title: Why stack unwind stops at CFRunLoopRunSpecific
body: <p>for example:below call stack</p>
  
  <p>call stack unwind stop at CFRunLoopRunSpecific, why it does not unwind until main, even empty?</p>
  
  <pre><code>Thread 0 name:  Dispatch queue: com.apple.main-thread
  Thread 0 Crashed:
  0   libsystem_kernel.dylib          0x36398848 __kill + 8
  1   FrogFinder                      0x000b5034 0x69000 + 311348
  2   CoreFoundation                  0x3447e980 __handleUncaughtException + 68
  3   libobjc.A.dylib                 0x346ce2ca _objc_terminate + 122
  4   libc++abi.dylib                 0x338a33be _ZL19safe_handler_callerPFvvE + 70
  5   libc++abi.dylib                 0x338a344a std::terminate() + 14
  6   libc++abi.dylib                 0x338a481e __cxa_rethrow + 82
  7   libobjc.A.dylib                 0x346ce22e objc_exception_rethrow + 6
  8   CoreFoundation                  0x343d453e CFRunLoopRunSpecific + 398
  9   CoreFoundation                  0x343d439e CFRunLoopRunInMode + 98
  10  GraphicsServices                0x30c58fc6 GSEventRunModal + 150
  11  UIKit                           0x3785573c UIApplicationMain + 1084
  12  FrogFinder                      0x0006a7e6 0x69000 + 6118
  13  FrogFinder                      0x0006a7a4 0x69000 + 6052
  </code></pre>