gistfile1.m

static void *volatile _dlManagerInstance = nil;                                                

+ (DownloadManager *) sharedInstance {                                                                    
    while (!_dlManagerInstance) {                                                                          
        DownloadManager *temp = [[self alloc] init];                                                                 
        if(!OSAtomicCompareAndSwapPtrBarrier(0x0, temp, &_dlManagerInstance)) {
            [temp release];                                                                                   
        }                                                                                                    
    }
    
    return _dlManagerInstance;
}