// ...
thehook = SetWindowsHookEx( WH_KEYBOARD_LL, hook_proc, hwnd, 0 );
// ...

/**
 *
 *  wParam, one of the: WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, or WM_SYSKEYUP
    lParam: pointer to a KBDLLHOOKSTRUCT structure

    (*) "The hook procedure should process a message in less time than the
    data entry specified in the LowLevelHooksTimeout value in the following registry key: 
    HKEY_CURRENT_USERControl PanelDesktop 

    The value is in milliseconds. If the hook procedure does not 
    return during this interval, the system will pass the message to the next hook."

 *
 */
LRESULT CALLBACK
hook_proc( int code, WPARAM wParam, LPARAM lParam )
{
  static long ctrl_cnt = 0;
  static bool mmode = false;
  static DWORD time;

  KBDLLHOOKSTRUCT*  kbd = (KBDLLHOOKSTRUCT*)lParam;

  if (  code < 0
  ||   (kbd->flags & 0x10) // ignore injected events
     ) return CallNextHookEx( thehook, code, wParam, lParam );

  long ret = 1; // by default I swallow the keys
  if (  mmode  ) { // macro mode is ON
    if (  WM_KEYDOWN == wParam  )
      PostMessage(mainwnd, WM_MCR_ACCUM, kbd->vkCode, 0);

    if (  WM_KEYUP == wParam  )
      switch (kbd->vkCode) {
        case VK_ESCAPE:
          mmode = false;
          keys.removeall();
          PostMessage(mainwnd, WM_MCR_HIDE, 0, 0);
          break;

        case VK_RETURN:
          PostMessage(mainwnd, WM_MCR_EXEC, 0, 0);
          break;

        case VK_LCONTROL:
          mmode = false;
          PostMessage(mainwnd, WM_MCR_HIDE, 0, 0);
          PostMessage(mainwnd, WM_MCR_EXEC, 0, 0);
          break;
      }

    /* Which non printable keys allow passing? */
    switch( kbd->vkCode ) {
      case VK_LCONTROL:
      case VK_CAPITAL:
      case VK_LSHIFT:
      case VK_RSHIFT:
        ret = CallNextHookEx( thehook, code, wParam, lParam );
    }
  }
  else { // macro mode is OFF
    /* Ctrl pressed */
    if (  kbd->vkCode == VK_LCONTROL && WM_KEYDOWN == wParam  ) {
      ctrl_cnt = 1;
      time = kbd->time;
    }

    /* Prevent ctrl combinations to activate macro mode */
    if (  kbd->vkCode != VK_LCONTROL  )
      ctrl_cnt = 0;

    /* Ctrl released */
    if (  ctrl_cnt == 1 && WM_KEYUP == wParam  ) {
      if (  kbd->time - time > 40  ) {
        mmode = true;
        PostMessage(mainwnd, WM_MCR_SHOW, 0, 0);
      }
    }

    ret = CallNextHookEx( thehook, code, wParam, lParam ); // let it pass
  }

  return ret;
}