I'm using Quartz CGEventTap in an attempt to globally intercept capslock presses and block them (to have them do something useful instead). I succesfully detect capslock presses but have so far been unable to block them. My code (originating from this stackoverflow answer) is something like this:

eventTap = CGEventTapCreate(kCGHIDEventTap,
                            kCGTailAppendEventTap, 
                            kCGEventTapOptionDefault, 
                            eventMask,
                            myCGEventCallback,
                            &oldFlags);

runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);

CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);
CGEventTapEnable(eventTap, true);

CGEventRef myCGEventCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef theEvent, void *refcon)
{
    CGEventFlags *oldFlags = (CGEventFlags *)refcon; 

    switch (type)
    {
        case kCGEventFlagsChanged:
        {
            CGEventFlags newFlags = CGEventGetFlags(theEvent);
            CGEventFlags changedFlags = *oldFlags ^ newFlags; 
            *oldFlags = newFlags;

            if (changedFlags == 65536)
            {
                NSLog(@"Capslock pressed. Let's not return the event");
                return NULL;
            }
            break;
        }
        default:
            break;
    }

    NSLog(@"Different modifier than capslock. Returning the event");
    return theEvent;
}

If I understand correctly returning NULL should effectively block the keypress from propagating. Indeed it also does for "normal" keyup and -down events. However capslock toggles regardless. Any ideas why that is? Am I making incorrect assumptions? And/or how can I do things differently to achieve my goal?

Thanks,

Thor