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copied!<p>According to the <a href="http://developer.apple.com/library/mac/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_once" rel="nofollow">apple documentation</a> <code>dispatch_once</code> should not be used in that case!</p> <blockquote> <p>The predicate must point to a variable stored in global or static scope. <strong>The result of using a predicate with automatic or dynamic storage is undefined.</strong></p> </blockquote> <p>An alternative approach would be to use <a href="https://developer.apple.com/library/mac/#documentation/System/Reference/OSAtomic_header_reference/Reference/reference.html" rel="nofollow">atomic operations</a>. That way it can also be ensured that code is only executed once during the lifetime of an object.</p> <pre><code>@implementation MyClass { // int32_t initialized; BOOL initialized; } - (void)myMethod { dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // if (OSAtomicTestAndSetBarrier(1, &initialized) == NO) { if (!initialized) { // do stuff once initialized = YES; } }); }; @end </code></pre> <p>EDIT: <code>initialized</code> has been of type BOOL, which has been corrected thanks to the comments of Chris Devereux and Stephen Canon.</p> <p>EDIT: As Catfish_Man pointed out using atomic operations likely leads to problems. Because of that you would have to wrap the whole stuff with a dispatch_sync or something like that, which then makes using atomic operations pointless.</p>

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