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 /skill 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:

Flag Effect

-d run

Run dynamic type resolution (slower but more accurate)

-T

Show types

-R

Show raw (unformatted) output

-D N

Limit depth of nested types to N levels

-P N

Limit pointer depth to N levels

-F

Flat output (no hierarchy)

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:

Flag Effect

-l objc

Evaluate as Objective-C

-l swift

Evaluate as Swift (default)

-O

Object description (same as po )

-i false

Stop on breakpoints hit during evaluation (default: ignore)

--

Separator between flags and expression

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:

• Use v instead (no compilation needed)

• Simplify the expression

• Try expr -l objc -- ... for ObjC-bridge types

• Clean derived data and rebuild

"variable not available"

Cause: Compiler optimized the variable out.

Fixes:

• Switch to Debug build configuration

• Set -Onone for the specific file (Build Settings → per-file compiler flags)

• 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:

• Don't evaluate expressions that modify state unless you intend to

• Use v for read-only inspection

• 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:

• Use v instead of p /po (no compilation)

• Reduce expression complexity

• If LLDB hangs during po , Ctrl+C to cancel and use v instead

Breakpoint Not Hit

Causes and fixes:

Cause Fix

Wrong file/line (code moved) Re-set breakpoint on current code

Breakpoint disabled breakpoint enable N

Code not executed Verify the code path is reached

Optimized out (Release) Switch to Debug configuration

In a framework/SPM package Set symbolic breakpoint by function name

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