Trying to constrain mouse movement in OS X (doesn't work).

constrain-event.cpp

// gcc -Wall -o constrain-event constrain-event.cpp -framework ApplicationServices

#include <ApplicationServices/ApplicationServices.h>

int leftBound;
int topBound;
int rightBound;
int bottomBound;

CGEventRef myCGEventCallback(CGEventTapProxy proxy, CGEventType type,
                  CGEventRef event, void *refcon) {
    CGPoint location = CGEventGetLocation(event);
    CGFloat x = location.x, y = location.y;
    if(x < leftBound || x > rightBound || y < topBound || y > bottomBound) {
        if(x < leftBound) x = leftBound;
        if(x > rightBound) x = rightBound;
        if(y < topBound) y = topBound;
        if(y > bottomBound) y = bottomBound;
        CGEventSetLocation(event, CGPointMake(x, y));
    }
    return event;
}

int main(int argc, const char * argv[]) {
    CFMachPortRef eventTap;
    CGEventMask eventMask;
    CFRunLoopSourceRef runLoopSource;
    
    if(argc != 5) {
        printf("Usage: constrain left top right bottom\n");
        return 0;
    }
    leftBound = strtol(argv[1], NULL, 10);
    topBound = strtol(argv[2], NULL, 10);
    rightBound = strtol(argv[3], NULL, 10);
    bottomBound = strtol(argv[4], NULL, 10);
    
    eventMask = (1 << kCGEventMouseMoved);
    eventTap = CGEventTapCreate(kCGHIDEventTap, kCGHeadInsertEventTap,
                                0, eventMask, myCGEventCallback, NULL);
    if (!eventTap) {
        fprintf(stderr, "failed to create event tap\n");
        exit(1);
    }
    runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);
    CGEventTapEnable(eventTap, true);
    CFRunLoopRun();
    CFRelease(runLoopSource);
    exit(0);
}