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POCommon queue for accessing shared resources in CGD
 

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  1. POCommon queue for accessing shared resources in CGD
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    <p>I have an array, <code>sharedArray</code>, of objects that are slow to create. I first create them and insert them in the array concurrently. After that, some readers concurrently access this <code>sharedArray</code>, get the objects in it and process it in their own way. I am experiencing crashes that I suppose come from not accessing the shared resource correctly. Am I doing it wrong?</p> <p>I am using ARC, with iOS target deployment version 5.1. I get the error <code>EXC_BAD_ACCESS</code> when setting the resources (marked in the code).</p> <p>Thank you very much! </p> <pre><code>//Creation of the array dispatch_queue_t constructionQueue = dispatch_queue_create("constructionQueue", DISPATCH_QUEUE_CONCURRENT); dispatch_apply(numberOfSlowObjects, constructionQueue, ^(size_t i) { __block SlowObject *slowOb = [[SlowObject alloc] init]; dispatch_sync(dispatch_get_main_queue(), ^{ [sharedArray setObject:slowOb atIndexedSubscript:i]; //HERE I GET THE ERROR }); }); dispatch_release(constructionQueue); //Accessing the array from the different readers dispatch_queue_t readersQueue = dispatch_queue_create("readersQueue", DISPATCH_QUEUE_CONCURRENT); dispatch_apply(readers.count, readersQueue, ^(size_t i) { Reader *reader = [readers objectAtIndex:i]; dispatch_queue_t processQueue = dispatch_queue_create("processQueue", DISPATCH_QUEUE_CONCURRENT); dispatch_apply(numberOfSlowObjects, processQueue, ^(size_t j) { __block SlowObject *slowOb; dispatch_sync(dispatch_get_main_queue(), ^{ slowOb = [sharedArray objectAtIndex:j]; }); [reader process:slowOb]; }); dispatch_release(processQueue); }); dispatch_release(readersQueue); </code></pre> <p>And the code of <code>sharedArray</code> (the shared resource) lazy initialization:</p> <pre><code>- (NSMutableArray *) sharedArray { if(!_sharedArray){ _sharedArray = [[NSMutableArray alloc] initWithCapacity:numberOfSlowObjects]; for(int i=0;i&lt;numberOfSlowObjects;i++) [_sharedArray addObject:[NSNumber numberWithInt:0]]; //null initialization; } return _sharedArray; } </code></pre>
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    1. COWhat message do you get when this crashes?
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      1. POCommon queue for accessing shared resources in CGD
      1. USSven
    2. COYou get the above error on this line slowOb = [sharedArray objectAtIndex:j];? Please mark it in the code
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      1. POCommon queue for accessing shared resources in CGD
      1. USBen Coffman
    3. COSince you are running everything concurrently you need to put some NSLogs to ensure everything is behaving. For instance, a NSLog for numberOfSlowObjects to make sure it's assigned a value and for J. Then you need to see if there is anything in sharedArray by the time it is being accessed.
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      1. POCommon queue for accessing shared resources in CGD
      1. USBen Coffman
 

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