Apple’s library technology has a long and glorious history, dating all the way back to the origins of Unix. This does, however, mean that it can be a bit confusing to newcomers. This is my attempt to clarify some terminology. If you have any questions or comments about this, start a new thread and tag it with Linker so that I see it. Share and Enjoy — Quinn “The Eskimo!” @ Developer Technical Support @ Apple let myEmail = "eskimo" + "1" + "@" + "apple.com" An Apple Library Primer Apple’s tools support two related concepts: Platform — This is the platform itself; macOS, iOS, iOS Simulator, and Mac Catalyst are all platforms. Architecture — This is a specific CPU architecture used by a platform. arm64 and x86_64 are both architectures. A given architecture might be used by multiple platforms. The most obvious example of this arm64, which is used by all of the platforms listed above. Code built for one platform will not work on another platform, even if both platforms use the same architecture. Code is usually packaged in either a Mach-O file or a static library. Mach-O is used for executables (MH_EXECUTE), dynamic libraries (MH_DYLIB), bundles (MH_BUNDLE), and object files (MH_OBJECT). These can have a variety of different extensions; the only constant is that .o is always used for a Mach-O containing an object file. Use otool and nm to examine a Mach-O file. Use vtool to quickly determine the platform for which it was built. Use size to get a summary of its size. Use dyld_info to get more details about a dynamic library. IMPORTANT All the tools mentioned here are documented in man pages. For information on how to access that documentation, see Reading UNIX Manual Pages. There’s also a Mach-O man page, with basic information about the file format. Many of these tools have old and new variants, using the -classic suffix or llvm- prefix, respectively. For example, there’s nm-classic and llvm-nm. If you run the original name for the tool, you’ll get either the old or new variant depending on the version of the currently selected tools. To explicitly request the old or new variants, use xcrun. The term Mach-O image refers to a Mach-O that can be loaded and executed without further processing. That includes executables, dynamic libraries, and bundles, but not object files. A dynamic library has the extension .dylib. You may also see this called a shared library. A framework is a bundle structure with the .framework extension that has both compile-time and run-time roles: At compile time, the framework combines the library’s headers and its stub library (stub libraries are explained below). At run time, the framework combines the library’s code, as a Mach-O dynamic library, and its associated resources. The exact structure of a framework varies by platform. For the details, see Placing Content in a Bundle. macOS supports both frameworks and standalone dynamic libraries. Other Apple platforms support frameworks but not standalone dynamic libraries. Historically these two roles were combined, that is, the framework included the headers, the dynamic library, and its resources. These days Apple ships different frameworks for each role. That is, the macOS SDK includes the compile-time framework and macOS itself includes the run-time one. Most third-party frameworks continue to combine these roles. A static library is an archive of one or more object files. It has the extension .a. Use ar, libtool, and ranlib to inspect and manipulate these archives. The static linker, or just the linker, runs at build time. It combines various inputs into a single output. Typically these inputs are object files, static libraries, dynamic libraries, and various configuration items. The output is most commonly a Mach-O image, although it’s also possible to output an object file. The linker may also output metadata, such as a link map (see Using a Link Map to Track Down a Symbol’s Origin). The linker has seen three major implementations: ld — This dates from the dawn of Mac OS X. ld64 — This was a rewrite started in the 2005 timeframe. Eventually it replaced ld completely. If you type ld, you get ld64. ld_prime — This was introduced with Xcode 15. This isn’t a separate tool. Rather, ld now supports the -ld_classic and -ld_new options to select a specific implementation. Note During the Xcode 15 beta cycle these options were -ld64 and -ld_prime. I continue to use those names because the definition of new changes over time (some of us still think of ld64 as the new linker ;–). The dynamic linker loads Mach-O images at runtime. Its path is /usr/lib/dyld, so it’s often referred to as dyld, dyld, or DYLD. Personally I pronounced that dee-lid, but some folks say di-lid and others say dee-why-el-dee. IMPORTANT Third-party executables must use the standard dynamic linker. Other Unix-y platforms support the notion of a statically linked executable, one that makes system calls directly. This is not supported on Apple platforms. Apple platforms provide binary compatibility via system dynamic libraries and frameworks, not at the system call level. Note Apple platforms have vestigial support for custom dynamic linkers (your executable tells the system which dynamic linker to use via the LC_LOAD_DYLINKER load command). This facility originated on macOS’s ancestor platform and has never been a supported option on any Apple platform. The dynamic linker has seen 4 major revisions. See WWDC 2017 Session 413 (referenced below) for a discussion of versions 1 through 3. Version 4 is basically a merging of versions 2 and 3. The dyld man page is chock-full of useful info, including a discussion of how it finds images at runtime. Every dynamic library has an install name, which is how the dynamic linker identifies the library. Historically that was the path where you installed the library. That’s still true for most system libraries, but nowadays a third-party library should use an rpath-relative install name. For more about this, see Dynamic Library Identification. Mach-O images are position independent, that is, they can be loaded at any location within the process’s address space. Historically, Mach-O supported the concept of position-dependent images, ones that could only be loaded at a specific address. While it may still be possible to create such an image, it’s no longer a good life choice. Mach-O images have a default load address, also known as the base address. For modern position-independent images this is 0 for library images and 4 GiB for executables (leaving the bottom 32 bits of the process’s address space unmapped). When the dynamic linker loads an image, it chooses an address for the image and then rebases the image to that address. If you take that address and subtract the image’s load address, you get a value known as the slide. Xcode 15 introduced the concept of a mergeable library. This a dynamic library with extra metadata that allows the linker to embed it into the output Mach-O image, much like a static library. Mergeable libraries have many benefits. For all the backstory, see WWDC 2023 Session 10268 Meet mergeable libraries. For instructions on how to set this up, see Configuring your project to use mergeable libraries. If you put a mergeable library into a framework structure you get a mergeable framework. Xcode 15 also introduced the concept of a static framework. This is a framework structure where the framework’s dynamic library is replaced by a static library. Note It’s not clear to me whether this offers any benefit over creating a mergeable framework. Earlier versions of Xcode did not have proper static framework support. That didn’t stop folks trying to use them, which caused all sorts of weird build problems. A universal binary is a file that contains multiple architectures for the same platform. Universal binaries always use the universal binary format. Use the file command to learn what architectures are within a universal binary. Use the lipo command to manipulate universal binaries. A universal binary’s architectures are either all in Mach-O format or all in the static library archive format. The latter is called a universal static library. A universal binary has the same extension as its non-universal equivalent. That means a .a file might be a static library or a universal static library. Most tools work on a single architecture within a universal binary. They default to the architecture of the current machine. To override this, pass the architecture in using a command-line option, typically -arch or --arch. An XCFramework is a single document package that includes libraries for any combination of platforms and architectures. It has the extension .xcframework. An XCFramework holds either a framework, a dynamic library, or a static library. All the elements must be the same type. Use xcodebuild to create an XCFramework. For specific instructions, see Xcode Help > Distribute binary frameworks > Create an XCFramework. Historically there was no need to code sign libraries in SDKs. If you shipped an SDK to another developer, they were responsible for re-signing all the code as part of their distribution process. Xcode 15 changes this. You should sign your SDK so that a developer using it can verify this dependency. For more details, see WWDC 2023 Session 10061 Verify app dependencies with digital signatures and Verifying the origin of your XCFrameworks. A stub library is a compact description of the contents of a dynamic library. It has the extension .tbd, which stands for text-based description (TBD). Apple’s SDKs include stub libraries to minimise their size; for the backstory, read this post. Use the tapi tool to create and manipulate stub libraries. In this context TAPI stands for a text-based API, an alternative name for TBD. Oh, and on the subject of tapi, I’d be remiss if I didn’t mention tapi-analyze! Stub libraries currently use YAML format, a fact that’s relevant when you try to interpret linker errors. If you’re curious about the format, read the tapi-tbdv4 man page. There’s also a JSON variant documented in the tapi-tbdv5 man page. Note Back in the day stub libraries used to be Mach-O files with all the code removed (MH_DYLIB_STUB). This format has long been deprecated in favour of TBD. Historically, the system maintained a dynamic linker shared cache, built at runtime from its working set of dynamic libraries. In macOS 11 and later this cache is included in the OS itself. Libraries in the cache are no longer present in their original locations on disk: % ls -lh /usr/lib/libSystem.B.dylib ls: /usr/lib/libSystem.B.dylib: No such file or directory Apple APIs, most notably dlopen, understand this and do the right thing if you supply the path of a library that moved into the cache. That’s true for some, but not all, command-line tools, for example: % dyld_info -exports /usr/lib/libSystem.B.dylib /usr/lib/libSystem.B.dylib [arm64e]: -exports: offset symbol … 0x5B827FE8 _mach_init_routine % nm /usr/lib/libSystem.B.dylib …/nm: error: /usr/lib/libSystem.B.dylib: No such file or directory When the linker creates a Mach-O image, it adds a bunch of helpful information to that image, including: The target platform The deployment target, that is, the minimum supported version of that platform Information about the tools used to build the image, most notably, the SDK version A build UUID For more information about the build UUID, see TN3178 Checking for and resolving build UUID problems. To dump the other information, run vtool. In some cases the OS uses the SDK version of the main executable to determine whether to enable new behaviour or retain old behaviour for compatibility purposes. You might see this referred to as compiled against SDK X. I typically refer to this as a linked-on-or-later check. Apple tools support the concept of autolinking. When your code uses a symbol from a module, the compiler inserts a reference (using the LC_LINKER_OPTION load command) to that module into the resulting object file (.o). When you link with that object file, the linker adds the referenced module to the list of modules that it searches when resolving symbols. Autolinking is obviously helpful but it can also cause problems, especially with cross-platform code. For information on how to enable and disable it, see the Build settings reference. Mach-O uses a two-level namespace. When a Mach-O image imports a symbol, it references the symbol name and the library where it expects to find that symbol. This improves both performance and reliability but it precludes certain techniques that might work on other platforms. For example, you can’t define a function called printf and expect it to ‘see’ calls from other dynamic libraries because those libraries import the version of printf from libSystem. To help folks who rely on techniques like this, macOS supports a flat namespace compatibility mode. This has numerous sharp edges — for an example, see the posts on this thread — and it’s best to avoid it where you can. If you’re enabling the flat namespace as part of a developer tool, search the ’net for dyld interpose to learn about an alternative technique. WARNING Dynamic linker interposing is not documented as API. While it’s a useful technique for developer tools, do not use it in products you ship to end users. Apple platforms use DWARF. When you compile a file, the compiler puts the debug info into the resulting object file. When you link a set of object files into a executable, dynamic library, or bundle for distribution, the linker does not include this debug info. Rather, debug info is stored in a separate debug symbols document package. This has the extension .dSYM and is created using dsymutil. Use symbols to learn about the symbols in a file. Use dwarfdump to get detailed information about DWARF debug info. Use atos to map an address to its corresponding symbol name. Different languages use different name mangling schemes: C, and all later languages, add a leading underscore (_) to distinguish their symbols from assembly language symbols. C++ uses a complex name mangling scheme. Use the c++filt tool to undo this mangling. Likewise, for Swift. Use swift demangle to undo this mangling. For a bunch more info about symbols in Mach-O, see Understanding Mach-O Symbols. This includes a discussion of weak references and weak definition. If your code is referencing a symbol unexpectedly, see Determining Why a Symbol is Referenced. To remove symbols from a Mach-O file, run strip. To hide symbols, run nmedit. It’s common for linkers to divide an object file into sections. You might find data in the data section and code in the text section (text is an old Unix term for code). Mach-O uses segments and sections. For example, there is a text segment (__TEXT) and within that various sections for code (__TEXT > __text), constant C strings (__TEXT > __cstring), and so on. Over the years there have been some really good talks about linking and libraries at WWDC, including: WWDC 2023 Session 10268 Meet mergeable libraries WWDC 2022 Session 110362 Link fast: Improve build and launch times WWDC 2022 Session 110370 Debug Swift debugging with LLDB WWDC 2021 Session 10211 Symbolication: Beyond the basics WWDC 2019 Session 416 Binary Frameworks in Swift — Despite the name, this covers XCFrameworks in depth. WWDC 2018 Session 415 Behind the Scenes of the Xcode Build Process WWDC 2017 Session 413 App Startup Time: Past, Present, and Future WWDC 2016 Session 406 Optimizing App Startup Time Note The older talks are no longer available from Apple, but you may be able to find transcripts out there on the ’net. Historically Apple published a document, Mac OS X ABI Mach-O File Format Reference, or some variant thereof, that acted as the definitive reference to the Mach-O file format. This document is no longer available from Apple. If you’re doing serious work with Mach-O, I recommend that you find an old copy. It’s definitely out of date, but there’s no better place to get a high-level introduction to the concepts. The Mach-O Wikipedia page has a link to an archived version of the document. For the most up-to-date information about Mach-O, see the declarations and doc comments in <mach-o/loader.h>. Revision History 2025-08-04 Added a link to Determining Why a Symbol is Referenced. 2025-06-29 Added information about autolinking. 2025-05-21 Added a note about the legacy Mach-O stub library format (MH_DYLIB_STUB). 2025-04-30 Added a specific reference to the man pages for the TBD format. 2025-03-01 Added a link to Understanding Mach-O Symbols. Added a link to TN3178 Checking for and resolving build UUID problems. Added a summary of the information available via vtool. Discussed linked-on-or-later checks. Explained how Mach-O uses segments and sections. Explained the old (-classic) and new (llvm-) tool variants. Referenced the Mach-O man page. Added basic info about the strip and nmedit tools. 2025-02-17 Expanded the discussion of dynamic library identification. 2024-10-07 Added some basic information about the dynamic linker shared cache. 2024-07-26 Clarified the description of the expected load address for Mach-O images. 2024-07-23 Added a discussion of position-independent images and the image slide. 2024-05-08 Added links to the demangling tools. 2024-04-30 Clarified the requirement to use the standard dynamic linker. 2024-03-02 Updated the discussion of static frameworks to account for Xcode 15 changes. Removed the link to WWDC 2018 Session 415 because it no longer works )-: 2024-03-01 Added the WWDC 2023 session to the list of sessions to make it easier to find. Added a reference to Using a Link Map to Track Down a Symbol’s Origin. Made other minor editorial changes. 2023-09-20 Added a link to Dynamic Library Identification. Updated the names for the static linker implementations (-ld_prime is no more!). Removed the beta epithet from Xcode 15. 2023-06-13 Defined the term Mach-O image. Added sections for both the static and dynamic linkers. Described the two big new features in Xcode 15: mergeable libraries and dependency verification. 2023-06-01 Add a reference to tapi-analyze. 2023-05-29 Added a discussion of the two-level namespace. 2023-04-27 Added a mention of the size tool. 2023-01-23 Explained the compile-time and run-time roles of a framework. Made other minor editorial changes. 2022-11-17 Added an explanation of TAPI. 2022-10-12 Added links to Mach-O documentation. 2022-09-29 Added info about .dSYM files. Added a few more links to WWDC sessions. 2022-09-21 First posted.

With Xcode 14, the device selection on the bottom of the storyboard canvas has changed, and the iPhone 12 and 13 mini screens are no longer there, as well as there only being one model of iPad. Is there a way to add more devices to this section? I have simulators installed for these devices, and this is the way I've been checking compatibility so far, but this is not sustainable. I appreciate any help that you can give me.

I am developing CarPlay addition on our app. Which is distributed with the Enterprise In distribution method, so we do not have a product in the App Store. I am wondering if CarPlay support can be provided in applications distributed with the Enterprise in distribution method? If this is not possible, I will inform management that this is not possible. I am waiting for your answers, thanks.

I have an iPhone 14 running iOS 16.1 and my series 5 watch running watchOS 9.1. I was able to turn on Developer Mode on the phone by going to Settings--> Privacy & Security --> Developer Mode. On the watch however (I'm doing this directly on the watch and not on the watch app on the phone) once I'm in Privacy & Security, there is no option to select Developer Mode. How do I get my watch in Developer Mode in order to get a successful build in xCode?

I've been happily building and deploying my app to my iPhone and Watch S8, and the app was ready to submit to App Store Connect last night. However, when archiving it I got an error saying that my DynamicEventSelectionIntent was in multiple extensions. It was, kind of. When I started working on the complications I copied the Widgets intents into the complications, and left the name the same, but they were not in multiple targets. It looks like the info plist only had one item in the IntentsSupported (because they're the same name), so I decided to rename them so I had a widget one and a complications one. The problem I have now is that I can't deploy to my iPhone and Watch anymore because I'm getting this error: This app contains a WatchKit app with one or more Siri Intents app extensions that declare IntentsSupported that are not declared in any of the companion app's Siri Intents app extensions. WatchKit Siri Intents extensions' IntentsSupported values must be a subset of the companion app's Siri Intents extensions' IntentsSupported values. All I've done is rename one intent, and locate every instance of it in the info plist files, and add the appropriate new one into the right places. Here's what I've got. Main App contains Widget and WidgetIntentHandler, plus Watch App, which contains Complications and ComplicationsIntentHandler. Target: Main app: (I've removed everything that has no bearing on extensions.) Target: Widget: Target: WidgetIntentHandler: Target: Watch App: Target: Complications: Target: ComplicationsIntentHandler: Please, can someone tell me what should and should not be in the various parts, as I've tried for 12 hours now and I cannot get this to deploy to my iPhone anymore :( Thanks.

It's maddening. I just opened a file and now it's gone because I had the audacity to open another file. I've learned that I can double click the tab of the first file to make it a permanent tab... but, of course, I'll need to do that every time I ever open any file ever, and if I close that file and come back to it later I'm going to be constantly having to double click tabs just to make Xcode stop replacing my tabs for me. It's nuts. It's driving me insane. Please. Is there any way to just DISABLE the temporary tabs behavior? I don't find it helpful in any way and I find it to be destructive to my development workflows.

I have had this notice for a while: Registration is being processed for these devices. They may become available for development and ad hoc distribution in 24 to 72 hours. Changes to device availability will appear in the Status column. The status for my new iPhone has been stuck in this "Processing" state. See image:

Recently, I have trouble with my Xcode Cloud builds, where they seemingly complete, but the very final step does not happen. I am building from the same codebase for both macOS and iOS (I have two “Archive” steps in my workflow, one for iOS and one for macOS). The iOS one usually completes fast without problems. The macOS log ending looks like this, and remains stuck forever, with the macOS build in the “spinning progress” state and never completing. Is there something I can do on my end to make this work better? I would not want to go back to manual builds, Xcode Cloud is convenient and was working well for a while, but this is really annoying lately.

Is there a way to use CFBundleVersion instead of CI_BUILD_NUMBER from XCode Cloud's workflow on Web ? We would like to have control over the major and minor version numbers of the build from the workflow.

Each year I get my renewal email from Apple, however, since 2019 (yes, this is the 4th year in a row) I've been unable to renew due to this infamous "Please enter fewer characters" error message. Each time I've had to open a support case, talk to someone on the phone, they take all of my details along with screenshots, etc. and manually renew my subscription. Once they've charged my card I never hear from them again and issue never gets resolved. IDK if any Apple Dev Support folks actually use this forum but this is ridiculous.

Hi I've downloaded the latest version of Xcode 14.2 but when I run the app in Simulator I can only test it on iOS16.2 Is there a way to run it in 16.3?

Hi, Our account was due for renewal and not getting renew option neither on the website nor on the app. While the banner shows renewal is due but no option to pay. Attaching the screenshot below need immediate help.

We are getting following error in xcode cloud "The step invocation hit a user timeout. The xcodebuild archive invocation timed out. No activity has been detected on stdout, stderr or the result bundle in 30 minutes" We are using following env to create a build. Xcode: 14.2 Macos: Ventura 13.2.1 We didn't face any issue in Ventura 13.2.0 and we are seeing this issue in our first build using ventura 13.2.1. The difference I see between successful and failure build, I don't see "Discovering Swift tasks after 'Compiling { list of swift file " in a failed build. I see compiling { list of swift file } and end of the compilation, the build is failing.

Hello , am facing issue in submitting my app to store I have submitted my case to apple developer team my case ids "101969263018","101975805043". they told me to submit the report from feedback assistance my case id : FB12141270. but still I don't get any replay form feedback assistance. after that I submitted my case to DTS engineer case id : 2394373. got email to submit some file which I have submitted after that still I don't get any reply from DTS team. please help me to short out this issue. last one month am trying to short out this issue with apple developer team. still I don't get solution.

This is probably a silly question, but I couldn't find the answer to it in the forums or in the documentation, though I may be missing something. I currently have an app with a deployment target of iOS 16 and a watchOS app (not independent) with a deployment target of watchOS 7. I understand what happens when I change the deployment target on the iOS app (e.g., users with iOS/iPadOS versions less than 16 will just never see the updates in the App Store). But what happens if I change the deployment target of the watchOS dependent app to something like watchOS 8? Will users who have iOS 16 and watchOS 7 (iOS meets deployment target/watchOS does not) get the app update, and it'll just uninstall the watchOS app automatically? Will they just not see the update? Does the old version of the Watch app somehow stay on their watch while the iOS app gets updated?

Hi everyone, (you can answer in french) I’m french-rookie in xcode, and I have a problem: I’m trying to choice my custom font to add it in my Main.storyboard button, but it not works. I have my « Font provided by application » line in my Info.plist, with the name of my font in [0] (See images below) When I’m on storyboard button, I chose « custom » in font selector, then display font list but my custom font doesn’t appear. I already tried to install the font in my mac, but still stucked, nothing change 😭 Could you help me please? Thanks a lot (I specify that I scrupulously followed the way of doing explained on the Apple official page: Adding a Custom Font to Your App )

Hi! I was wondering when the maintenance of the Apple Developer website will end as I need to register a new App ID for my project. I know that WWDC just ended, but is there an estimate for this?

I have a repo with hundreds of tags (we make a tag for every build and for every version, and it has a browser extension project pulled in as a subtree where all those commits can and do have version tags of their own). I am trying to create a simple tag-triggered workflow to release my app to the external users and the app store, so I want to trigger it every time I update and push an e.g. latest tag. I created a workflow and setup the tag trigger accordingly, but, in the workflow config, it says it can't find the tag. I saved the workflow anyway, however, when I try to manually trigger the workflow and browse the Tags tab, I only see a subset of the repo's tags. I suspect Xcode Cloud might be limiting the number of tags it pulls and consequently shows when searching tags. I think this is a bug/shortcoming of Xcode Cloud. Is there any way to work around this?

Hello Team, I try to delete photo from Photos for that i used this method, [[PHPhotoLibrary sharedPhotoLibrary] performChanges:^{ [PHAssetChangeRequest deleteAssets:@[assetToDelete]]; completionHandler:^(BOOL success, NSError *error) { }]; This method pops up a dialog with Don't Allow or Delete. But some time in some iPhones not respond PHAssetChangeRequest deleteAssets method that's why that completionHandler not called because of that i can't perform any operation of PHPhotoLibrary then after. If I restart my iPhone then it works. Many users of my app complained about this issue. I have an iPhone 11 with iOS 15.3. But some iOS 12,14,16 users also face the same issue. So what exact issue is there? Is it related to iOS or a method? Thanks, Ankur

Anyone know how to set Side By Side Comparison in the new commit UI? New UI seems to missing a lot of features like multi-select of files to discard/stage...