Is ARC really supported in iOS 4? The iOS 4.2 SDK is missing ARC-related symbols at linking time

Asked 12/10, 2011 at 23:30 Answered 28/8, 2012 at 7:50

Solved iphone ios ios4 automatic-ref-counting xcode4.2

I've read and heard since ARC was first announced that it was a compile-time thing and would be backwards-compatible with iOS 4. I have successfully refactored my project to ARC using Xcode 4.2's automatic refactoring, and when compiled against the iOS 5.0 SDK, it works fine. However, if I try to compile against my iOS 4.2 SDK, it fails at link time, missing the following symbols:

_objc_retainAutoreleaseReturnValue
_objc_autoreleaseReturnValue
_objc_storeStrong
_objc_retain
_objc_release
_objc_retainAutoreleasedReturnValue

I checked, and these symbols are present in 5.0 but not 4.2:

iPhoneOS5.0.sdk/usr/lib $ find . -type f|xargs nm|grep -i _objc_retain$
00005ed0 T _objc_retain
000061d0 T _objc_retain

iPhoneOS4.2.sdk/usr/lib $ find . -type f|xargs nm|grep -i _objc_retain$
[... *crickets* ...]

Does this mean that Apple lied? I assume instead that I'm confused and doing something wrong, but I can't figure out what.

This is with the GM release of Xcode 4.2 (Build 4C199)

Intemerate answered 12/10, 2011 at 23:30 Comment(2)

In addition to what others have said, don't install the 4.2 SDK. Use the 5.0 SDK. Set your deployment target. I don't mean for this, I mean in general; older SDKs should not be used. – Ziwot 12/10, 2011 at 23:40

Wow, I can't believe I've gone this long without knowing that newer SDKs could build for older OS versions. Here I've been mad at Apple that they've been deleting all my old SDK versions :/ Thank you! – Intemerate 12/10, 2011 at 23:46

ARC is supported on iOS 4.3 and above. You need to use the iOS 5.x SDK but can select iOS 4.3 for the Deployment Target. The one thing that is not supported in 4.x is automatic weak reference zeroing.

See Apple's documentation here for the Objective-C Feature Availability Index.

Xcode 4.4 and 4.5 can be used to submit apps.

Note that NSDictionary and NSArray subscripting deploys back to OS X 10.6 and iOS 4, be sure to set the deployment target back as well.

Branton answered 12/10, 2011 at 23:34 Comment(10)

Thanks (facepalm). It actually seems to be working with 4.2 as a deployment target as well, however... going to test on a real device now. – Intemerate 12/10, 2011 at 23:47

It seems to run fine on my device as well, running iOS 4.2.1. – Intemerate 13/10, 2011 at 0:24

Sorry, looks like @CocoaFu was correct. After I restarted Xcode, it seems to only work in 4.3. Xcode 4 seems easily confused :( – Intemerate 13/10, 2011 at 1:17

@CocoaFu what version reference are you referring to? Everything I can find just says it supports iOS 4. – Leandra 13/10, 2011 at 21:2

I can't find a good reference so I am basing on what I remember from WWDC11. – Branton 13/10, 2011 at 22:26

@CocoaFu - This isn't limited to iOS 4.3. I was just able to successfully deploy an ARC-enabled application to an iPhone 3G running 4.2. I'm pretty sure the floor on the deployment target here is 4.0, from what I saw at WWDC. All that is required is the modern runtime, which is present in iOS 4.0 (and 64-bit on Snow Leopard). – Embodiment 19/10, 2011 at 14:18

@Doug - If you are attempting to deploy to an older device (say an iPhone 3G running 4.2), make sure that you've added an armv6 architecture to your build settings. Xcode 4.2 doesn't have a default setting for this any more, so you need to manually add it in order to target older devices. I just successfully ran an ARC-enabled application on an iPhone 3G on 4.2 here. – Embodiment 19/10, 2011 at 14:20

Apple itself says that ARC is available on "iOS 4": developer.apple.com/library/ios/#releasenotes/ObjectiveC/… Personally I've ran ARC app on iPhone 3G with iOS 4.2 and iPad with iOS 4.2. If you use the "weak" keyword however you will get an error trying to build for iOS 4.x. – Cachalot 23/10, 2011 at 22:34

I can confirm that ARC without weak reference support works on at least iOS 4.2.1, I have nothing older to test on. – Dilatant 15/1, 2012 at 19:54

Apple's documentation provided in the answer clearly says that "NSDictionary and NSArray subscripting deploys back to iOS 5", not iOS 4. I have submitted an update that is waiting for approval. – Guile 10/5, 2013 at 22:0

A little bit late, but this is important information. The accepted answer is correct, Apple states iOS 4.0 and above as the minimum OS for ARC support.

However, it can break in a few situations. One of them is of course the __weak keyword and it's derivatives. You will see an error like the following.

dyld: lazy symbol binding failed: Symbol not found: _objc_initWeak

A second and very dangerous condition is when you use the +load method of NSObject. If you do this using ARC on iOS 4.x you will have runtime crashes that are very hard to find. If you get runtime errors like the following, check your +load methods. For me it worked to set the -fno-objc-arc flag for that particular file.

dyld: lazy symbol binding failed: Symbol not found: _objc_retainAutoreleasedReturnValue

Rental answered 28/8, 2012 at 7:50 Comment(1)

That second part you mention has come back to haunt me, crash in +load. We will drop iOS 4.3 soon but I have to provide one more working update for some people refusing to update 4.3 to 5 or 6, gaaaah. – Malpractice 18/5, 2013 at 19:1

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