Get the UDID of the current iOS device.

getUDID.m

- (NSString*)UDID {
    NSString *uuidString = nil;
    // get os version 
    NSUInteger currentOSVersion = [[[[[UIDevice currentDevice] systemVersion] componentsSeparatedByString:@"."] objectAtIndex:0] integerValue];
    
    if(currentOSVersion <= 5) {
        if([[NSUserDefaults standardUserDefaults] valueForKey:@"udid"]) {
            uuidString = [[NSUserDefaults standardDefaults] valueForKey:@"udid"];
        } else {
            CFUUIDRef uuidRef = CFUUIDCreate(kCFAllocatorDefault);
            uuidString = (NSString *)CFBridgingRelease(CFUUIDCreateString(NULL,uuidRef));
            CFRelease(uuidRef);
            [[NSUserDefaults standardUserDefaults] setObject:uuidString ForKey:@"udid"];
            [[NSUserDefaults standardUserDefaults] synchronize];
        }
        return uuidString;
    } else {
        return [UIDevice identifierForVendor];
    }
}