HOWTO: Toggle the NUM LOCK, CAPS LOCK, and SCROLL LOCK Keys

The information in this article applies to:

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The documentation for SetKeyboardState() correctly says that you cannot use this API to toggle the NUM LOCK, CAPS LOCK, and SCROLL LOCK keys.

You can use keybd_event() to toggle the NUM LOCK, CAPS LOCK, and SCROLL LOCK keys under Windows NT. The same technique works for toggling CAPS LOCK and SCROLL LOCK under Windows 95, but it will not work for NUM LOCK.

MORE INFORMATION

The following sample program turns the NUM LOCK light on if it is off. The SetNumLock function defined here simulates pressing the NUM LOCK key, using keybd_event() with a virtual key of VK_NUMLOCK. It takes a boolean value that indicates whether the light should be turned off (FALSE) or on (TRUE).

The same technique can be used for the CAPS LOCK key (VK_CAPITAL) and the SCROLL LOCK key (VK_SCROLL).

Sample Code

/* Compile options needed:
*/

#include <windows.h>

void SetNumLock( BOOL bState )

   BYTE keyState[256];

   GetKeyboardState((LPBYTE)&keyState);
   if( (bState && !(keyState[VK_NUMLOCK] & 1)) ||
       (!bState && (keyState[VK_NUMLOCK] & 1)) )
   {
   // Simulate a key press
      keybd_event( VK_NUMLOCK,
                   0x45,
                   KEYEVENTF_EXTENDEDKEY | 0,
                   0 );

   // Simulate a key release
      keybd_event( VK_NUMLOCK,
                   0x45,
                   KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP,
                   0);
   }
}

void main()

   SetNumLock( TRUE );