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Determining Why a Symbol is Referenced
Recently a bunch of folks have asked about why a specific symbol is being referenced by their app. This is my attempt to address that question. If you have questions or comments, please start a new thread. 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" Determining Why a Symbol is Referenced In some situations you might want to know why a symbol is referenced by your app. For example: You might be working with a security auditing tool that flags uses of malloc. You might be creating a privacy manifest and want to track down where your app is calling stat. This post is my attempt at explaining a general process for tracking down the origin of these symbol references. This process works from ‘below’. That is, it works ‘up’ from you app’s binary rather than ‘down’ from your app’s source code. That’s important because: It might be hard to track down all of your source code, especially if you’re using one or more package management systems. If your app has a binary dependency on a static library, dynamic library, or framework, you might not have access to that library’s source code. IMPORTANT This post assumes the terminology from An Apple Library Primer. Read that before continuing here. The general outline of this process is: Find all Mach-O images. Find the Mach-O image that references the symbol. Find the object files (.o) used to make that Mach-O. Find the object file that references the symbol. Find the code within that object file. Those last few steps require some gnarly low-level Mach-O knowledge. If you’re looking for an easier path, try using the approach described in the A higher-level alternative section as a replacement for steps 3 through 5. This post assumes that you’re using Xcode. If you’re using third-party tools that are based on Apple tools, and specifically Apple’s linker, you should be able to adapt this process to your tooling. If you’re using a third-party tool that has its own linker, you’ll need to ask for help via your tool’s support channel. Find all Mach-O images On Apple platforms an app consists of a number of Mach-O images. Every app has a main executable. The app may also embed dynamic libraries or frameworks. The app may also embed app extensions or system extensions, each of which have their own executable. And a Mac app might have embedded bundles, helper tools, XPC services, agents, daemons, and so on. To find all the Mach-O images in your app, combine the find and file tools. For example: % find "Apple Configurator.app" -print0 | xargs -0 file | grep Mach-O Apple Configurator.app/Contents/MacOS/Apple Configurator: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64] … Apple Configurator.app/Contents/MacOS/cfgutil: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64] … Apple Configurator.app/Contents/Extensions/ConfiguratorIntents.appex/Contents/MacOS/ConfiguratorIntents: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64] … Apple Configurator.app/Contents/Frameworks/ConfigurationUtilityKit.framework/Versions/A/ConfigurationUtilityKit: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit dynamically linked shared library x86_64] [arm64] … This shows that Apple Configurator has a main executable (Apple Configurator), a helper tool (cfgutil), an app extension (ConfiguratorIntents), a framework (ConfigurationUtilityKit), and many more. This output is quite unwieldy. For nicer output, create and use a shell script like this: % cat FindMachO.sh #! /bin/sh # Passing `-0` to `find` causes it to emit a NUL delimited after the # file name and the `:`. Sadly, macOS `cut` doesn’t support a nul # delimiter so we use `tr` to convert that to a DLE (0x01) and `cut` on # that. # # Weirdly, `find` only inserts the NUL on the primary line, not the # per-architecture Mach-O lines. We use that to our advantage, filtering # out the per-architecture noise by only passing through lines # containing a DLE. find "$@" -type f -print0 \ | xargs -0 file -0 \ | grep -a Mach-O \ | tr '\0' '\1' \ | grep -a $(printf '\1') \ | cut -d $(printf '\1') -f 1 Find the Mach-O image that references the symbol Once you have a list of Mach-O images, use nm to find the one that references the symbol. The rest of this post investigate a test app, WaffleVarnishORama, that’s written in Swift but uses waffle management functionality from the libWaffleCore.a static library. The goal is to find the code that calls calloc. This app has a single Mach-O image: % FindMachO.sh "WaffleVarnishORama.app" WaffleVarnishORama.app/WaffleVarnishORama Use nm to confirm that it references calloc: % nm "WaffleVarnishORama.app/WaffleVarnishORama" | grep "calloc" U _calloc The _calloc symbol has a leading underscore because it’s a C symbol. This convention dates from the dawn of Unix, where the underscore distinguish C symbols from assembly language symbols. The U prefix indicates that the symbol is undefined, that is, the Mach-O images is importing the symbol. If the symbol name is prefixed by a hex number and some other character, like T or t, that means that the library includes an implementation of calloc. That’s weird, but certainly possible. OTOH, if you see this then you know this Mach-O image isn’t importing calloc. IMPORTANT If this Mach-O isn’t something that you build — that is, you get this Mach-O image as a binary from another developer — you won’t be able to follow the rest of this process. Instead, ask for help via that library’s support channel. Find the object files used to make that Mach-O image The next step is to track down which .o file includes the reference to calloc. Do this by generating a link map. A link map is an old school linker feature that records the location, size, and origin of every symbol added to the linker’s output. To generate a link map, enable the Write Link Map File build setting. By default this puts the link map into a text (.txt) file within the derived data directory. To find the exact path, look at the Link step in the build log. If you want to customise this, use the Path to Link Map File build setting. A link map has three parts: A simple header A list of object files used to build the Mach-O image A list of sections and their symbols In our case the link map looks like this: # Path: …/WaffleVarnishORama.app/WaffleVarnishORama # Arch: arm64 # Object files: [ 0] linker synthesized [ 1] objc-file [ 2] …/AppDelegate.o [ 3] …/MainViewController.o [ 4] …/libWaffleCore.a[2](WaffleCore.o) [ 5] …/Foundation.framework/Foundation.tbd … # Sections: # Address Size Segment Section 0x100008000 0x00001AB8 __TEXT __text … The list of object files contains: An object file for each of our app’s source files — That’s AppDelegate.o and MainViewController.o in this example. A list of static libraries — Here that’s just libWaffleCore.a. A list of dynamic libraries — These might be stub libraries (.tbd), dynamic libraries (.dylib), or frameworks (.framework). Focus on the object files and static libraries. The list of dynamic libraries is irrelevant because each of those is its own Mach-O image. Find the object file that references the symbol Once you have list of object files and static libraries, use nm to each one for the calloc symbol: % nm "…/AppDelegate.o" | grep calloc % nm "…/MainViewController.o" | grep calloc % nm "…/libWaffleCore.a" | grep calloc U _calloc This indicates that only libWaffleCore.a references the calloc symbol, so let’s focus on that. Note As in the Mach-O case, the U prefix indicates that the symbol is undefined, that is, the object file is importing the symbol. Find the code within that object file To find the code within the object file that references the symbol, use the objdump tool. That tool takes an object file as input, but in this example we have a static library. That’s an archive containing one or more object files. So, the first step is to unpack that archive: % mkdir "libWaffleCore-objects" % cd "libWaffleCore-objects" % ar -x "…/libWaffleCore.a" % ls -lh total 24 -rw-r--r-- 1 quinn staff 4.1K 8 May 11:24 WaffleCore.o -rw-r--r-- 1 quinn staff 56B 8 May 11:24 __.SYMDEF SORTED There’s only a single object file in that library, which makes things easy. If there were a multiple, run the following process over each one independently. To find the code that references a symbol, run objdump with the -S and -r options: % xcrun objdump -S -r "WaffleCore.o" … ; extern WaffleRef newWaffle(void) { 0: d10083ff sub sp, sp, #32 4: a9017bfd stp x29, x30, [sp, #16] 8: 910043fd add x29, sp, #16 c: d2800020 mov x0, #1 10: d2800081 mov x1, #4 ; Waffle * result = calloc(1, sizeof(Waffle)); 14: 94000000 bl 0x14 <ltmp0+0x14> 0000000000000014: ARM64_RELOC_BRANCH26 _calloc … Note the ARM64_RELOC_BRANCH26 line. This tells you that the instruction before that — the bl at offset 0x14 — references the _calloc symbol. IMPORTANT The ARM64_RELOC_BRANCH26 relocation is specific to the bl instruction in 64-bit Arm code. You’ll see other relocations for other instructions. And the Intel architecture has a whole different set of relocations. So, when searching this output don’t look for ARM64_RELOC_BRANCH26 specifically, but rather any relocation that references _calloc. In this case we’ve built the object file from source code, so WaffleCore.o contains debug symbols. That allows objdump include information about the source code context. From that, we can easily see that calloc is referenced by our newWaffle function. To see what happens when you don’t have debug symbols, create an new object file with them stripped out: % cp "WaffleCore.o" "WaffleCore-stripped.o" % strip -x -S "WaffleCore-stripped.o" Then repeat the objdump command: % xcrun objdump -S -r "WaffleCore-stripped.o" … 0000000000000000 <_newWaffle>: 0: d10083ff sub sp, sp, #32 4: a9017bfd stp x29, x30, [sp, #16] 8: 910043fd add x29, sp, #16 c: d2800020 mov x0, #1 10: d2800081 mov x1, #4 14: 94000000 bl 0x14 <_newWaffle+0x14> 0000000000000014: ARM64_RELOC_BRANCH26 _calloc … While this isn’t as nice as the previous output, you can still see that newWaffle is calling calloc. A higher-level alternative Grovelling through Mach-O object files is quite tricky. Fortunately there’s an easier approach: Use the -why_live option to ask the linker why it included a reference to the symbol. To continue the above example, I set the Other Linker Flags build setting to -Xlinker / -why_live / -Xlinker / _calloc and this is what I saw in the build transcript: _calloc from /usr/lib/system/libsystem_malloc.dylib _newWaffle from …/libWaffleCore.a[2](WaffleCore.o) _$s18WaffleVarnishORama18MainViewControllerC05tableE0_14didSelectRowAtySo07UITableE0C_10Foundation9IndexPathVtFTf4dnn_n from …/MainViewController.o _$s18WaffleVarnishORama18MainViewControllerC05tableE0_14didSelectRowAtySo07UITableE0C_10Foundation9IndexPathVtF from …/MainViewController.o Demangling reveals a call chain like this: calloc newWaffle WaffleVarnishORama.MainViewController.tableView(_:didSelectRowAt:) WaffleVarnishORama.MainViewController.tableView(_:didSelectRowAt:) and that should be enough to kick start your investigation. IMPORTANT The -why_live option only works if you dead strip your Mach-O image. This is the default for the Release build configuration, so use that for this test. Revision History 2025-07-18 Added the A higher-level alternative section. 2024-05-08 First posted.
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1.4k
Jul ’25
How to implement background notifications with action buttons (Accept/Decline) in iOS Flutter app?
I am developing a Flutter app for food delivery (a multivendor e-commerce restaurant app). In the vendor app (Android), I successfully implemented a background notification that stays active until the vendor responds with either Accept or Decline. This works fine on Android, but I cannot get the same functionality working on iOS. My requirements: Vendor should receive a background notification. The notification should include action buttons (Accept / Decline). It should remain active until the vendor takes action. My questions: Is this possible to implement in iOS with Flutter? If yes, what is the recommended way (e.g., firebase_messaging, flutter_local_notifications, flutter_foreground_task, or native iOS integration)? Are there any iOS restrictions I should consider compared to Android background services? I built this for Android using firebase_messaging + flutter_foreground_task + flutter_local_notifications. On iOS, I tried setting up firebase_messaging and flutter_local_notifications, but I’m unable to keep the notification persistent with Accept/Decline action buttons. I expected similar behavior to Android, but it seems iOS has more restrictions around background services and notification handling. Dependencies I am using (relevant ones): firebase_core: ^3.8.0 firebase_messaging: ^15.1.5 flutter_local_notifications: ^17.2.2 flutter_foreground_task: ^8.17.0 get: ^4.7.2 shared_preferences: ^2.3.2
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282
Sep ’25
Apple 403 detected - Access forbidden
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85
3w
App crashing on startup
I am a developer working on an iOS enterprise application. When I deployed the release version to my device, it crashes upon startup. It works fine in debug mode on my device. The app is an upgraded version of Xamarin Form to .NET MAUI. I have attached a crash report. I would appreciate any help/insights on how to resolve. --John
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122
Apr ’25
Is there a way for two users to make development builds on separate accounts for one app?
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Dec ’25
Package created with pkgbuild installs zero-byte file
Just recently, any pkg file that I create with pkgbuild will install the Payload's application as a zero-byte file in the /Applications directory. This has been working for years without issue for me. Here are the commands I am using with company specific items replaced: pkgbuild --analyze --root MyApplicationRootDirectory standalone.plist plutil -replace BundleIsRelocatable -bool NO standalone.plist pkgbuild --identifier MyIdentifier --version 1.0 --install-location /Applications --root MyApplicationRootDirectory --component-plist standalone.plist --sign 'Developer ID Installer: MyCompany (MySignId)' --timestamp installer.pkg Any ideas on what could be causing the issue? I have verified the following: The application being added to the pkg is both signed and notarized using the correct Developer ID Application certificate. The resultant pkg file is both signed and notarized using the Developer ID Installer certificate. Verified the pkg contents using "pkgutil --expand" to dump the contents. Verified the pkg's Payload contents by extracting the data using "cat Payload | gunzip | cpio -1". This results in an application file that is a binary match for file added in the "pkgbuild --root" argument. My application is the only file within the directory passed to the "pkgbuild --root" argument. There are no warnings in the System Settings / Privacy & Security Panel when running the package installer. I have a valid Mac Developer account. I am building the application and the pkg file on the same computer. Thank you for any insight.
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257
Dec ’25
Need Step-by-Step Instructions for Submitting React Native/Expo App to TestFlight & App Store (Individual Account)
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91
Nov ’25
Running tests on multiple simulators and local mock servers
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301
Feb ’26
iOS 18.5 crash with iPad 7 only
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130
Jun ’25
Attach a debugger to app launched via `devicectrl`
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194
Oct ’25
I made a browser plugin to do something Apple should've done themselves.
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255
Apr ’25
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414
Nov ’25
Unstable behavior of xcodebuild -showdestinations
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333
Jun ’25
Unable to Enable “Media Services (MusicKit, ShazamKit)” When Creating a Key
I am trying to integrate Apple Music API using MusicKit and need to generate a Developer Token. However, when I try to create a new key from the Certificates, Identifiers &amp; Profiles section, the “Media Services (MusicKit, ShazamKit, Apple Music Feed)” option is grayed out. We are getting the error 'there are no identifiers available that can be associated with the key.' Although we did checkmark 'musickit' in app services. I have already: Enrolled in the paid Apple Developer Program Created a valid App ID under Identifiers Logged in as the Account Holder Tried multiple browsers and devices Despite this, the option remains disabled. Could you please enable this or let me know what further steps I need to take? Thank you!
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228
Apr ’25
Side loading an app
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87
Jun ’25