Raj Raj - 1 year ago 186
Objective-C Question

NSMutableDictionary thread safety

I have a question on thread safety while using


The main thread is reading data from

  • key is

  • value is

An asynchronous thread is writing data to above dictionary (using

How do I make the above dictionary thread safe?

Should I make the
? Or do I need to make any additional changes?

@property(retain) NSMutableDictionary *dicNamesWithPhotos;

Answer Source

NSMutableDictionary isn't designed to be thread-safe data structure, and simply marking the property as atomic, doesn't ensure that the underlying data operations are actually performed atomically (in a safe manner).

To ensure that each operation is done in a safe manner, you would need to guard each operation on the dictionary with a lock:

// 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];

You should consider rolling your own NSDictionary that simply delegates calls to NSMutableDictionary while holding a lock:

@interface SafeMutableDictionary : NSMutableDictionary
    NSLock *lock;
    NSMutableDictionary *underlyingDictionary;


@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];


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.

If you want to use a proper threaded library, you can use TransactionKit library as they have TKMutableDictionary 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.