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    <p>If all you want is a simple global lock, look at NSLock.</p> <p>eg:</p> <pre><code>static NSLock * myGlobalLock = [[NSLock alloc] init]; if ([myGlobalLock tryLock]) { // do something [myGlobalLock unlock]; } else { // couldn't acquire lock } </code></pre> <p>However, this is going to incur a performance penalty because it requires a kernel call. If you want to serialize access to a resource, using Grand Central Dispatch and a private queue is going to perform much better--these are scheduled without requiring a kernel interrupt.</p> <p>eg:</p> <pre><code>// alloc a dispatch queue for controlling access to your shared resource static dispatch_queue_t mySharedResourceQueue = nil; static dispatch_once_t onceToken; dispatch_once(&amp;onceToken, ^{ mySharedResourceQueue = dispatch_queue_create("com.myidentifier.MySharedResourceQueue", DISPATCH_QUEUE_SERIAL); // pick a unique identifier }); // don't forget to call dispatch_release() to clean up your queue when done with it! // to serialize access to your shared resource and block the current thread... dispatch_sync(mySharedResourceQueue, ^{ // access my shared resource }); // or to access it asynchronously, w/o blocking the current thread... dispatch_async(mySharedResourceQueue, ^{ // access my shared resource }); </code></pre> <p>Dispatch queues are highly awesome things and if you're getting in to iOS development you should learn how to use them in order to make apps with great performance.</p> <p>There are different types of locks besides NSLock too. Read up on synchronization in the Threaded Programming reference for more info...</p> <p>Threaded Programming reference: <a href="https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/Multithreading/Introduction/Introduction.html#//apple_ref/doc/uid/10000057i" rel="nofollow">https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/Multithreading/Introduction/Introduction.html#//apple_ref/doc/uid/10000057i</a></p> <p>NSLock reference: <a href="https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSLock_Class/Reference/Reference.html" rel="nofollow">https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSLock_Class/Reference/Reference.html</a></p> <p>Grand Central Dispatch reference: <a href="https://developer.apple.com/library/ios/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html" rel="nofollow">https://developer.apple.com/library/ios/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html</a></p>
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    1. COThat queue should be created with the explicit DISPATCH_.._SERIAL option.
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    2. COFrom Apple's docs: "By default, queues created with dispatch_queue_create() wait for the previously dequeued block to complete before dequeuing the next block. This FIFO completion behavior is sometimes simply described as a "serial queue.""
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    3. COSure -- but that doesn't mean you shouldn't be explicit in your code. The more explicit concurrency code is, the easier it is for someone new (including yourself six months from now ;) to maintain it in the future. Obviously, that is a default behavior that can't be changed, but why make future maintainers look up that behavior?
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