How to restart app if it unexpectedly shutdown

As far as I know, some sort of apps can be run in background and can be restarted in specific case. This is from Apple docs

https://developer.apple.com/library/ios/#documentation/CoreLocation/Reference/CLLocationManager_Class/CLLocationManager/CLLocationManager.html#//apple_ref/doc/uid/TP40007125

If you start this service and your application is subsequently terminated, the system automatically relaunches the application into the background if a new event arrives. In such a case, the options dictionary passed to the application:didFinishLaunchingWithOptions: method of your application delegate contains the key UIApplicationLaunchOptionsLocationKey to indicate that your application was launched because of a location event.

My opinion is: Skype is relaunched with remote notifications mechanism.

Update

Well, I think I found something. @Malek_Jundi was half-right. Your app should fail with non-zero exit code, but it should be registered as VoIP application.

Because VoIP applications need to stay running in order to receive incoming calls, the system automatically relaunches the application if it exits with a nonzero exit code. (This could happen in cases where there is memory pressure and your application is terminated as a result.) However, terminating the application also releases all of its sockets, including the one used to maintain the VoIP service connection. Therefore, when the application is launched, it always needs to create its sockets from scratch.

http://developer.apple.com/library/ios/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/AdvancedAppTricks/AdvancedAppTricks.html#//apple_ref/doc/uid/TP40007072-CH7-SW12

Skype does exactly this:

1. Registers it's TCP sockets for VoIP.
2. Logs in.
3. Upon nonzero exit code the app will be relaunched by iOS (Because VoIP applications need to stay running in order to receive incoming calls, the system automatically relaunches the application if it exits with a nonzero exit code.).
4. It then immediately starts a background task (Relaunched apps remain suspended).
5. Next it registers it's login socket for VoIP.
6. It logs back in.

as far as I know its not possible to restart your app .. you could kill your app using exit(1); but this is not allowed by Apple and will rejected. maybe you do some workaround by remove some views from the mainViewController and add it again and reset the data and content .. but making a real restart I don't think so.