Static library with ARC support linked to non-ARC

问题:

I have a non-ARC project that uses an ARC-enabled static library. This is a supported scenario, so that everything works fine. That is, until I run the code on a 4.x device, including the Simulator. In that case the code blows up with the following linker error:

dyld: lazy symbol binding failed: Symbol not found: _objc_storeStrong
  Referenced from: /Users/zoul/Library/Application Support/iPhone Simulator/4.3.2/Applications/…/Demo.app/Demo
  Expected in: /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk/System/Library/Frameworks/Foundation.framework/Foundation

This happens as soon as some of the ARC-enabled code attempts to call _objc_storeStrong function, like in an init method (self = [super init]). Converting the main project to ARC solves the problem, but I’d like to know if there are other solutions.

回答1:

I assumed that the toolchain may have added the necessary libraries to link to, in order for ARC to work properly. So the linker transcript may contain this piece of information. If the project of the app itself is not ARC-enabled, you may not get these by default, but you could still link to them by defining them explicitly.

Looking at the build transcript you can indeed find the appropriate linker flag there: it’s called -fobjc-arc (just as the related compiler flag). When you add this setting to Other Linker Flags, the linker will include the ARC library with the main build product and the code should run fine.

回答2:

I'm adding a new answer to this as the previous accepted solution no longer appears to work with Xcode 4.3.2. I can only assume that the -fobjc-arc linker flag was never supposed to be exposed and has now been removed.

This appears to be a known issue although the only thread I can find on this with somebody from Apple commenting on the devforums dates back to mid-2011. From that thread, it is suggested that manually linking the following file solves the issue:

${DEVROOT}/Platforms/iPhoneOS.platform/Developer/usr/lib/arc/libarclite_iphoneos.a

This requires you to be compiling using the latest compiler/SDK though. I'm submitting this answer without testing, please upvote if it works, downvote if it doesn't!