Toggling the Num Lock, Caps Lock, and Scroll Lock keys

Armen Hakobyan

4.84/5 (23 votes)

May 25, 2002

CPOL

3 min read

255458

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How to toggle the Num Lock, Caps Lock, and Scroll Lock keys programmatically

Introduction

I have a status bar that toggles the Num Lock, Caps Lock and Scroll Lock keys in one application, but some time ago, I discovered that the API function SendInput I used for it requires Windows 98 or later and Windows NT 4.0 with Service Pack 3.0 or later. I have browsed MSDN and found that there are two more functions that can toggle keyboard state but each of them has some specific limitations like SendInput does. To make it easier for others, I have posted all possible variants with their descriptions and limitations and a simple solution for toggling these keys avoiding compatibility problems.

Variant A - using keybd_event

Description

The keybd_event function synthesizes a keystroke. The system can use such a synthesized keystroke to generate a WM_KEYUP or WM_KEYDOWN message. The keyboard driver's interrupt handler calls the keybd_event function.

Version Computability

Windows NT/2000/XP: Requires Windows NT 3.1 or later
Windows 95/98/ME: Requires Windows 95 or later
Windows CE: Requires version 1.0 or later

Important Notes

An application can simulate a press of the PRINTSCRN key in order to obtain a screen snapshot and save it to the clipboard. To do this, call keybd_event with the first parameter set to VK_SNAPSHOT.
Windows 95/98/Me: The keybd_event function can toggle only the CAPS LOCK and SCROLL LOCK keys. It cannot toggle the NUM LOCK key. We can do this with SendInput but it requires Windows NT 4.0 SP3 and later or Windows 98 and later.
Windows NT/2000/XP: The keybd_event function can toggle the NUM LOCK, CAPS LOCK, and SCROLL LOCK keys.

Sample Usage

When setting key state, be sure to generate both key press and key release (see below). Notes: Windows NT/2000: This function has been superseded. Use SendInput instead.

//
    // Toggle Caps Lock key:
    ::keybd_event( VK_CAPITAL, 0x45, KEYEVENTF_EXTENDEDKEY, 0 );
    ::keybd_event( VK_CAPITAL, 0x45, KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP, 0 );
//

Variant B - using SendInput

Description

The SendInput function synthesizes keystrokes, mouse motions, and button clicks.

Version Compatibility

Windows NT/2000/XP: Requires Windows NT 4.0 SP3 or later
Windows 95/98/Me: Requires Windows 98
Windows CE: Requires version 2.0 or later

Sample Usage

When setting key state, be sure to generate both key press and key release (see below). And do not forget to include WinAble.h, not WinUser.h as MSDN says.

//
    #include <WinAble.h> // Required for the ::SendInput function
//...
    // Toggle Caps Lock key:
    INPUT input[2];
    ::ZeroMemory(input, sizeof(input));        
    input[0].type = input[1].type = INPUT_KEYBOARD;
    input[0].ki.wVk  = input[1].ki.wVk = VK_CAPITAL;        
    input[1].ki.dwFlags = KEYEVENTF_KEYUP;  // THIS IS IMPORTANT
    ::SendInput(2, input, sizeof(INPUT));
//

Variant C - using SetKeyboardState

Description

The SetKeyboardState function copies a 256-byte array of keyboard key states into the calling thread's keyboard-input state table.

Version Compatibility

Windows NT/2000/XP: Requires Windows NT 3.1 or later
Windows 95/98/Me: Requires Windows 95 or later
Windows CE: Unsupported

Important Notes

Because the SetKeyboardState function alters the input state of the calling thread and not the global input state of the system, an application cannot use SetKeyboardState to set the NUM LOCK, CAPS LOCK, or SCROLL LOCK indicator lights on the keyboard.
SetKeyboardState function sets keyboard state for calling thread only, not the global input state of the system.

Sample Usage

//
    // Toggle Caps Lock key:
    BYTE byKeybState[256];
    ::GetKeyboardState(byKeybState);         
    byKeybState[nVirtKey] = !(BOOL)::GetKeyState(VK_CAPITAL);
    ::SetKeyboardState(byKeybState);
//

Support for Setting Num Lock, Caps Lock and Scroll Lock Keys State

Function\OS	Win CE 1.0	Win CE 2.0	Win 95	Win 98/Me	Win NT	Win 2000/XP
`keybd_event`	Yes	Yes	All but Num Lock	All but Num Lock	Yes	Yes
`SendInput`		Yes		Yes	SP3*	Yes
`SetKeyboardState`			Yes**	Yes**	Yes**	Yes**

* - Requires Service Pack 3 or later.
** - Does not set indicator lights on the keyboard and toggles input state of the calling thread only.

Simple Solution

Sample Image

I have provided a simple StatusBar class that toggles the keys when the appropriate pane is double-clicked. See below:

//
#include <WinAble.h> // Required for the ::SendInput function
//...

void CStatusBarEx::OnNMDblclk(NMHDR* pNMHDR, LRESULT *pResult)
{
    // Get the double-clicked pane index:
    
    NMMOUSE* pNMMOUSE = (NMMOUSE*)pNMHDR;
    INT nInd = GetItemID((int)pNMMOUSE->dwItemSpec);

    if((nInd >= ID_INDICATOR_CAPS) && 
       (nInd <= ID_INDICATOR_OVR))
    {        
        UINT nVirtKey = 0;
        switch(nInd)
        {
        case ID_INDICATOR_CAPS: nVirtKey = VK_CAPITAL; break;
        case ID_INDICATOR_NUM:  nVirtKey = VK_NUMLOCK; break;
        case ID_INDICATOR_SCRL: nVirtKey = VK_SCROLL;  break;
        case ID_INDICATOR_OVR:  nVirtKey = VK_INSERT;  break;
        }
        
        ASSERT(nVirtKey >= 1 && nVirtKey<= 254);
    
        DWORD dwVersion = ::GetVersion();

        // Windows NT/2000/XP: The keybd_event function can toggle the 
        // NUM LOCK, CAPS LOCK, and SCROLL LOCK keys although
        // this function has been superseded by ::SendInput.

        if( dwVersion < 0x80000000) // Win NT
        {            
            ::keybd_event( nVirtKey, 0x45, KEYEVENTF_EXTENDEDKEY, 0 );
            ::keybd_event( nVirtKey, 0x45, KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP, 0 );
        }
        else
        {
            // Windows 95/98/Me: The keybd_event function can toggle only the 
            // CAPS LOCK and SCROLL LOCK keys. It cannot toggle the NUM LOCK key.
            // We can do this with SendInput but it requires 
            // Windows NT 4.0 SP3 and later or Windows 98 and later.

            DWORD dwMin = (DWORD)(HIBYTE(LOWORD(dwVersion)));
            if(dwMin >= 10) // Windows 98 and later
            {
                INPUT input[2];
                ::ZeroMemory(input, sizeof(input));        
                input[0].type = input[1].type = INPUT_KEYBOARD;
                input[0].ki.wVk  = input[1].ki.wVk = nVirtKey;        
                input[1].ki.dwFlags = KEYEVENTF_KEYUP;  // THIS IS IMPORTANT
                ::SendInput(2, input, sizeof(INPUT));
            }
            else // Windows  95
            {
                // Because the SetKeyboardState function alters the input state 
                // of the calling thread and not the global input state of the system, 
                // an application cannot use SetKeyboardState to set the 
                // NUM LOCK, CAPS LOCK, or SCROLL LOCK (or the Japanese KANA) indicator
                // lights on the keyboard.

                BYTE byKeybState[256];
                ::GetKeyboardState(byKeybState);         
                byKeybState[nVirtKey] = !(BOOL)::GetKeyState(nVirtKey);
                ::SetKeyboardState(byKeybState);
            }
        }
        ::Beep(500, 100);
    }
    else
    {
        CWnd* pWnd = GetParent();
        if(pWnd != NULL)
        {             
            //TODO: SendNotifyMessage; 
        }
    }

    *pResult = 0;
}
//

Please send me all bugs, fixes or better solutions.

History

25^th May, 2002: Initial version