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    <p>Comparing the current queue against the main queue is not a valid way to check whether you are running on the main thread.</p> <p>Use <code>[NSThread isMainThread]</code> or <code>pthread_main_np()</code> to explicitly check whether you are on the main thread if that is what you want to know.</p> <p>You can be on the main thread without the current queue being the main queue, and you can be on the main queue without the current thread being the main thread (the latter only if <code>dispatch_main()</code> has been called, but still).</p> <p>In recent releases this is documented explicitly in the CAVEATS section of the dispatch_get_main_queue(3) manpage:</p> <blockquote> <p>The result of dispatch_get_main_queue() may or may not equal the result of dispatch_get_current_queue() when called on the main thread. Comparing the two is not a valid way to test whether code is executing on the main thread. Foundation/AppKit programs should use [NSThread isMainThread]. POSIX programs may use pthread_main_np(3).</p> </blockquote> <p>In general you should avoid using queue pointer comparison to influence program logic. Dispatch queues exist in a dependency tree (the target queue hierarchy) and comparing individual leaves in that tree without taking their interdependency into account does not provide sufficient information to make safe decisions.</p> <p>If you really need program logic based on queue interdependency, use the <code>dispatch_get_specific(3)</code>/<code>dispatch_queue_set_specific(3)</code> APIs which are target-queue aware and much more explicit.</p>
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    1. POIs this the right way to compare two GCD Queues?
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    1. POIs this the right way to compare two GCD Queues?
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    1. COThank you for your detailed answer, one point to clarify... I am making this check in order to ensure UI code is run on the correct queue. Withstanding the flaws in my method of checking, am I correct in understanding UI code needs to be run on the main queue rather than the main thread? I always assumed the two were the same, clearly not always.
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    2. COthe other way around, unless otherwise documented any code interacting with current AppKit/UIKit objects needs to run on the main thread.
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    3. COOkay, that's what I originally thought... So how come every tutorial in the world (including Apples documentation about dispatch queues) says to call `dispatch_sync(dispatch_get_main_queue(), ^{...});` in order to run UI code from within a GCD queue, if the main queue won't always be the main thread?
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