exit(X), where X is a number (according to the doc) should work.
But it is not recommended by Apple and won't be accepted by the AppStore.
Why? Because of these guidelines (one of my app got rejected):
We found that your app includes a UI control for quitting the app. This is not in compliance with the iOS Human Interface Guidelines, as required by the App Store Review Guidelines.
Please refer to the attached screenshot/s for reference.
The iOS Human Interface Guidelines specify,
"Always Be Prepared to Stop iOS applications stop when people press the Home button to open a different application or use a device feature, such as the phone. In particular, people don’t tap an application close button or select Quit from a menu. To provide a good stopping experience, an iOS application should:
Save user data as soon as possible and as often as reasonable because an exit or terminate notification can arrive at any time.
Save the current state when stopping, at the finest level of detail possible so that people don’t lose their context when they start the application again. For example, if your app displays scrolling data, save the current scroll position."
> It would be appropriate to remove any mechanisms for quitting your app.
Plus, if you try to hide that function, it would be understood by the user as a crash.