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POblocks and ARC - copy or crash with release build (caused by optimization level)
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copied!<p>I'm using Xcode 4.3.3 and developing for iOS 5.0+. In development of an ARC iOS application, I've started using blocks as a callback mechanism for asynchronous operations. The app works fine in the simulator and on the device. </p> <p>Then I ran it the profiler for the first time, and it started crashing on me nearly right away - in particular, an EXC_BAD_ACCESS when trying to invoke the first callback block. </p> <p>After a little investigation, it was clear the difference in behavior was because the profiler runs in "Release mode" by default - in particular, with Optimization Level set to "Fastest, Smallest [-Os]" instead of "None [-O0]".</p> <p>For example, the following code (simplified for this question) would crash when trying to execute the callbackBlock: </p> <pre><code>- (void) setCallbackBlock:(void (^)(NSString *input))block { callbackBlock = block; } - (void) invokeCallbackWithInput:(NSString *)input { if (callbackBlock) { callbackBlock(input); } } </code></pre> <p>Debugging into it, calling setCallbackBlock with optimization level set to "None", the incoming block would be an <code>NSStackBlock</code>, and the callbackBlock would become an <code>NSMallocBlock</code>.</p> <p>However, with Optimization Level "Fastest, Smallest", it remained an <code>NSStackBlock</code>.</p> <p>Changing the setter code to use <code>[block copy]</code> fixes the crashing problem (based on <a href="https://stackoverflow.com/questions/10263110/ios-5-blocks-crash-only-with-release-build">iOS 5 blocks crash only with Release Build</a>). </p> <p>However, another related question indicates that this shouldn't be necessary with ARC - block variables are copied to the heap in ARC - <a href="https://stackoverflow.com/questions/11433943/why-does-objective-c-block-still-work-without-copying-it-to-the-heap">Why does Objective-C block still work without copying it to the heap?</a> </p> <p>So my question: What's going on here, and why? (Also, how can both of those answers be correct...?)</p> <p><strong>Edit</strong>: To clarify how callbackBlock is being declared - just above my @implementation where those methods are is this: </p> <pre><code>@interface MyClass () { void (^callbackBlock)(NSString *input); } @end </code></pre>

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