Axiom Lldb Ref

LLDB Command Reference

Complete command reference for LLDB in Xcode. Organized by task so you can find the exact command you need.

For debugging workflows and decision trees, see $axiom-lldb.

Part 1: Variable Inspection

v / frame variable

Reads memory directly. No compilation. Most reliable for Swift values.

(lldb) v                              # All variables in current frame
(lldb) v self                         # Self in current context
(lldb) v self.propertyName            # Specific property
(lldb) v localVariable                # Local variable
(lldb) v self.array[0]               # Collection element
(lldb) v self._showDetails            # SwiftUI @State backing store (underscore prefix)

Flags:

Limitations: Cannot evaluate expressions, computed properties, or function calls. Use p for those.

p / expression (with format)

Compiles and executes an expression. Shows formatted result.

(lldb) p self.computedProperty
(lldb) p items.count
(lldb) p someFunction()
(lldb) p String(describing: someValue)
(lldb) p (1...10).map { $0 * 2 }

Result stored in numbered variables:

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

po / expression --object-description

Calls debugDescription (or description) on the result.

(lldb) po myObject
(lldb) po error
(lldb) po notification.userInfo
(lldb) po NSHomeDirectory()

When po adds value: Classes with CustomDebugStringConvertible, NSError, NSNotification, collections of objects.

When po fails: Swift structs without CustomDebugStringConvertible, protocol-typed values (use v instead — it performs iterative dynamic type resolution that po doesn't).

expression (full form)

Full expression evaluation with all options.

(lldb) expression self.view.backgroundColor = UIColor.red
(lldb) expression self.debugFlag = true
(lldb) expression myArray.append("test")
(lldb) expression CATransaction.flush()           # Force UI update
(lldb) expression Self._printChanges()             # SwiftUI debug

Flags:

ObjC expressions for Swift debugging:

(lldb) expr -l objc -- (void)[[[UIApplication sharedApplication] keyWindow] recursiveDescription]
(lldb) expr -l objc -- (void)[CATransaction flush]
(lldb) expr -l objc -- (int)[[UIApplication sharedApplication] _isForeground]

register read

Low-level register inspection:

(lldb) register read
(lldb) register read x0 x1                # Specific registers (ARM64)
(lldb) register read --all                # All register sets

Part 2: Breakpoints

Setting Breakpoints

(lldb) breakpoint set -f File.swift -l 42                # File + line
(lldb) b File.swift:42                                    # Short form
(lldb) breakpoint set -n methodName                       # By function name
(lldb) breakpoint set -n "MyClass.myMethod"               # Qualified name
(lldb) breakpoint set -S layoutSubviews                   # ObjC selector
(lldb) breakpoint set -r "viewDid.*"                      # Regex on name
(lldb) breakpoint set -a 0x100abc123                      # Memory address

Conditional Breakpoints

(lldb) breakpoint set -f File.swift -l 42 -c "value == nil"
(lldb) breakpoint set -f File.swift -l 42 -c "index > 100"
(lldb) breakpoint set -f File.swift -l 42 -c 'name == "test"'

Ignore Count

(lldb) breakpoint set -f File.swift -l 42 -i 50          # Skip first 50 hits

One-Shot Breakpoints

(lldb) breakpoint set -f File.swift -l 42 -o             # Delete after first hit

Breakpoint Commands (Logpoints)

Add commands that execute when breakpoint hits:

(lldb) breakpoint command add 1
> v self.state
> p self.items.count
> continue
> DONE

Or in one line:

(lldb) breakpoint command add 1 -o "v self.state"

Exception Breakpoints

(lldb) breakpoint set -E swift                            # All Swift errors
(lldb) breakpoint set -E objc                             # All ObjC exceptions
# Filtering by exception name requires Xcode's GUI (Edit Breakpoint → Exception field)

Symbolic Breakpoints

(lldb) breakpoint set -n UIViewAlertForUnsatisfiableConstraints    # Auto Layout
(lldb) breakpoint set -n "-[UIApplication _run]"                    # App launch
(lldb) breakpoint set -n swift_willThrow                            # Swift throw

Managing Breakpoints

(lldb) breakpoint list                     # List all
(lldb) breakpoint list -b                  # Brief format
(lldb) breakpoint enable 3                 # Enable breakpoint 3
(lldb) breakpoint disable 3                # Disable breakpoint 3
(lldb) breakpoint delete 3                 # Delete breakpoint 3
(lldb) breakpoint delete                   # Delete ALL (asks confirmation)
(lldb) breakpoint modify 3 -c "x > 10"    # Add condition to existing

Watchpoints

Break when a variable's memory changes:

(lldb) watchpoint set variable self.count                  # Watch for write
(lldb) watchpoint set variable -w read_write myGlobal      # Watch for read or write
(lldb) watchpoint set expression -- &myVariable            # Watch memory address
(lldb) watchpoint list                                     # List all
(lldb) watchpoint delete 1                                 # Delete watchpoint 1
(lldb) watchpoint modify 1 -c "self.count > 10"            # Add condition

Note: Hardware watchpoints are limited (~4 per process). Use sparingly.

Part 3: Thread & Backtrace

Backtraces

(lldb) bt                              # Current thread backtrace
(lldb) bt 10                           # Limit to 10 frames
(lldb) bt all                          # All threads
(lldb) thread backtrace all            # Same as bt all

Thread Navigation

(lldb) thread list                     # List all threads with state
(lldb) thread info                     # Current thread details + stop reason
(lldb) thread select 3                 # Switch to thread 3

Frame Navigation

(lldb) frame info                      # Current frame details
(lldb) frame select 5                  # Jump to frame 5
(lldb) up                              # Go up one frame (toward caller)
(lldb) down                            # Shortcut: go down one frame

Thread Return (Skip Code)

Force an early return from the current function:

(lldb) thread return                   # Return void
(lldb) thread return 42                # Return specific value

Use with caution — skips cleanup code, can leave state inconsistent.

Part 4: Expression Evaluation

Swift Expressions

(lldb) expr let x = 42; print(x)
(lldb) expr self.view.backgroundColor = UIColor.red
(lldb) expr UIApplication.shared.windows.first?.rootViewController
(lldb) expr UserDefaults.standard.set(true, forKey: "debug")

Objective-C Expressions

Switch to ObjC when Swift expression parser fails:

(lldb) expr -l objc -- (void)[CATransaction flush]
(lldb) expr -l objc -- (id)[[UIApplication sharedApplication] keyWindow]
(lldb) expr -l objc -- (void)[[NSNotificationCenter defaultCenter] postNotificationName:@"test" object:nil]

UI Debugging Expressions

(lldb) expr -l objc -- (void)[[[UIApplication sharedApplication] keyWindow] recursiveDescription]
(lldb) po UIApplication.shared.windows.first?.rootViewController?.view.recursiveDescription()

SwiftUI Debugging

(lldb) expr Self._printChanges()                # Print what triggered body re-eval (inside view body only)

Runtime Type Information

(lldb) expr type(of: someValue)
(lldb) expr String(describing: type(of: someValue))

Part 5: Process Control

Execution Control

(lldb) continue                        # Resume execution (c)
(lldb) c                               # Short form
(lldb) process interrupt               # Pause running process
(lldb) thread step-over                # Step over (n / next)
(lldb) n                               # Short form
(lldb) thread step-in                  # Step into (s / step)
(lldb) s                               # Short form
(lldb) thread step-out                 # Step out (finish)
(lldb) finish                          # Short form
(lldb) thread step-inst                # Step one instruction (assembly-level)
(lldb) ni                              # Step over one instruction

Process Management

(lldb) process launch                  # Launch/restart
(lldb) process attach --pid 1234       # Attach to running process
(lldb) process attach --name MyApp     # Attach by name
(lldb) process detach                  # Detach without killing
(lldb) kill                            # Kill debugged process

Part 6: Memory & Image

Memory Reading

(lldb) memory read 0x100abc123                    # Read memory at address
(lldb) memory read -c 64 0x100abc123              # Read 64 bytes
(lldb) memory read -f x 0x100abc123               # Format as hex
(lldb) memory read -f s 0x100abc123               # Format as string

Memory Search

(lldb) memory find -s "searchString" -- 0x100000000 0x200000000

Image/Module Inspection

(lldb) image lookup -a 0x100abc123                # Lookup symbol at address
(lldb) image lookup -n myFunction                 # Find function by name
(lldb) image lookup -rn "MyClass.*"               # Regex search
(lldb) image list                                 # List all loaded images/frameworks
(lldb) image list -b                              # Brief format

Common use: Finding which framework a crash address belongs to:

(lldb) image lookup -a 0x1a2b3c4d5

Part 7: .lldbinit & Customization

File Location

LLDB reads ~/.lldbinit at startup. Per-project init files are also supported when configured in Xcode's scheme settings.

Useful Aliases

Add to ~/.lldbinit:

# Quick reload — flush UI changes made via expression
command alias flush expr -l objc -- (void)[CATransaction flush]

# Print view hierarchy
command alias views expr -l objc -- (void)[[[UIApplication sharedApplication] keyWindow] recursiveDescription]

# Print auto layout constraints
command alias constraints po [[UIWindow keyWindow] _autolayoutTrace]

Custom Type Summaries

# Show CLLocationCoordinate2D as "lat, lon"
type summary add CLLocationCoordinate2D --summary-string "${var.latitude}, ${var.longitude}"

Settings

(lldb) settings show target.language              # Current language
(lldb) settings set target.language swift          # Force Swift mode
(lldb) settings set target.max-children-count 100  # Show more collection items

Per-Project .lldbinit

In Xcode: Edit Scheme → Run → Options → "LLDB Init File" field.

Put project-specific aliases and breakpoints in a .lldbinit file in your project root.

Part 8: Troubleshooting LLDB Itself

"expression failed to parse"

Cause: Swift expression parser can't resolve types from the current module.

Fixes:

1. Use v instead (no compilation needed)
2. Simplify the expression
3. Try expr -l objc -- ... for ObjC-bridge types
4. Clean derived data and rebuild

"variable not available"

Cause: Compiler optimized the variable out.

Fixes:

1. Switch to Debug build configuration
2. Set -Onone for the specific file (Build Settings → per-file compiler flags)
3. Use register read to check if the value is in a register

"wrong language mode"

Cause: LLDB defaults to ObjC in some contexts (especially in frameworks).

Fix:

(lldb) settings set target.language swift
(lldb) expr -l swift -- mySwiftExpression

"expression caused a crash"

Cause: The expression you evaluated had a side effect that crashed.

Fix:

1. Don't evaluate expressions that modify state unless you intend to
2. Use v for read-only inspection
3. If the crash corrupted state, restart the debug session

LLDB Hangs or Is Slow

Cause: Usually compiling a complex expression or resolving types in a large project.

Fix:

1. Use v instead of p/po (no compilation)
2. Reduce expression complexity
3. If LLDB hangs during po, Ctrl+C to cancel and use v instead

Breakpoint Not Hit

Causes and fixes:

Resources

WWDC: 2019-429, 2018-412, 2022-110370, 2015-402

Docs: /xcode/stepping-through-code-and-inspecting-variables-to-isolate-bugs, /xcode/setting-breakpoints-to-pause-your-running-app

Skills: axiom-lldb, axiom-xcode-debugging