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mohitmishra786/low-level-dev-skills/lldb

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version: "1.0.1" name: lldb description: LLDB debugger skill for C/C++/Swift/Objective-C programs. Use when debugging with LLDB on macOS, FreeBSD, or Linux-clang environments, mapping GDB mental models to LLDB commands, using LLDB in Xcode or VS Code, or debugging Swift/Objective-C. Activates on queries about LLDB commands, GDB to LLDB migration, Apple platform debugging, LLDB Python scripting, or IDE-integrated debugging with clang-built binaries. user-invocable: true triggers:

debug with LLDB
GDB to LLDB migration
LLDB commands
debug Swift with LLDB
LLDB in Xcode
LLDB Python scripting
Apple platform debugging LLDB
LLDB VS Code integration

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

bash

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

lldb

# 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

lldb

# 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

lldb

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

lldb

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

lldb

# 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:

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

python

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

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