iOS 4 app crashes at startup on iOS 3.1.3: Symbol not found: __NSConcreteStackBlock

Question

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 build an app for both iOS 4 and 3.

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):

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

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:

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  <d5f0ff6f233b4b034c222c16438c88d9> /var/mobile/Applications/192B30ED-16AC-431E-B0E9-67C1F41FD5DA/MyApp.app/MyApp
0x2fe00000 - 0x2fe26fff  dyld armv6  <544395a4b5546114b878d5131a84fd7f> /usr/lib/dyld
0x30410000 - 0x30536fff  libSystem.B.dylib armv6  <0373fd64e915a17160732b29d343f95f> /usr/lib/libSystem.B.dylib

Thanks for any advice!

Answer 1

Ben Gottlieb pointed out yesterday that if you use blocks anywhere in your application, you'll see a crash similar to this on a pre-4.0 OS while building with the LLVM compiler. To work around this, you can specify the linker flag -weak-lSystem in your Xcode build settings.