Apple Platform Crash Log .NET Symbolication

Resolves native backtrace frames from .NET MAUI and Mono app crashes on Apple platforms (iOS, tvOS, Mac Catalyst, macOS) to function names, source files, and line numbers using Mach-O UUIDs and dSYM debug symbol bundles.

Inputs: Crash log file (.ips JSON format, iOS 15+ / macOS 12+), atos (from Xcode), optionally a connected iOS device to pull crash logs from.

Do not use when: The crashing library is not a .NET component (e.g., pure Swift/UIKit), or the crash log is an Android tombstone.

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Workflow

Step 1: Parse the .ips Crash Log

Format check: Before proceeding, verify the file is .ips JSON format. The first line must be valid JSON. If the file is plain text (e.g., Android tombstone with #NN pc frame lines, or legacy Apple .crash text format), stop immediately — this workflow does not apply. Report the format mismatch to the user and do not attempt any symbolication.

The .ips file is two-part JSON: line 1 is a metadata header; the remaining lines are a separate JSON crash body. Parse them separately:

lines = open('crash.ips').readlines()
metadata = json.loads(lines[0])           # app_name, bundleID, os_version, slice_uuid
crash    = json.loads(''.join(lines[1:])) # Full crash report

Key fields in the crash body:

• usedImages[N] has name, base (load address), uuid, arch for each loaded binary
• threads[N].frames[M] has imageOffset, imageIndex; frame address = usedImages[imageIndex].base + imageOffset
• exception.type, exception.signal (e.g., EXC_CRASH / SIGABRT)
• asi (Application Specific Information) often contains the managed exception message
• lastExceptionBacktrace has frames from the exception that triggered the crash
• faultingThread is the index into the threads array

Parsing gotcha: Some .ips files have case-conflicting duplicate keys (vmRegionInfo / vmregioninfo). Pre-process the raw JSON to rename the lowercase duplicate before parsing. The asi field may be absent.

Step 2: Identify .NET Runtime Libraries

Filter usedImages to .NET runtime libraries:

Library	Runtime
libcoreclr	CoreCLR runtime
libmonosgen-2.0	Mono runtime
libSystem.Native	.NET BCL native component
libSystem.Globalization.Native	.NET BCL globalization
libSystem.Security.Cryptography.Native.Apple	.NET BCL crypto
libSystem.IO.Compression.Native	.NET BCL compression
libSystem.Net.Security.Native	.NET BCL net security

On Apple platforms these ship as .framework bundles, so image names may omit .dylib. Match using substring (e.g., libcoreclr not libcoreclr.dylib). The app binary may appear twice in usedImages with different UUIDs.

Key bridge functions in the app binary: xamarin_process_managed_exception (managed exception bridged to ObjC NSException), xamarin_main, mono_jit_exec, coreclr_execute_assembly.

NativeAOT: Runtime is statically linked into the app binary. libSystem.* BCL libraries remain separate. The app binary needs its own dSYM from the build output.

Skip libsystem_kernel.dylib, UIKitCore, and other Apple system frameworks unless specifically asked.

Step 3: Interpret the Crash

Start with asi (Application Specific Information) — for .NET crashes, it often contains the managed exception type and message (e.g., XamlParseException, NullReferenceException). The root cause may already be visible here.

Then examine the faulting thread (threads[faultingThread]). Explain what frames #0 and #1 mean before examining other threads. Cross-thread context (GC state, thread pool) is useful for validation but not evidence of causation.

Also check lastExceptionBacktrace for the managed exception path through bridge functions like xamarin_process_managed_exception.

Sometimes the .NET runtime version is visible in image paths in usedImages, particularly on macOS when using shared-framework installs or NuGet-pack-style layouts (e.g., .../Microsoft.NETCore.App/10.0.4/libcoreclr.dylib). On iOS, however, image paths are typically inside the app bundle (for example, .../Frameworks/libcoreclr.framework/libcoreclr) and do not embed the runtime version, so you usually need to infer it via the Mach-O UUID by matching against SDK packs or symbol-server downloads rather than relying on the path alone.

Step 4: Locate dSYMs

For each .NET library needing symbolication, locate a UUID-matched dSYM:

1. Microsoft symbol server (automatic): Download .dwarf via https://msdl.microsoft.com/download/symbols/_.dwarf/mach-uuid-sym-{UUID}/_.dwarf (UUID lowercase, no dashes). Convert to .dSYM bundle (use the image name from usedImages[].name, e.g., libcoreclr):

   mkdir -p libcoreclr.dSYM/Contents/Resources/DWARF
   cp _.dwarf libcoreclr.dSYM/Contents/Resources/DWARF/libcoreclr
   

2. Build output: bin/Debug/net*-ios/ios-arm64/<App>.app.dSYM/
3. SDK packs: $DOTNET_ROOT/packs/Microsoft.NETCore.App.Runtime.<rid>/<version>/runtimes/<rid>/native/
4. NuGet cache: ~/.nuget/packages/microsoft.netcore.app.runtime.<rid>/<version>/runtimes/<rid>/native/
5. dotnet-symbol: dotnet-symbol --symbols -o symbols-out <path-to-binary.dylib>

Always verify: dwarfdump --uuid <dsym> must match the UUID from the crash log exactly.

Step 5: Symbolicate with atos

atos -arch arm64 -o <path.dSYM/Contents/Resources/DWARF/binary_name> -l <load_address> <frame_addresses...>

• -o points to the DWARF binary inside the .dSYM bundle (Contents/Resources/DWARF/), not the bundle itself
• -l is the load address from usedImages[N].base
• Use the arch from usedImages[N].arch (usually arm64, may be arm64e)
• Pass multiple addresses per invocation for batch symbolication

# Example: symbolicate libcoreclr frames
atos -arch arm64 -o libcoreclr.dSYM/Contents/Resources/DWARF/libcoreclr -l 0x104000000 0x104522098 0x1043c0014

Strip the /__w/1/s/ CI workspace prefix from output — meaningful paths start at src/runtime/, mapping to the dotnet/dotnet VMR.

Automation Script

scripts/Symbolicate-Crash.ps1 automates the full workflow (parsing, dSYM lookup, symbol download, and symbolication). Resolve the path relative to this SKILL.md file.

# $SKILL_DIR is the directory containing this SKILL.md
pwsh "$SKILL_DIR/scripts/Symbolicate-Crash.ps1" -CrashFile MyApp-2026-02-25.ips

Start with -ParseOnly for a fast overview without requiring atos. The script automatically downloads symbols from the Microsoft symbol server when local dSYMs are missing.

Flags: -CrashingThreadOnly, -OutputFile path, -ParseOnly, -SkipVersionLookup, -SkipSymbolDownload, -SymbolCacheDir path, -DsymSearchPaths path1,path2.

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Retrieving Crash Logs

Pull crash logs from a connected iOS device using idevicecrashreport (from libimobiledevice):

idevicecrashreport -e /tmp/crashlogs/
find /tmp/crashlogs/ -iname '*MyApp*' -name '*.ips'

Also available in Xcode > Window > Devices and Simulators > View Device Logs, or at ~/Library/Logs/CrashReporter/ (Mac Catalyst), ~/Library/Logs/DiagnosticReports/ (macOS).

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Validation

1. dwarfdump --uuid <dsym> matches UUID from the crash log
2. At least one .NET frame resolves to a function name (not a raw address)
3. Resolved paths contain recognizable .NET runtime structure (e.g., src/coreclr/, mono/metadata/, mono/mini/)

Stop Signals

• Wrong file format: If the file is not .ips JSON (e.g., Android tombstone with #NN pc stack frames, legacy .crash text format), stop immediately — report the format mismatch to the user and do not proceed with any symbolication. Do not attempt to symbolicate using other tools or workflows.
• No .NET frames found: Report parsed frames and stop.
• All frames resolved: Present symbolicated backtrace with brief crash analysis (faulting thread, exception type, likely area). If the user asks for deeper investigation, proceed.
• dSYM not available / UUID mismatch: Report unsymbolicated frames with UUIDs and addresses. Suggest locating the original build artifacts.
• atos not available: Present the manual atos commands for the user to run. Do not install Xcode. atos ships with Xcode Command Line Tools (xcode-select --install).

References

• IPS format details: See references/ips-crash-format.md for additional .ips parsing details and macOS symbol package differences.