Question

I want to do like, when the app start, app check for iPhone GPS ON or OFF? if GPS off then app quite.

Answer 1

What do you want exactly to know, location service or does user granted access to the location service?
If first, than depending on iOS version you can use:

on iOS pre 4.x:

CLLocationManager *testLocationManager = [[CLLocationManager alloc] init];
BOOL servicesEnabled = [testLocationManager locationServicesEnabled];
[testLocationManager release];

on iOS 4.x+:

BOOL servicesEnabled = [CLLocationManager locationServicesEnabled];

If second, than:

on iOS pre 4.x: there is no quick snippet, you have to wait if didFailWithError or didUpdateToLocation will be called and if second, than user granted access

on iOS 4.x+:

BOOL accessGranted = [CLLocationManager authorizationStatus] == kCLAuthorizationStatusAuthorized;

And than easily use exit(0) if you option not reached. And yes, try notify user about what you is about to quit the app.