# Missing symbol: makeUniqueAndReserveCapacityIfNot in libswiftcore.dylib

**URL:** <https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439>\
**Category:** Using Swift\
**Tags:** macos\
**Created:** [August 1, 2025, 11:27am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439 "2025-08-01T11:27:16Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Juraj](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/juraj/32/30398_2.png) [@Juraj](https://forums.swift.org/u/Juraj)\
**Post date:** [August 1, 2025, 11:27am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/1 "2025-08-01T11:27:16Z")

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We did hit strange error lately. After building the product(rather big macos app, C, C++, ObjC, Swift codebase), it runs well on macos 14+. On lower targets we get crash during startup about missing symbol.

> Termination Reason: Namespace DYLD, Code 4 Symbol missing  
> Symbol not found: \_$sSa034\_makeUniqueAndReserveCapacityIfNotB0yyFyXl\_Ts5  
> Referenced from: \<4A1BC342-E608-3A85-A754-5CDB86D5CA16\> /Applications/-redacted-  
> Expected in: /usr/lib/swift/libswiftCore.dylib  
> (terminated at launch; ignore backtrace)

It seems to be private symbol in swift core library, google shows nothing.  
Weakly linking libswiftcore helps, but why such runtime error is there in the first place? What is causing it?

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**Author:** ![eskimo](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/eskimo/32/13558_2.png) [@eskimo](https://forums.swift.org/u/eskimo)\
**Post date:** [August 4, 2025, 7:58am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/2 "2025-08-04T07:58:07Z")

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In my experience, most issues like this are caused by one of two things:

- Setting your deployment target lower than the minimum supported by your tools.
- Building different chunks of your code with different deployment targets.

Recent versions of Xcode support back to macOS 10.13 [1], which rules out the first possibility.

The second possibility happens when one or more of the Mach-O object files (`.o`) that were linked together to create the Mach-O image were built with a higher deployment target that the image itself. I most commonly see this when static libraries are in play.

Sometimes you can figure this out by carefully reviewing your build settings. Failing that, there’s two paths forward:

- Review the deployment target of every `.o` files that was linked into the Mach-O image. Remember to dig inside static libraries. Use `vtool` with the `-show-build` subcommand to get the deployment target.
- In many cases you can use the `-why_live` option as a shortcut to see where the reference came from. See the discussion at the bottom of [Determining Why a Symbol is Referenced](https://developer.apple.com/forums/thread/751329).

ps I’m using the terminology from [An Apple Library Primer](https://developer.apple.com/forums/thread/715385). If you’re gonna work at this level, I strongly recommend that you review that post.

* * *

Finally, there is a third possible cause here:

- A bug in the compiler or libraries.

I’ve definitely seen this. It’s less common than the other two possibilities, so my initial advice is that you rule those out before heading down this third path.

Share and Enjoy

Quinn “The Eskimo!” @ DTS @ Apple

[1] Although Xcode 26 beta raises this to macOS 11. I highly recommend that you bookmark [Developer \> Support \> Xcode](https://developer.apple.com/support/xcode).

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<div class="post-metadata">

**Author:** ![pyrtsa](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/pyrtsa/32/740_2.png) [@pyrtsa](https://forums.swift.org/u/pyrtsa)\
**Post date:** [August 5, 2025, 5:00am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/3 "2025-08-05T05:00:33Z")

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> [@eskimo](#):
>
> Finally, there is a third possible cause here:
> 
> - A bug in the compiler or libraries.

I have a bit of evidence suggesting this third cause could be the issue, the feedback (FB19040850) we filed a couple of weeks ago:

> **Symbol references in \`if #available(iOS 16, \*) {…}\` blocks crash program at launch: system frameworks are no longer weak-linked by default**
>
> Swift code referencing conditionally available `NSString` constants inside guarded by `if #available(iOS 16, *)` checks stopped working in the latest Xcode 26 beta. Expecting the app to launch without crashing in these cases.
> 
> Consider this simple iOS app (also attached as Xcode project):
> 
> ```swift
> import AVFoundation
> import MetricKit
> import StoreKit
> import SwiftUI
> 
> @main
> struct SymbolNotFoundApp: App {
> var strings: [String] = []
> init() {
> // Comment out these lines to avoid the crash:
> if #available(iOS 16.0, *) { strings.append(AVURLAssetHTTPUserAgentKey) }
> if #available(iOS 16.0, *) { strings.append(MXErrorDomain) }
> if #available(iOS 16.1, *) { strings.append(SKStoreProductParameterAdNetworkSourceIdentifier) }
> }
> var body: some Scene {
> WindowGroup {
> Text(strings.isEmpty ? "No strings." : strings.joined(separator: "\n")).padding()
> }
> }
> }
> 
> ```
> 
> Build the attached project with Xcode 16.4 and run on iOS 15.5 simulator or device: the app launches displaying the message "No strings." as expected.
> 
> Then repeat the same with Xcode 26 beta 4 and iOS 15.5: the app now crashes at launch with the following error:
> 
> ```swift
> Symbol not found: _AVURLAssetHTTPUserAgentKey
> Referenced from:
> /Users/<me>/Library/Developer/CoreSimulator/Devices/<uuid>/data/Containers/Bundle/Application/<uuid>/Xcode26-iOS15-AVURLAssetHTTPUserAgentKe
> .app/Xcode26-iOS15-AVURLAssetHTTPUserAgentKey
> Expected in: /Library/Developer/CoreSimulator/Profiles/Runtimes/iOS
> 15.5.simruntime/Contents/Resources/RuntimeRoot/System/Library/Frameworks/AVFoundation.framework/AVFoundation
> 
> ```
> 
> With Xcode 16.4 and prior, system frameworks would get automatically linked weakly, causing conditionally available symbol references to only be looked up by dyld when needed. But when compiled with the latest beta SDK, the system frameworks are suddenly linked strongly, as seen when comparing the `dyld_info` output of the app binary:
> 
> ```swift
> -linked_dylibs:
> ...
> - weak-link /System/Library/Frameworks/AVFoundation.framework/AVFoundation
> - weak-link /System/Library/Frameworks/MetricKit.framework/MetricKit
> - weak-link /System/Library/Frameworks/StoreKit.framework/StoreKit
> + /System/Library/Frameworks/AVFoundation.framework/AVFoundation
> + /System/Library/Frameworks/MetricKit.framework/MetricKit
> + /System/Library/Frameworks/StoreKit.framework/StoreKit
> 
> ```
> 
> The workaround is to mark the frameworks as "optional" in the "Link Binary With Libraries" phase, but this shouldn't be required by the developer, AFAICT.

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<div class="post-metadata">

**Author:** ![eskimo](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/eskimo/32/13558_2.png) [@eskimo](https://forums.swift.org/u/eskimo)\
**Post date:** [August 5, 2025, 8:01am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/4 "2025-08-05T08:01:00Z")

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> the feedback (`FB19040850`) we filed a couple of weeks ago

Oh, thanks for that.

Looking at the bug it seems to be ‘real’, that is, something in the toolchain rather than something in your project.

Your bug was filed against Xcode 26.0b4. @Juraj, are you also seeing this on the beta? I’d assumed that you were on a production Xcode, but looking over your original post I don’t see any evidence for that assumption.

Share and Enjoy

Quinn “The Eskimo!” @ DTS @ Apple

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<div class="post-metadata">

**Author:** ![Juraj](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/juraj/32/30398_2.png) [@Juraj](https://forums.swift.org/u/Juraj)\
**Post date:** [August 5, 2025, 8:51am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/5 "2025-08-05T08:51:32Z")

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We are using Xcode 16.4 to do the builds. Deployment target macOS 12 and up.

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<div class="post-metadata">

**Author:** ![Finagolfin](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/finagolfin/32/34388_2.png) [@Finagolfin](https://forums.swift.org/u/Finagolfin)\
**Post date:** [August 5, 2025, 3:49pm UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/6 "2025-08-05T15:49:18Z")

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I just saw [a similar issue with running the OSS macOS Aug. 2 trunk snapshot toolchain on macOS 13](https://github.com/swiftlang/swift-package-manager/issues/8991), could be related, though that is a missing Foundation symbol instead.

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<div class="post-metadata">

**Author:** ![eskimo](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/eskimo/32/13558_2.png) [@eskimo](https://forums.swift.org/u/eskimo)\
**Post date:** [August 6, 2025, 8:20am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/7 "2025-08-06T08:20:58Z")

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There are limits to how much I can share about the state of this )-: What I can say is that:

- It remains unfixed in Xcode 26.0b5 that we seeded yesterday.
- As always, I encourage you to retest with future Xcode betas as we seed them.

Share and Enjoy

Quinn “The Eskimo!” @ DTS @ Apple

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<div class="post-metadata">

**Author:** ![Juraj](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/juraj/32/30398_2.png) [@Juraj](https://forums.swift.org/u/Juraj)\
**Post date:** [September 1, 2025, 8:51am UTC](https://forums.swift.org/t/missing-symbol-makeuniqueandreservecapacityifnot-in-libswiftcore-dylib/81439/8 "2025-09-01T08:51:14Z")

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Compilation with Xcode 26 beta 7 seems to fix iOS availability problems in @pyrtsa sample code.