HOWTO: Set the CAPS LOCK, SCROLL LOCK, and NUM LOCK States (94606)

SUMMARY

The current state of the CAPS LOCK, SCROLL LOCK, and NUM LOCK keys can be queried and retrieved by the GetKeyboardState function. Although the SetKeyboardState function works on Windows 3.1x, it does not work correctly on Windows 95 or Windows NT. SetKeyboardState retrieves a byte array describing the state of every key. If the low order bit of a toggle key is set, the key is "on." By changing this bit, the toggle key state can be changed.

MORE INFORMATION

The following sample code demonstrates how to change the state of any key on all platforms. Simply pass the virtual key code (VK_CAPITAL, VK_NUMLOCK or VK_SCROLL) and a flag to set or reset the key state to the PostVirtualKeyEvent:

Sample Code

   #include <windows.h>

   #define MAKEWORD(low, high)   ((WORD)(((BYTE)(low)) | \ 
                                 (((WORD)((BYTE)(high))) << 8)))

   void FAR PASCAL Keybd_Event(void);

   void FAR PASCAL PostVirtualKey(  BYTE bVirtualKey,
                                    BOOL fKeyUp)
   {
   WORD wARegister, wBRegister;

   // Convert the virtual-key code to the scan code
   BRegister = MAKEWORD((BYTE) MapVirtualKey(bVirtualKey, 0), 0);
   ARegister = MAKEWORD(bVirtualKey, (fKeyUp ? 0x80 : 0x00));
   _asm mov bx, wBRegister;   // BH = 0 (No prefix)
   _asm mov ax, wARegister;
   Keybd_Event();
   }

   int PASCAL WinMain(  HINSTANCE hInst,
                        HINSTANCE hPrevInst,
                        LPSTR szCmdLine,
                        int nCmdShow)
   {
   // Toggle the caps lock key
   PostVirtualKey(VK_CAPITAL, FALSE);
   PostVirtualKey(VK_CAPITAL, TRUE);

   return 0;
   }