LLDB Debugger

TL;DR

LLDB is the LLVM-project debugger, native on macOS and preferred with Clang on Linux. Same capabilities as GDB — different command syntax. If you know GDB, see the mapping table at the bottom.

Starting LLDB

bash

# Compile with debug info
clang++ -g -O0 myapp.cpp -o myapp

# Launch
lldb myapp
lldb -- myapp arg1 arg2        # with arguments
lldb -p <PID>                  # attach to running process
lldb myapp -c core.dump        # open core dump

Essential Commands

cpp

Godbolt

(lldb) run [args]               start the program
(lldb) continue                 continue after stop
(lldb) next                     step over (source line)
(lldb) step                     step into
(lldb) finish                   step out of current function
(lldb) quit                     exit LLDB

(lldb) breakpoint set -n main               break at function main
(lldb) breakpoint set -f file.cpp -l 42     break at file:line
(lldb) breakpoint set -n "ClassName::method"
(lldb) breakpoint list                       show all breakpoints
(lldb) breakpoint delete 1                   delete breakpoint #1
(lldb) breakpoint disable 2                  disable without deleting

(lldb) watchpoint set variable myVar        stop when myVar changes
(lldb) watchpoint set expression -w read_write -- &obj.field

(lldb) bt                       backtrace (call stack)
(lldb) bt all                   backtrace all threads
(lldb) frame select 2           select frame 2
(lldb) frame info               info about current frame

(lldb) print expr               evaluate and print expression
(lldb) p myvar                  shorthand for print
(lldb) po obj                   print object (calls description)
(lldb) expression myvar = 42    modify variable in debuggee

Inspecting Variables

cpp

Godbolt

(lldb) frame variable                       all local variables
(lldb) frame variable myvar                 specific variable
(lldb) frame variable -r myobj             recursive (expand object)
(lldb) p *ptr                              dereference pointer
(lldb) p arr[0]                            array element
(lldb) memory read --size 4 --count 10 0x7fff5fc0  read raw memory

Thread Commands

cpp

Godbolt

(lldb) thread list                          all threads
(lldb) thread select 2                      switch to thread 2
(lldb) thread backtrace all                 all thread stacks
(lldb) thread info                          current thread info

Conditional Breakpoints

cpp

Godbolt

(lldb) breakpoint set -n compute -c "(x > 100)"
(lldb) breakpoint modify -c "(errno != 0)" 1
(lldb) breakpoint command add 1
  Enter commands, end with 'DONE':
  > p x
  > continue
  > DONE

Expression Language

cpp

Godbolt

(lldb) p (int)myChar            cast expression
(lldb) p &myVar                 address of variable
(lldb) p sizeof(MyStruct)       sizeof
(lldb) expression --language c++ -- std::vector<int>{1,2,3}.size()

LLDB vs GDB Command Mapping

Action	GDB	LLDB
Start	`run`	`run`
Breakpoint on function	`break main`	`breakpoint set -n main` or `b main`
Breakpoint on line	`break file.cpp:42`	`breakpoint set -f file.cpp -l 42`
Print variable	`print x`	`print x` or `p x`
Backtrace	`bt`	`bt`
Next line	`next`	`next`
Step in	`step`	`step`
Watch variable	`watch x`	`watchpoint set variable x`
List breakpoints	`info breakpoints`	`breakpoint list`
Delete breakpoint	`delete 1`	`breakpoint delete 1`
All threads bt	`thread apply all bt`	`bt all`

.lldbinit Configuration

python

# ~/.lldbinit
settings set target.x86-disassembly-flavor intel
settings set thread-format "thread #${thread.index}: ${thread.name}\n"

# Alias GDB-style commands
command alias gdb-bt bt