lldb

LLDB

Purpose

Guide agents through LLDB sessions and map existing GDB knowledge to LLDB. Covers command differences, Apple specifics, Python scripting, and IDE integration.

Triggers

• "I'm on macOS and need to debug a C++ program"
• "How does LLDB differ from GDB?"
• "How do I do [GDB command] in LLDB?"
• "LLDB shows <unavailable> for variables"
• "How do I use LLDB in VS Code?"
• "How do I write an LLDB Python script?"

Workflow

1. Start LLDB

lldb ./prog                         # load binary
lldb ./prog -- arg1 arg2            # with arguments
lldb -p 12345                       # attach to PID
lldb -c core.1234                   # load core dump
lldb ./prog core.1234               # binary + core

2. GDB → LLDB command map

Source: https://lldb.llvm.org/use/map.html

GDB	LLDB	Notes
run [args]	process launch [args] / r
continue	process continue / c
next	thread step-over / n
step	thread step-in / s
nexti	thread step-inst-over / ni
stepi	thread step-inst / si
finish	thread step-out / finish
break main	breakpoint set -n main / b main
break file.c:42	breakpoint set -f file.c -l 42 / b file.c:42
break *0x400abc	breakpoint set -a 0x400abc / b -a 0x400abc
watch x	watchpoint set variable x / wa s v x
print x	frame variable x / p x
print/x x	p/x x
info locals	frame variable / fr v
info args	frame variable --arguments
backtrace	thread backtrace / bt
frame N	frame select N / f N
info threads	thread list
thread N	thread select N
thread apply all bt	thread backtrace all
x/10wx addr	memory read -s4 -fx -c10 addr / x/10xw addr
set var = 42	expression var = 42 / expr var = 42
quit	quit / q

3. Breakpoints

# By name
b main
breakpoint set --name foo
breakpoint set --name foo --condition 'x > 0'

# By file:line
b file.c:42
breakpoint set --file file.c --line 42

# By address
b -a 0x100003f20

# By regex
breakpoint set --func-regex '^MyClass::'

# List
breakpoint list / br l

# Delete
breakpoint delete 2

# Disable/enable
breakpoint disable 1
breakpoint enable 1

# Commands on hit
breakpoint command add 1
  > p x
  > continue
  > DONE

4. Inspect state

# Print variable
p x
frame variable x
p *ptr
p arr[0]

# Print expression
expression x * 2 + 1
expr (int)sqrt(9.0)

# All locals
frame variable
fr v -a          # include arguments

# Registers
register read
register read rip rsp

# Memory
memory read --size 4 --format x --count 10 0x7fff0000
x/10xw 0x7fff0000          # GDB-compatible syntax

# Type info
image lookup --type MyClass
type lookup MyClass

5. Watchpoints

watchpoint set variable x           # write watchpoint
watchpoint set variable -w read x   # read watchpoint
watchpoint set variable -w read_write x
watchpoint set expression -- &x     # by address

watchpoint list
watchpoint delete 1

6. Threads

thread list
thread select 3
thread backtrace all
thread backtrace --count 5           # limit depth

# Per-thread stepping
thread step-over                     # step this thread only

7. macOS / Apple specifics

# Symbol lookup in shared cache
image lookup --address 0x18ab12345
image lookup --name objc_msgSend

# Objective-C method breakpoint
b "-[NSArray objectAtIndex:]"
b "+[NSString stringWithFormat:]"

# Inspect Objective-C object
po myObject                          # print-object (calls -description)
po [arr count]

# Show loaded libraries
image list
image list -b                        # brief (names only)

8. VS Code integration

Install the CodeLLDB extension. .vscode/launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug (lldb)",
      "type": "lldb",
      "request": "launch",
      "program": "${workspaceFolder}/build/prog",
      "args": [],
      "cwd": "${workspaceFolder}",
      "preLaunchTask": "build"
    }
  ]
}

9. LLDB Python scripting

import lldb

def print_all_threads(debugger, command, result, internal_dict):
    target = debugger.GetSelectedTarget()
    process = target.GetProcess()
    for thread in process:
        print(f"Thread {thread.GetIndexID()}: {thread.GetName()}")
        for frame in thread:
            print(f"  {frame}")

def __lldb_init_module(debugger, internal_dict):
    debugger.HandleCommand('command script add -f myscript.print_all_threads pthreads')

Load: command script import /path/to/myscript.py

For a full GDB↔LLDB command map, see references/gdb-lldb-map.md.

Related skills

• Use skills/debuggers/gdb for GDB workflows
• Use skills/debuggers/core-dumps for core dump analysis
• Use skills/compilers/clang for building with debug info