title: iOS 4 app crashes at startup on iOS 3.1.3: Symbol not found: __NSConcreteStackBlock
body: <p>I'm running Xcode 3.2.3 with the iOS 4.0 SDK. I built my app with Base SDK = iphoneos4.0, Active SDK = iphoneos4.0, Deployment Target = 3.1.3, and Architecture = standard (arm6 arm7). Compiler = GCC 4.2. As I understand it, this is the correct way to <em>build</em> an app for both iOS 4 and 3.</p>
  
  <p>The app runs fine on devices running iOS 4. But it crashes on startup when you try to run it on a device with iOS 3.1.3 (an iPod Touch 1G):</p>
  
  <pre><code>dyld: Symbol not found: __NSConcreteStackBlock
    Referenced from: /var/mobile/Applications/192B30ED-16AC-431E-B0E9-67C1F41FD5DA/MyApp.app/MyApp
    Expected in: /usr/lib/libSystem.B.dylib
  </code></pre>
  
  <p>It appears to be an issue with a fairly "low level" dynamically-linked library, BEFORE my main() function even gets called. I have even tried re-starting the device, etc., with no luck. Here's part of the the crash log:</p>
  
  <pre><code>Process:         MyApp [60]
  Path:            /var/mobile/Applications/192B30ED-16AC-431E-B0E9-67C1F41FD5DA/MyApp.app/MyApp
  Identifier:      MyApp
  Version:         ??? (???)
  Code Type:       ARM (Native)
  Parent Process:  launchd [1]
  
  Date/Time:       2010-07-22 17:16:17.942 -0400
  OS Version:      iPhone OS 3.1.3 (7E18)
  Report Version:  104
  
  Exception Type:  EXC_BREAKPOINT (SIGTRAP)
  Exception Codes: 0x00000001, 0xe7ffdefe
  Crashed Thread:  0
  
  Dyld Error Message:
    Symbol not found: __NSConcreteStackBlock
    Referenced from: /var/mobile/Applications/192B30ED-16AC-431E-B0E9-67C1F41FD5DA/MyApp.app/MyApp
    Expected in: /usr/lib/libSystem.B.dylib
    Dyld Version: 149
  
  Binary Images:
      0x1000 -    0x80fff +MyApp armv6  &lt;d5f0ff6f233b4b034c222c16438c88d9&gt; /var/mobile/Applications/192B30ED-16AC-431E-B0E9-67C1F41FD5DA/MyApp.app/MyApp
  0x2fe00000 - 0x2fe26fff  dyld armv6  &lt;544395a4b5546114b878d5131a84fd7f&gt; /usr/lib/dyld
  0x30410000 - 0x30536fff  libSystem.B.dylib armv6  &lt;0373fd64e915a17160732b29d343f95f&gt; /usr/lib/libSystem.B.dylib
  </code></pre>
  
  <p>Thanks for any advice!</p>