Complete LLDB command reference — variable inspection, breakpoints, threads, expression evaluation, process control, memory commands, and .lldbinit customization
Provides comprehensive LLDB command references for debugging tasks including breakpoints, variable inspection, and process control.
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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.
v / frame variableReads 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-descriptionCalls 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 readLow-level register inspection:
(lldb) register read
(lldb) register read x0 x1 # Specific registers (ARM64)
(lldb) register read --all # All register sets
(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
(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"'
(lldb) breakpoint set -f File.swift -l 42 -i 50 # Skip first 50 hits
(lldb) breakpoint set -f File.swift -l 42 -o # Delete after first hit
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"
(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)
(lldb) breakpoint set -n UIViewAlertForUnsatisfiableConstraints # Auto Layout
(lldb) breakpoint set -n "-[UIApplication _run]" # App launch
(lldb) breakpoint set -n swift_willThrow # Swift throw
(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
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.
(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
(lldb) thread list # List all threads with state
(lldb) thread info # Current thread details + stop reason
(lldb) thread select 3 # Switch to thread 3
(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
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.
(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")
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]
(lldb) expr -l objc -- (void)[[[UIApplication sharedApplication] keyWindow] recursiveDescription]
(lldb) po UIApplication.shared.windows.first?.rootViewController?.view.recursiveDescription()
(lldb) expr Self._printChanges() # Print what triggered body re-eval (inside view body only)
(lldb) expr type(of: someValue)
(lldb) expr String(describing: type(of: someValue))
(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
(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
(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
(lldb) memory find -s "searchString" -- 0x100000000 0x200000000
(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
LLDB reads ~/.lldbinit at startup. Per-project init files are also supported when configured in Xcode's scheme settings.
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]
# Show CLLocationCoordinate2D as "lat, lon"
type summary add CLLocationCoordinate2D --summary-string "${var.latitude}, ${var.longitude}"
(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
In Xcode: Edit Scheme → Run → Options → "LLDB Init File" field.
Put project-specific aliases and breakpoints in a .lldbinit file in your project root.
Cause: Swift expression parser can't resolve types from the current module.
Fixes:
v instead (no compilation needed)expr -l objc -- ... for ObjC-bridge typesCause: Compiler optimized the variable out.
Fixes:
-Onone for the specific file (Build Settings → per-file compiler flags)register read to check if the value is in a registerCause: LLDB defaults to ObjC in some contexts (especially in frameworks).
Fix:
(lldb) settings set target.language swift
(lldb) expr -l swift -- mySwiftExpression
Cause: The expression you evaluated had a side effect that crashed.
Fix:
v for read-only inspectionCause: Usually compiling a complex expression or resolving types in a large project.
Fix:
v instead of p/po (no compilation)po, Ctrl+C to cancel and use v insteadCauses 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 |
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
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