LLDB Debugging

Interactive debugging with LLDB. The debugger freezes time so you can interrogate your running app — inspect variables, evaluate expressions, navigate threads, and understand exactly why something went wrong.

Core insight: "LLDB is useless" really means "I don't know which command to use for Swift types." This is a knowledge-gap problem, not a tool problem.

Red Flags — Check This Skill When

LLDB vs Other Tools

digraph tool_selection {
    "What do you need?" [shape=diamond];

    "axiom-testflight-triage" [shape=box];
    "axiom-hang-diagnostics" [shape=box];
    "axiom-memory-debugging" [shape=box];
    "axiom-performance-profiling" [shape=box];
    "LLDB (this skill)" [shape=box, style=bold];

    "What do you need?" -> "axiom-testflight-triage" [label="Crash log from field,\ncan't reproduce locally"];
    "What do you need?" -> "axiom-hang-diagnostics" [label="App frozen,\nneed diagnosis approach"];
    "What do you need?" -> "axiom-memory-debugging" [label="Memory growing,\nneed leak pattern"];
    "What do you need?" -> "axiom-performance-profiling" [label="Need to measure\nCPU/memory over time"];
    "What do you need?" -> "LLDB (this skill)" [label="Need to inspect state\nat a specific moment"];
}

Rule of thumb: Instruments measures. LLDB inspects. If you need to understand what's happening at a specific moment in time, use LLDB. If you need to understand trends over time, use Instruments.

Response Format

When helping with LLDB debugging, structure your output as:

1. Immediate diagnosis (1-3 bullets, confidence-tagged: HIGH/MEDIUM/LOW)
2. Commands to run (numbered, copy-paste ready, with (lldb) prefix)
3. What to look for (command → expected output → interpretation)
4. Likely root causes (ranked by probability)
5. Next breakpoint plan (catch it earlier next time)
6. If no debugger attached (crash-log-only fallback path)

Playbook 1: Crash Triage

Goal: Understand why the app crashed, starting from the stop point.

Step 1: Read the Stop Reason

When the debugger stops, the first thing to check:

(lldb) thread info

This shows the stop reason. Common stop reasons:

Step 2: Get the Backtrace

(lldb) bt

Read top-to-bottom. Find the first frame in YOUR code (not system frameworks). That's where to start investigating.

(lldb) bt 10

Limit to 10 frames if the full trace is noisy.

Step 3: Navigate to Your Frame

(lldb) frame select 3

Jump to frame 3 (or whichever frame is in your code).

Step 4: Inspect State

(lldb) v
(lldb) v self.someProperty
(lldb) v localVariable

Use v (not po) for reliable Swift value inspection. See Playbook 3 for details.

Step 5: Classify and Fix

Step 6: Set a Conditional Breakpoint to Catch It Earlier

(lldb) breakpoint set -f MyFile.swift -l 42 -c "value == nil"

This breaks only when value is nil at line 42 — catches the problem before the crash.

Playbook 2: Hang/Deadlock Diagnosis

Goal: Understand why the app is frozen by inspecting all thread states.

Step 1: Pause the App

If the app is hung, press the pause button in Xcode (⌃⌘Y) or:

(lldb) process interrupt

Step 2: Get All Thread Backtraces

(lldb) thread backtrace all

Or the shorthand:

(lldb) bt all

Step 3: Classify Thread States

Look at Thread 0 (main thread) — it processes all UI events. If it's blocked, the app is frozen.

Main thread blocked on synchronous wait:

frame #0: libsystem_kernel.dylib`__psynch_mutexwait
frame #1: libsystem_pthread.dylib`_pthread_mutex_firstfit_lock_wait
...
frame #5: MyApp`ViewController.viewDidLoad()

Translation: Main thread is waiting for a mutex lock. Something else holds it.

Main thread blocked on dispatch_sync:

frame #0: libdispatch.dylib`_dispatch_sync_f_slow
...
frame #3: MyApp`DataManager.fetchData()

Translation: DispatchQueue.main.sync called from background → classic deadlock.

Main thread busy (CPU-bound):

frame #0: MyApp`ImageProcessor.processAllImages()
frame #1: MyApp`ViewController.viewDidLoad()

Translation: Expensive work on main thread. Move to background.

Step 4: Check for Deadlocks

If two threads are both waiting on something the other holds:

(lldb) thread list

Look for multiple threads with state waiting that reference each other's locks.

Step 5: Inspect Specific Thread

(lldb) thread select 3
(lldb) bt
(lldb) v

Switch to another thread to inspect its state.

Cross-reference: For fix patterns once you've identified the hang cause → $axiom-hang-diagnostics

Playbook 3: Swift Value Inspection

This is the core value of this skill. Most developers abandon LLDB because po doesn't work reliably with Swift types. Here's what actually works.

The Four Print Commands

When to Use Each

Start with v — it's fastest and most reliable for stored properties:

(lldb) v self.userName
(lldb) v self.items[0]
(lldb) v localStruct

v works by reading memory directly. It doesn't compile anything, so it can't fail due to expression compilation errors.

v limitation: It only reads stored properties — computed properties, lazy var (before first access), and property wrapper projected values ($binding) won't show meaningful values. If a field looks wrong or missing with v, try p instead.

Use p when v can't reach it:

(lldb) p self.computedProperty
(lldb) p self.items.count
(lldb) p someFunction()

p compiles and executes the expression. Needed for computed properties and function calls.

Use po for class descriptions:

(lldb) po myObject
(lldb) po error
(lldb) po notification

po calls debugDescription on the result. Best for objects that have meaningful descriptions (NSError, Notification, etc.).

The "LLDB Is Broken" Moments

Inspecting Optionals

(lldb) v optionalValue

Shows: (String?) some = "hello" or (String?) none

Don't use po optionalValue — it may show just Optional("hello") which is less useful.

Inspecting Collections

(lldb) v myArray
(lldb) v myArray[2]
(lldb) v myDict

For large collections, limit output:

(lldb) p Array(myArray.prefix(5))

Inspecting SwiftUI State

SwiftUI @State is backed by stored properties with underscore prefix:

(lldb) v self._isPresented
(lldb) v self._items

For @Observable models:

(lldb) v self.viewModel.propertyName

Diagnosing "view doesn't update": If a property changes (confirmed with v) but the SwiftUI view doesn't re-render, check which thread the mutation happens on with bt. @Observable mutations must happen on @MainActor for SwiftUI to observe them — mutations on a background actor won't trigger view updates. Use Self._printChanges() inside a view body to see which property triggered (or didn't trigger) a re-render:

(lldb) expr Self._printChanges()

For the full observation diagnostic tree → $axiom-swiftui-debugging

Inspecting Actors

Actor state is best inspected with v, which reads memory directly without isolation concerns:

(lldb) v actor

Shows all stored properties. This works because LLDB pauses the entire process — you can read any memory regardless of actor isolation (which is a compile-time concept).

Modifying Values at Runtime

(lldb) expr self.debugFlag = true
(lldb) expr myArray.append("test")
(lldb) expr self.view.backgroundColor = UIColor.red

Modify values without rebuilding. Useful for testing theories.

Referencing Previous Results

LLDB assigns result variables ($R0, $R1, etc.):

(lldb) p someValue
$R0 = 42
(lldb) p $R0 + 10
$R1 = 52

Playbook 4: Breakpoint Strategies

Source Breakpoints (Basic)

(lldb) breakpoint set -f ViewController.swift -l 42
(lldb) b ViewController.swift:42

Short form b works for simple cases.

Conditional Breakpoints

Break only when a condition is true:

(lldb) breakpoint set -f MyFile.swift -l 42 -c "index > 100"
(lldb) breakpoint set -f MyFile.swift -l 42 -c "name == \"test\""

Iteration-based: Break after N hits:

(lldb) breakpoint set -f MyFile.swift -l 42 -i 50

Ignores the first 50 hits, then breaks.

Logpoints (Action + Auto-Continue)

Log without stopping — like a print statement but no rebuild needed:

(lldb) breakpoint set -f MyFile.swift -l 42
(lldb) breakpoint command add 1
> v self.value
> continue
> DONE

Or in Xcode: Edit breakpoint → Add Action → "Log Message" → use @self.value@ token syntax → Check "Automatically continue"

Symbolic Breakpoints

Break on ANY call to a method by name:

(lldb) breakpoint set -n viewDidLoad
(lldb) breakpoint set -n "MyClass.myMethod"

Break on all ObjC messages to a selector:

(lldb) breakpoint set -S "layoutSubviews"

Exception Breakpoints

Swift errors (break on throw):

(lldb) breakpoint set -E swift

Objective-C exceptions (break on throw):

(lldb) breakpoint set -E objc

In Xcode: Breakpoint Navigator → + → Swift Error Breakpoint / Exception Breakpoint

This is the single most useful breakpoint for crash debugging. It stops at the throw site instead of the catch/crash site.

Watchpoints

Break when a variable's value changes:

(lldb) watchpoint set variable self.count
(lldb) watchpoint set variable -w read_write myGlobal

Watchpoints are hardware-backed — limited to ~4 per process but very fast.

One-Shot Breakpoints

Break once, then auto-delete:

(lldb) breakpoint set -f MyFile.swift -l 42 -o

Managing Breakpoints

(lldb) breakpoint list
(lldb) breakpoint disable 3
(lldb) breakpoint enable 3
(lldb) breakpoint delete 3
(lldb) breakpoint delete

Playbook 5: Async/Concurrency Debugging

Identifying Async Frames

Swift concurrency backtraces are noisy — expect swift_task_switch, _dispatch_call_block_and_release, and executor internals mixed in with your code. Don't be discouraged by 40+ frames of runtime noise. Focus on frames from YOUR module.

In Swift concurrency backtraces, look for swift-task frames:

Thread 3:
frame #0: MyApp`MyActor.doWork()
frame #1: swift_task_switch
frame #2: MyApp`closure #1 in ViewController.loadData()

The swift_task_switch frame indicates an async suspension point. Your code frames are the ones prefixed with your module name (MyApp above).

Inspecting Task State

(lldb) thread backtrace all

Look for threads with swift_task in their frames. Each represents an active Swift task.

Actor-Isolated Code

When stopped inside an actor:

(lldb) v self

Shows all actor state. This works because LLDB pauses the entire process — actor isolation is a compile-time concept, not a runtime lock (for default actors).

Task Group Inspection

When debugging task groups, break inside the group closure and inspect:

(lldb) v
(lldb) bt

Each child task runs on its own thread. Use bt all to see them.

Cross-reference: For Swift concurrency patterns and fix strategies → $axiom-swift-concurrency. For profiling async performance → $axiom-concurrency-profiling

Pressure Scenarios

Scenario 1: "Release-Only Crash — LLDB Is Useless in Release"

Situation: Crash happens in Release builds but not Debug. Team says "we can't debug it."

Why this fails: Release optimizations change timing, memory layout, and can eliminate variables — making the crash non-reproducible in Debug.

Correct approach:

1. Build with Debug configuration but Release-like settings:
   • Optimization Level: -O (not -Onone)
   • Still include debug symbols (DEBUG_INFORMATION_FORMAT = dwarf-with-dsym)
2. Enable Address Sanitizer (-fsanitize=address) — catches memory errors with 2-3x overhead
3. Use the crash report to set breakpoints at the crash site
4. Set exception breakpoints to catch the error before the crash:

   (lldb) breakpoint set -E swift
   (lldb) breakpoint set -E objc
   

5. If variable shows <optimized out>, reduce optimization for that one file:
   • Build Settings → Per-file flags → -Onone for the specific file
6. Last resort — read register values directly (variables live in registers before being optimized out):

   (lldb) register read
   (lldb) register read x0 x1 x2
   

   On ARM64: x0 = self, x1-x7 = first 7 arguments. Check $axiom-lldb-ref Part 1 for details.

Scenario 2: "Just Add Print Statements"

Situation: Developer adds print() calls to debug, rebuilds, runs, reads console. Repeat.

Why this fails: Each print-debug cycle costs 3-5 minutes (edit → build → run → navigate to state → read output). An LLDB breakpoint costs 30 seconds.

Correct approach:

1. Set a breakpoint at the line you'd add a print():

   (lldb) b MyFile.swift:42
   

2. Add a logpoint for "print-like" behavior without rebuilding:
   • Edit breakpoint → Add Action → Log Message → Check "Auto continue"
3. Inspect variables directly: v self.someValue
4. Modify variables at runtime to test theories: expr self.debugMode = true
5. One breakpoint session replaces 5-10 print-debug cycles.

Time comparison (typical control-flow debugging):

Exception: In tight loops (thousands of hits/sec), logpoints add per-hit overhead. Use -i to skip to the iteration you care about, or use a temporary print() for that specific loop.

Scenario 3: "po Doesn't Work So LLDB Is Broken"

Situation: Developer types po myStruct and gets garbage. Concludes LLDB is broken for Swift. Goes back to print debugging.

This is the #1 reason developers abandon LLDB.

Why po fails with Swift structs: po calls debugDescription which requires compiling an expression in the debugger context. For Swift structs, this compilation often fails due to missing type metadata, generics, or module resolution issues.

Correct approach:

1. Use v instead of po — reads memory directly, no compilation:

   (lldb) v myStruct
   (lldb) v myStruct.propertyName
   

2. Use p for computed properties:

   (lldb) p myStruct.computedValue
   

3. Use po only for classes with CustomDebugStringConvertible
4. If p also fails, try specifying the language:

   (lldb) expr -l objc -- (id)0x12345
   

5. If everything fails, v self always works inside a method.

Anti-Patterns

Debugging Checklist

Before starting a debug session:

• Debug build configuration (not Release)
• Exception breakpoints enabled (Swift Error + ObjC Exception)
• Breakpoint set before suspected problem area
• Know which command to use: v for values, p for computed, po for descriptions

During debug session:

• Read stop reason (thread info) before anything else
• Get backtrace (bt) — find your frame
• Navigate to your frame (frame select N)
• Inspect relevant state (v self, v localVar)
• Understand the cause before writing any fix

After finding the issue:

• Set conditional breakpoint to catch recurrence
• Consider adding assertion/precondition for this case
• Remove temporary breakpoints

Resources

WWDC: 2019-429, 2018-412, 2022-110370

Docs: /xcode/stepping-through-code-and-inspecting-variables-to-isolate-bugs, /xcode/setting-breakpoints-to-pause-your-running-app, /xcode/diagnosing-memory-thread-and-crash-issues-early

Skills: axiom-lldb-ref, axiom-testflight-triage, axiom-hang-diagnostics, axiom-memory-debugging, axiom-swift-concurrency, axiom-concurrency-profiling