title: GCD dispatch_async DISPATCH_QUEUE_PRIORITY_DEFAULT EXC_BAD_ACCESS crash
body: <p>I want to write log to a file in global default queue. It works well in iOS 6 but crash in iOS 5 even if the block is empty.</p>
  
  <pre><code>dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
  //fprintf(s_logfp, "[%s] %s\n", [[[NSDate date] hipuClientLogDescription] UTF8String], [log UTF8String]);
  });
  </code></pre>
  
  <p>It goes all right if my code is like this:</p>
  
  <pre><code>logQueue = dispatch_queue_create("com.hipu.clientlog", NULL);
  dispatch_async(logQueue, ^{
      fprintf(s_logfp, "[%s] %s\n", [[[NSDate date] hipuClientLogDescription] UTF8String], [log UTF8String]);
  });
  </code></pre>
  
  <p>Call stack:</p>
  
  <pre><code>#0  0x01dff0b0 in objc_msgSend ()
  #1  0x01dffd40 in objc_retain ()
  #2  0x0044a712 in HpWriteLogToDisk at /Users/tanqiyu/Documents/hipu_work/iOS/trunk/HipuClient/HpFoundation/HpFoundation/HpLog.m:60
  #3  0x003420b6 in -[HpEngine initWithRootDir:appVersion:] at /Users/tanqiyu/Documents/hipu_work/iOS/trunk/HipuClient/HpEngine/HpEngine/Engine/HpEngine.m:69
  #4  0x00341a8d in +[HpEngine initEngineWithRootDir:appVersion:] at /Users/tanqiyu/Documents/hipu_work/iOS/trunk/HipuClient/HpEngine/HpEngine/Engine/HpEngine.m:28
  #5  0x0000abcd in -[AppDelegate application:didFinishLaunchingWithOptions:] at /Users/tanqiyu/Documents/hipu_work/iOS/trunk/HipuClient/HipuClient/HipuClient/AppDelegate.m:35
  #6  0x00f5a9d6 in -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] ()
  #7  0x00f5b8a6 in -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] ()
  #8  0x00f6a743 in -[UIApplication handleEvent:withNewEvent:] ()
  #9  0x00f6b1f8 in -[UIApplication sendEvent:] ()
  #10 0x00f5eaa9 in _UIApplicationHandleEvent ()
  #11 0x02325fa9 in PurpleEventCallback ()
  #12 0x021661c5 in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ ()
  #13 0x020cb022 in __CFRunLoopDoSource1 ()
  #14 0x020c990a in __CFRunLoopRun ()
  #15 0x020c8db4 in CFRunLoopRunSpecific ()
  #16 0x020c8ccb in CFRunLoopRunInMode ()
  #17 0x00f5b2a7 in -[UIApplication _run] ()
  #18 0x00f5ca9b in UIApplicationMain ()
  #19 0x0000a97d in main at 
  </code></pre>