LLDB Cheatsheet

LLDB is a debugger for C/C++ that runs in the terminal.

(lldb) denotes a function being run in lldb.

Summary

# Opening
c++ -g -std=c++17 -o <output_exe> <file_name> # With debug flags (-g)
lldb main # Open executable main

# Breakpoints
(lldb) b main.cpp : <line_number> # Set breakpoint online number
(lldb) b set -n <function_name> # e.g. Foo(int)
(lldb) br l # List breakpoints
(lldb) br del <line_number> # Delete breakpoint

# Run
(lldb) r

# Watch variable
(lldb) watch set var <variable_name>
(lldb) watch modify -c '(<variable_name> == 5)' # Break on condition
(lldb) watch list

# Navigation
(lldb) n # Next line
(lldb) s # Step into function on current line
(lldb) c # Continue until next breakpoint (or end)

# Variables
(lldb) p <variable_name> # Print variable value
(lldb) fr v # Print all variables in current frame

# Frames
(lldb) fr s # Current point of execution
(lldb) fr v # Print all variables in current frame
(lldb) fr s <frame_id> # Go to specific frame
(lldb) bt # List frames up until point

# Exiting
(lldb) kill # Kill the process (but stay in lldb)
(lldb) quit # Exit lldb

Open File

lldb main # Where main is your executable
# Note main must be compiled with -g flag
# E.g. c++ -g -std=c++17 -o main main.cpp

# or
lldb
(lldb) file "path/to/file"

# or if arguments are supplied 
lldb -- main arg1 arg2 # note the -- before the executable

Breakpoints

Executable must be compiled with the -g flag.

Set

By line number:

(lldb) breakpoint set -f <filename> -l <line_number>
# or
(lldb) b <filename> : <line_number>
# or
(lldb) break <filename> : <line_number>

By function:

(lldb) breakpoint set --name <function_name>
# or
(lldb) b set -n <function_name> # e.g. Foo(int)

On a class method:

(lldb) b Foo::foo()

Inside a namespace:

(lldb) b NameSpace::foo(int)

List

(lldb) breakpoint list
# or
(lldb) br l

Delete

Deletion is based on the breakpoint ID (obtained using br l).

(lldb) br del # All
(lldb) br del <line_number> # Specific

Watch

Watch a specific variable to see it changes (also based on conditionals). Must be running before setting.

(lldb) watch set var <variable_name>
# Stop based on condition
(lldb) watch modify -c '(<variable_name> == 5)'
# List watch
(lldb) watch list

Run

process launch
# or
(lldb) run
# or
(lldb) r

Navigation

Step Over

Go to next line.

(lldb) next
# or 
(lldb) n

Step Into

Go into function on current line. If multiple (functions) exist it will go into them in order.

(lldb) step
# or
(lldb) s

Continue

Run until next breakpoint or end of program.

(lldb) continue
# or
(lldb) c

Variables

Inspecting

View variable contents.

(lldb) print <variable_name>
# or
(lldb) p <variable_name>

Frames

Inspect all variables around a specific call site.

(lldb) frame <variable_name>
# or
(lldb) fr v 

See what frame the debugger is up to. Or select a specific frame.

(lldb) frame select
# or
(lldb) fr s # Current point of execution

# Or to see a specific frame
(lldb) frame select <frame_id>
# or
(lldb) fr s <frame_id>

Back-tracing

Lists the current call stack. Most recent function at the top of the stack (descending order).

(lldb) bt

Useful for debugging crashes by seeing the programs state before the crash occurred, for example:

lldb main
(lldb) r
# Error occurs
(lldb) bt # List frames of execution
(llsb) fr s <frame_id> # Select frame before crash
(llsb) fr v # List varaibles prior to crash

Terminating

# Kill the process (but stay in lldb)
(lldb) kill
# Exit lldb
(lldb) quit