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    <p><code>NSMutableDictionary</code> isn't designed to be thread-safe data structure, and simply marking the property as <code>atomic</code>, doesn't ensure that the underlying data operations are actually performed atomically (in a safe manner).</p> <p>To ensure that each operation is done in a safe manner, you would need to guard each operation on the dictionary with a lock:</p> <pre><code>// in initialization self.dictionary = [[NSMutableDictionary alloc] init]; // create a lock object for the dictionary self.dictionary_lock = [[NSLock alloc] init]; // at every access or modification: [object.dictionary_lock lock]; [object.dictionary setObject:image forKey:name]; [object.dictionary_lock unlock]; </code></pre> <p>You should consider rolling your own <code>NSDictionary</code> that simply delegates calls to NSMutableDictionary while holding a lock:</p> <pre><code>@interface SafeMutableDictionary : NSMutableDictionary { NSLock *lock; NSMutableDictionary *underlyingDictionary; } @end @implementation SafeMutableDictionary - (id)init { if (self = [super init]) { lock = [[NSLock alloc] init]; underlyingDictionary = [[NSMutableDictionary alloc] init]; } return self; } - (void) dealloc { [lock_ release]; [underlyingDictionary release]; [super dealloc]; } // forward all the calls with the lock held - (retval_t) forward: (SEL) sel : (arglist_t) args { [lock lock]; @try { return [underlyingDictionary performv:sel : args]; } @finally { [lock unlock]; } } @end </code></pre> <p>Please note that because each operation requires waiting for the lock and holding it, it's not quite scalable, but it might be good enough in your case.</p> <p>If you want to use a proper threaded library, you can use <a href="http://transactionkit.sourceforge.net/" rel="noreferrer">TransactionKit library</a> as they have <code>TKMutableDictionary</code> which is a multi-threaded safe library. I personally haven't used it, and it seems that it's a work in progress library, but you might want to give it a try.</p>
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    1. COThis looks like a great method but I can't get it to compile. I'm getting `"expected ')' before 'retval_t'"` on the line `- (retval_t) forward: (SEL) sel : (arglist_t) args` Any ideas?
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    2. COFabulous answer. Now obsolete. Use a queue instead. I have a dead simple serialized dictionary somewhere. I should post it. Message forwarding is slow and fragile.
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    3. COFollow-up to bbum's comment. There's a useful write-up on using GCD concurrent queues with NSMutableDictionary on Mike Ash's blog here: http://mikeash.com/pyblog/friday-qa-2011-10-14-whats-new-in-gcd.html
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