Question

Here is the link to the sample code http://developer.apple.com/library/ios/#samplecode/MVCNetworking/Introduction/Intro.html

Below is the code snippet from the file NetworkManager.m

+ (NetworkManager *)sharedManager
// See comment in header.
{
    static NetworkManager * sNetworkManager;

    // This can be called on any thread, so we synchronise.  We only do this in 
    // the sNetworkManager case because, once sNetworkManager goes non-nil, it can 
    // never go nil again.

    if (sNetworkManager == nil) {
        @synchronized (self) {
            sNetworkManager = [[NetworkManager alloc] init];
            assert(sNetworkManager != nil);
        }
    }
    return sNetworkManager;
}

Obviously there are thread safe issues here. Two NetworkManager instance may be created when there are more than one thread. So Apple made a mistake, right?

Answer 1

Yes, it’s wrong. Start with sNetworkManager as nil, and consider two threads T1 and T2.

One possible, if unlikely, scenario is:

T1: Determines (sNetworkManager == nil) is true
T2: Determines (sNetworkManager == nil) is true
T1: Takes the @synchronized lock
    Creates a NetworkManager 
    Sets sNetworkManager
    Releases the lock
T2: Takes the @synchronized lock
    Creates a NetworkManager 
    Sets sNetworkManager, LEAKING the first one
    Releases the lock

This question has some safer ways of doing it.