Question

I’m trying to make a CGEventTap work. I think I’ve done everything correctly but when I go to run I get this error:

Apple Mach-O Linker (id) Error

Undefined symbols for architecture x86_64:
  "_tapCallback", referenced from:
      -[TestAppDelegate applicationDidFinishLaunching:] in TestAppDelegate.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Answer 1

It sounds like you have defined your tapCallback() function, but haven’t actually implemented it. In other words, your TestAppDelegate class probably looks something like this:

#import "TestAppDelegate.h"

CGEventRef tapCallback(CGEventTapProxy proxy,
            CGEventType type,
           CGEventRef event,
              void *userInfo);


@implementation TestAppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    CFMachPortRef eventTapRef = CGEventTapCreate(kCGSessionEventTap, 
                                                 kCGTailAppendEventTap,
                                                 kCGEventTapOptionListenOnly,
                                                 kCGEventMaskForAllEvents,
                                                 tapCallback,
                                                 NULL);
    // additional code

}

@end

Somewhere along the lines, you’ll need to actually implement the tapCallback() function. If you have actually implemented it, say, in another file, make sure that file is included in your built target.

Otherwise, doing something like this should allow the program to run:

CGEventRef tapCallback(CGEventTapProxy proxy,
                             CGEventType type,
                                CGEventRef event,
                                   void *userInfo) {
    NSLog(@"tapCallback()");
    return event;
}