xCode 4.4.1 OSX 10.8.2, parece que [operación cancelAllOperations]; no está funcionando

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    NSOperationQueue *operation = [[NSOperationQueue alloc] init];
    [operation setMaxConcurrentOperationCount: 1];
    [operation addOperationWithBlock: ^{
        for (unsigned i=0; i < 10000000; i++) {
            printf("%i\n",i);
           }
    }];
    sleep(1);
    if ([operation operationCount] > 0) {
        [operation cancelAllOperations];
    }
}

resultados 9999999