I am executing a block using dispatch_sync and the block is executed correctly. But this block is executed on the main thread. As per the Apple Doc:

Serial queues (also known as private dispatch queues) execute one task at a time in the order in which they are added to the queue. The currently executing task runs on a distinct thread (which can vary from task to task) that is managed by the dispatch queue.

which means (or what I understood) that current process that is being executed will run on a separate thread.

Below is the code that I am using to judge what's going on. It is being called inside NSURLConnection's didReceiveData: delegate method (I know I should not do that inside the didReceiveData: delegate method - but this is just a sample to focus on dispatch_sync). Following are the different ways that I can assume as a proof of my conclusion:

  1. Using dispatch_sync on a Global Concurrent Queue

       dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        if ([NSThread isMainThread]) {
            NSLog(@"Main Thread");
        }
        else
            NSLog(@"Not on Main Thread");

        //Some Process
    });

Output - 

         Main Thread
         Main Thread 
         Main Thread
         // Main Thread printed till didReceiveData: gets called

  1. Using dispatch_sync on a self created queue using dispatch_queue_create

    // Create queue somewhere else like this
dispatch_queue_t newQueue = dispatch_queue_create("WriteQueue", DISPATCH_QUEUE_SERIAL);


   dispatch_sync(newQueue, ^{

        if ([NSThread isMainThread]) {
            NSLog(@"Main Thread");
        }
        else
            NSLog(@"Not on Main Thread");

        //Some Process
    });
    2. ios objective-c cocoa-touch cocoa grand-central-dispatch
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