOffice/Openoffice
generatekey(2949,FALSE); //tamil character "a" works in Wordpad/Msoffice/Openoffice

generatekey(8,FALSE); //backspace - This is NOT working in Wordpad/Msoffice/Openoffice

Full calling code from Qt Exe is here: http://code.google.com/p/ekalappai/source/browse/trunk/ekalappai/window.cpp

I tried searching in google but could not fine a solution yet. If anyone has clue on resolving this pls help. Thanks.

Answer 1

You are mixing up the virtual key and the scan code. The wVk member is the important one, the scan code will only be used it the virtual key is ambiguous. Fix:

kb.wVk   = vk;
kb.wScan = 0;   // TODO: look at VkKeyScanEx()

Answer 2

The documentation says:

If KEYEVENTF_UNICODE is specified, SendInput sends a WM_KEYDOWN or WM_KEYUP message to the foreground thread's message queue with wParam equal to VK_PACKET. Once GetMessage or PeekMessage obtains this message, passing the message to TranslateMessage posts a WM_CHAR message with the Unicode character originally specified by wScan.

Just a guess, but the word processing programs might be triggering the backspace behavior off of WM_KEYDOWN/WM_KEYUP messages and not WM_CHAR. Thus they may be expecting VK_BACKSPACE (not VK_PACKET) as the wParam of those messages. It might even be done with accelerators based on VKEYs not characters...heck, you're on Windows, so pretty much anything is possible. :)

Have you tried not using KEYEVENTF_UNICODE, and doing kb.wVk = VK_BACKSPACE ?

(Also, you might use Spy++ to get a better clue of what key messages are sent to the target application and how it differs from when you hit a literal backspace.)