//in mainAppDelegate.m

 - (void)applicationDidFinishLaunching:(UIApplication *)application {

//

//init code

//

  atexit(applicationWillTerminate);

} 

void applicationWillTerminate(){

//some stuff

} 

//in mainAppDelegate.h

@interface mainAppDelegate : NSObject <UIApplicationDelegate> {

//.....

}

void applicationWillTerminate();