Undefined lookup, ObjC, and 64-bit

FROM : Nick Zitzmann
DATE : Mon May 12 19:39:04 2008

I asked this on the xcode-users list recently, but either nobody knew 
the answer, or someone did and didn't want to tell me, so maybe I'll 
have better luck asking on a related list:

I've got an application that loads some bundles. The bundles each 
contain an ObjC class that subclasses an abstract class that exists in 
the application. The bundles are compiled with the "-undefined 
dynamic_lookup" linker flag so that the linker will link the code 
without the presence of the superclass in the bundle.

This works fine when the app is running as 32-bit, but when it's 
running in 64-bit, the app fails to load the bundles, and generates a 
warning like this:

2008-05-10 00:45:42.856 TestApp[28170:813] Error loading /Users/
nickzman/Build/Debug/TestApp.app/Contents/Resources/MyPlugin.bundle/
Contents/MacOS/MyBundle:  dlopen(/Users/nickzman/Build/Debug/
TestApp.app/Contents/Resources/MyPlugin.bundle/Contents/MacOS/
MyBundle, 265): Symbol not found: _OBJC_CLASS_$_PluginSuperclass
  Referenced from: /Users/nickzman/Build/Debug/TestApp.app/Contents/
Resources/MyPlugin.bundle/Contents/MacOS/MyBundle
  Expected in: dynamic lookup

Yes, both the app and the plugins are being built using the same 
architecture (x86_64). And yes, PluginSuperclass does exist in the 
app. So now I need to know:

1. Why does this happen in 64-bit but not 32-bit?
2. How do I fix this without building the abstract superclass into 
each bundle (which is wasteful)?

Nick Zitzmann
<http://www.chronosnet.com/>