Debugging with LLDB

?Why Learn LLDB?

If you're on macOS, LLDB is your primary debugger. It's also becoming the standard for modern C/C++ development across all platforms.

macOS Default

No setup needed on Mac

Faster

Modern architecture

Python Scripting

Automate debugging tasks

Note: Most LLDB commands are similar to GDB. If you know GDB, you can learn LLDB quickly!

01What is LLDB?

LLDB is a modern, high-performance debugger built as part of the LLVM project. It's the default debugger on macOS and is increasingly popular on Linux and Windows due to its modern architecture and excellent performance.

LLDB Advantages

✓Default on macOS — no code signing needed
✓Faster startup and execution than GDB
✓Built-in Python scripting support
✓Better C++, Objective-C, Swift support
✓Modern, extensible architecture

When to Use Which?

macOS →LLDB

Linux →GDB or LLDB

Windows →GDB (MinGW)

Clang compiler →LLDB

GCC compiler →GDB

GDB → LLDB Command Cheat Sheet

Action	GDB	LLDB
Run program	run	run
Set breakpoint	break main	b main
Next line	next	next
Step into	step	step
Print variable	print x	p x
Backtrace	backtrace	bt
Continue	continue	c
List breakpoints	info breakpoints	br list
Local variables	info locals	frame variable
Quit	quit	quit

Most common commands are similar! LLDB just uses shorter aliases.

02Installing LLDB

LLDB installation varies by platform. Choose your operating system below:

macOS Installation (Easiest!)

Good news: LLDB comes pre-installed with Xcode Command Line Tools!

Install Xcode Command Line Tools (if not already installed):

xcode-select --install

Verify installation:

lldb --version

lldb-1500.0.22.8

Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1)

Or install via Homebrew (for latest version):

brew install llvm

# LLDB is at /opt/homebrew/opt/llvm/bin/lldb

Linux Installation

Ubuntu / Debian:

sudo apt update

sudo apt install lldb

# Or for a specific version:

sudo apt install lldb-15

Fedora / Red Hat / CentOS:

sudo dnf install lldb

Arch Linux:

sudo pacman -S lldb

From LLVM official repository (latest version):

# Add LLVM repository (Ubuntu example)

wget https://apt.llvm.org/llvm.sh

chmod +x llvm.sh

sudo ./llvm.sh 17 # Install LLVM 17

Verify installation:

lldb --version

lldb version 15.0.7

Windows Installation

Note: LLDB on Windows is less mature than on macOS/Linux. For Windows, GDB with MinGW or Visual Studio Debugger may be better choices.

Option 1: Pre-built LLVM binaries

Download LLVM from releases.llvm.org
Run the installer (choose "Add LLVM to PATH")
LLDB is included in the installation

Option 2: Using Visual Studio

Install Visual Studio with "C++ Clang tools for Windows"
LLDB is included with the Clang toolset

Option 3: Using MSYS2

pacman -S mingw-w64-x86_64-lldb

Option 4: WSL (Recommended for Windows)

# In WSL (Ubuntu):

sudo apt install lldb

03Compiling for LLDB

Like GDB, LLDB needs debug information. Compile with -g:

Using Clang (Recommended)

clang -g -O0 program.c -o program

Best debug experience with LLDB

Using GCC

gcc -g -O0 program.c -o program

Works, but Clang debug info is better for LLDB

# Recommended flags for debugging

clang -g -O0 -Wall -Wextra program.c -o program

# Start LLDB

lldb ./program

Why -O0?

-O0 disables optimization. With optimization enabled, variables may be optimized away, code may be reordered, and debugging becomes confusing. Use -O0 for debugging, -O2 for release.

04Starting LLDB

# Basic: Start LLDB with your program

lldb ./program

# With arguments

lldb -- ./program arg1 arg2

# Attach to running process by PID

lldb -p 12345

# Attach to running process by name

lldb -n myprogram

# Debug a core dump

lldb ./program -c core

LLDB Interface

$ lldb ./program

(lldb) target create "./program"

Current executable set to './program' (x86_64).

(lldb) _

The (lldb) prompt means LLDB is ready for commands.

05Essential LLDB Commands

LLDB uses a noun-verb command structure. Many commands have GDB-compatible aliases.

LLDB Command	GDB Alias	Description
Running
process launch	run, r	Start program
process launch -- arg1	run arg1	Run with arguments
process continue	continue, c	Continue execution
process kill	kill	Stop the program
quit	q	Exit LLDB
Breakpoints
breakpoint set -n main	b main	Break at function
breakpoint set -l 25	b 25	Break at line 25
breakpoint set -f file.c -l 10	b file.c:10	Break at file:line
breakpoint set -n foo -c 'x==5'	—	Conditional breakpoint
breakpoint list	info break	List breakpoints
breakpoint delete 1	delete 1	Delete breakpoint
Stepping
thread step-over	next, n	Step over function
thread step-in	step, s	Step into function
thread step-out	finish	Step out of function
thread step-inst	si	Step one instruction
Examining
frame variable	info locals	Show local variables
frame variable x	p x	Print variable x
expression x	print x, p x	Evaluate expression
thread backtrace	bt	Show call stack
source list	list, l	Show source code

Good News: GDB Commands Work!

LLDB includes GDB-compatible aliases, so commands like run,break, next,print work just like in GDB!

06Complete Debugging Session

Let's debug a buggy program using LLDB:

buggy.c

1#include <stdio.h>
2
3int calculate_average(int arr[], int size) {
4    int sum = 0;
5    for (int i = 0; i <= size; i++) {  // BUG: should be i < size
6        sum += arr[i];
7    }
8    return sum / size;
9}
10
11int main() {
12    int scores[] = {85, 92, 78, 95, 88};
13    int avg = calculate_average(scores, 5);
14    printf("Average = %d\n", avg);
15    return 0;
16}

Step-by-Step Debugging

# Compile and start LLDB

$ clang -g -O0 buggy.c -o buggy

$ lldb ./buggy

(lldb) target create "./buggy"

# Set breakpoint at the function

(lldb) b calculate_average

Breakpoint 1: where = buggy`calculate_average + 4 at buggy.c:4

# Run the program

(lldb) run

Process 12345 launched: './buggy'

Process 12345 stopped

* thread #1, stop reason = breakpoint 1.1

frame #0: calculate_average at buggy.c:4

# Check parameters

(lldb) frame variable

(int *) arr = 0x7fff5fbff8c0

(int) size = 5

(int) sum = 0

# Print the array

(lldb) p *arr@5

(int[5]) $0 = {85, 92, 78, 95, 88}

# Set conditional breakpoint: when i reaches 5

(lldb) breakpoint set -l 6 -c 'i == 5'

Breakpoint 2: where = buggy`calculate_average + 32 at buggy.c:6

(lldb) continue

Process stopped at breakpoint 2.1

# Check the index value

(lldb) p i

(int) $1 = 5

# i = 5, but valid indices are 0-4!

# What's at arr[5]? Out of bounds!

(lldb) p arr[5]

(int) $2 = 32767

Bug Found!

Loop condition i <= size should be i < size. When i=5, we access arr[5] which is out of bounds (array has indices 0-4 only).

07Watchpoints

Watchpoints pause execution when a variable's value changes — perfect for finding where a value gets corrupted:

# Watch when a variable changes

(lldb) watchpoint set variable sum

Watchpoint created: Watchpoint 1: addr = 0x7fff5fbff8c4 size = 4

watchpoint spec = 'sum'

# Watch a memory address

(lldb) watchpoint set expression -- &x

# List watchpoints

(lldb) watchpoint list

# Delete a watchpoint

(lldb) watchpoint delete 1

08GUI Mode

LLDB has a built-in curses-based GUI that shows source, variables, and threads:

# Enable GUI mode

(lldb) gui

# Exit GUI mode: press Escape

GUI Layout

┌─ Source ────────────────────────┬─ Variables ─────────┐

│ 3 int calculate_average(...) │ arr = 0x7fff... │

│ 4 int sum = 0; │ size = 5 │

│> 5 for (int i = 0; ... │ sum = 0 │

│ 6 sum += arr[i]; │ i = 0 │

├─ Threads ───────────────────────┴─────────────────────┤

│ thread #1: stop reason = breakpoint 1.1 │

└────────────────────────────────────────────────────────┘

Use Tab to switch between panes, arrow keys to navigate.

09Debugging Crashes

$ lldb ./crash_program

(lldb) run

Process launched...

Process stopped

* thread #1, stop reason = EXC_BAD_ACCESS (code=1, address=0x0)

# See where we crashed

(lldb) bt

* frame #0: crash`main at crash.c:5

frame #1: libdyld.dylib`start + 1

# Look at the code

(lldb) frame select 0

(lldb) source list

4 int *ptr = NULL;

-> 5 *ptr = 42;

# Check the pointer

(lldb) p ptr

(int *) $0 = 0x0000000000000000

# NULL pointer — found the bug!

10GDB to LLDB Translation

If you're coming from GDB, here's a quick translation guide:

GDB	LLDB (Canonical)	LLDB (Short)
run	process launch	r
break main	breakpoint set -n main	b main
break 25	breakpoint set -l 25	b 25
next	thread step-over	n
step	thread step-in	s
finish	thread step-out	finish
continue	process continue	c
print x	expression x	p x
info locals	frame variable	fr v
backtrace	thread backtrace	bt
list	source list	l
x/10xb addr	memory read -c 10 addr	x/10xb addr

11Quick Reference Card

Running

run — start
run arg1 — with args
c — continue
kill — stop
q — quit

Breakpoints

b main
b 25
br list
br del 1
watch var

Stepping

n — step over
s — step into
finish — step out
si — instruction

Examining

p x — print
fr v — locals
bt — backtrace
l — source
gui — visual

!Code Pitfalls: Common Mistakes & What to Watch For

copied code may work on paper but crash at runtime — LLDB is your essential tool for understanding why.

On macOS, LLDB is the default debugger, and it's crucial for diagnosing issues in copied C code. When you copy code that segfaults, you can't just know "why did this crash?" — you need to run it in LLDB, set breakpoints, and trace through the execution yourself.

The Trap: Copied code can produce syntactically correct code that contains logic errors, memory bugs, or race conditions that only manifest at runtime. Without debugger skills, you're reduced to adding printf statements everywhere — a slow and ineffective approach.

The Reality: LLDB mastery makes you independent of the limitations of copied code. Use thread backtrace to find where crashes happen, frame variable to inspect state, and watchpoint to catch memory corruption. These skills let you fix common mistakes efficiently.

13Frequently Asked Questions

Q:How do I switch from GDB to LLDB?

A: LLDB supports most GDB shortcuts! Commands like b,r, n,s, p, and bt work the same. The main difference is in longer commands like breakpoint set.

Q:Why is LLDB the default on macOS?

A: LLDB is part of the LLVM project (same as Clang). Apple uses Clang as the default compiler, so LLDB integrates better with macOS and Xcode. It also supports Swift debugging natively.

Q:How do I print an array in LLDB?

A: Usep *arr@count to print count elements, or p (int[10])*arr to cast and print. For frame variable: fr v -Z 10 arr.

Q:Can I use LLDB on Linux?

A: Yes! Install withapt install lldb or similar. It works great, especially with Clang-compiled code. Many developers prefer it for its modern syntax and Python scripting.

13LLDB vs GDB: When to Use Which?

Use LLDB When:

✓Working on macOS
✓Using Xcode for iOS/macOS development
✓Compiling with Clang/LLVM
✓Debugging Swift or Objective-C
✓Need faster startup

Use GDB When:

✓Working on Linux servers
✓Debugging embedded systems
✓Using GCC toolchain
✓Need remote debugging
✓Working with legacy projects

Pro Tip

Learn both! The core concepts (breakpoints, stepping, examining) are identical. If you know GDB, you can use LLDB immediately with the GDB-compatible commands!

?Why Learn LLDB?

If you're on macOS, LLDB is your primary debugger. It's also becoming the standard for modern C/C++ development across all platforms.

macOS Default

No setup needed on Mac

Faster

Modern architecture

Python Scripting

Automate debugging tasks

Note: Most LLDB commands are similar to GDB. If you know GDB, you can learn LLDB quickly!

01What is LLDB?

LLDB Advantages

✓Default on macOS — no code signing needed
✓Faster startup and execution than GDB
✓Built-in Python scripting support
✓Better C++, Objective-C, Swift support
✓Modern, extensible architecture

When to Use Which?

macOS →LLDB

Linux →GDB or LLDB

Windows →GDB (MinGW)

Clang compiler →LLDB

GCC compiler →GDB

GDB → LLDB Command Cheat Sheet

Action	GDB	LLDB
Run program	run	run
Set breakpoint	break main	b main
Next line	next	next
Step into	step	step
Print variable	print x	p x
Backtrace	backtrace	bt
Continue	continue	c
List breakpoints	info breakpoints	br list
Local variables	info locals	frame variable
Quit	quit	quit

Most common commands are similar! LLDB just uses shorter aliases.

02Installing LLDB

LLDB installation varies by platform. Choose your operating system below:

macOS Installation (Easiest!)

Good news: LLDB comes pre-installed with Xcode Command Line Tools!

Install Xcode Command Line Tools (if not already installed):

xcode-select --install

Verify installation:

lldb --version

lldb-1500.0.22.8

Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1)

Or install via Homebrew (for latest version):

brew install llvm

# LLDB is at /opt/homebrew/opt/llvm/bin/lldb

Linux Installation

Ubuntu / Debian:

sudo apt update

sudo apt install lldb

# Or for a specific version:

sudo apt install lldb-15

Fedora / Red Hat / CentOS:

sudo dnf install lldb

Arch Linux:

sudo pacman -S lldb

From LLVM official repository (latest version):

# Add LLVM repository (Ubuntu example)

wget https://apt.llvm.org/llvm.sh

chmod +x llvm.sh

sudo ./llvm.sh 17 # Install LLVM 17

Verify installation:

lldb --version

lldb version 15.0.7

Windows Installation

Note: LLDB on Windows is less mature than on macOS/Linux. For Windows, GDB with MinGW or Visual Studio Debugger may be better choices.

Option 1: Pre-built LLVM binaries

Download LLVM from releases.llvm.org
Run the installer (choose "Add LLVM to PATH")
LLDB is included in the installation

Option 2: Using Visual Studio

Install Visual Studio with "C++ Clang tools for Windows"
LLDB is included with the Clang toolset

Option 3: Using MSYS2

pacman -S mingw-w64-x86_64-lldb

Option 4: WSL (Recommended for Windows)

# In WSL (Ubuntu):

sudo apt install lldb

03Compiling for LLDB

Like GDB, LLDB needs debug information. Compile with -g:

Using Clang (Recommended)

clang -g -O0 program.c -o program

Best debug experience with LLDB

Using GCC

gcc -g -O0 program.c -o program

Works, but Clang debug info is better for LLDB

# Recommended flags for debugging

clang -g -O0 -Wall -Wextra program.c -o program

# Start LLDB

lldb ./program

Why -O0?

-O0 disables optimization. With optimization enabled, variables may be optimized away, code may be reordered, and debugging becomes confusing. Use -O0 for debugging, -O2 for release.

04Starting LLDB

# Basic: Start LLDB with your program

lldb ./program

# With arguments

lldb -- ./program arg1 arg2

# Attach to running process by PID

lldb -p 12345

# Attach to running process by name

lldb -n myprogram

# Debug a core dump

lldb ./program -c core

LLDB Interface

$ lldb ./program

(lldb) target create "./program"

Current executable set to './program' (x86_64).

(lldb) _

The (lldb) prompt means LLDB is ready for commands.

05Essential LLDB Commands

LLDB uses a noun-verb command structure. Many commands have GDB-compatible aliases.

LLDB Command	GDB Alias	Description
Running
process launch	run, r	Start program
process launch -- arg1	run arg1	Run with arguments
process continue	continue, c	Continue execution
process kill	kill	Stop the program
quit	q	Exit LLDB
Breakpoints
breakpoint set -n main	b main	Break at function
breakpoint set -l 25	b 25	Break at line 25
breakpoint set -f file.c -l 10	b file.c:10	Break at file:line
breakpoint set -n foo -c 'x==5'	—	Conditional breakpoint
breakpoint list	info break	List breakpoints
breakpoint delete 1	delete 1	Delete breakpoint
Stepping
thread step-over	next, n	Step over function
thread step-in	step, s	Step into function
thread step-out	finish	Step out of function
thread step-inst	si	Step one instruction
Examining
frame variable	info locals	Show local variables
frame variable x	p x	Print variable x
expression x	print x, p x	Evaluate expression
thread backtrace	bt	Show call stack
source list	list, l	Show source code

Good News: GDB Commands Work!

LLDB includes GDB-compatible aliases, so commands like run,break, next,print work just like in GDB!

06Complete Debugging Session

Let's debug a buggy program using LLDB:

buggy.c

1#include <stdio.h>
2
3int calculate_average(int arr[], int size) {
4    int sum = 0;
5    for (int i = 0; i <= size; i++) {  // BUG: should be i < size
6        sum += arr[i];
7    }
8    return sum / size;
9}
10
11int main() {
12    int scores[] = {85, 92, 78, 95, 88};
13    int avg = calculate_average(scores, 5);
14    printf("Average = %d\n", avg);
15    return 0;
16}

Step-by-Step Debugging

# Compile and start LLDB

$ clang -g -O0 buggy.c -o buggy

$ lldb ./buggy

(lldb) target create "./buggy"

# Set breakpoint at the function

(lldb) b calculate_average

Breakpoint 1: where = buggy`calculate_average + 4 at buggy.c:4

# Run the program

(lldb) run

Process 12345 launched: './buggy'

Process 12345 stopped

* thread #1, stop reason = breakpoint 1.1

frame #0: calculate_average at buggy.c:4

# Check parameters

(lldb) frame variable

(int *) arr = 0x7fff5fbff8c0

(int) size = 5

(int) sum = 0

# Print the array

(lldb) p *arr@5

(int[5]) $0 = {85, 92, 78, 95, 88}

# Set conditional breakpoint: when i reaches 5

(lldb) breakpoint set -l 6 -c 'i == 5'

Breakpoint 2: where = buggy`calculate_average + 32 at buggy.c:6

(lldb) continue

Process stopped at breakpoint 2.1

# Check the index value

(lldb) p i

(int) $1 = 5

# i = 5, but valid indices are 0-4!

# What's at arr[5]? Out of bounds!

(lldb) p arr[5]

(int) $2 = 32767

Bug Found!

Loop condition i <= size should be i < size. When i=5, we access arr[5] which is out of bounds (array has indices 0-4 only).

07Watchpoints

Watchpoints pause execution when a variable's value changes — perfect for finding where a value gets corrupted:

# Watch when a variable changes

(lldb) watchpoint set variable sum

Watchpoint created: Watchpoint 1: addr = 0x7fff5fbff8c4 size = 4

watchpoint spec = 'sum'

# Watch a memory address

(lldb) watchpoint set expression -- &x

# List watchpoints

(lldb) watchpoint list

# Delete a watchpoint

(lldb) watchpoint delete 1

08GUI Mode

LLDB has a built-in curses-based GUI that shows source, variables, and threads:

# Enable GUI mode

(lldb) gui

# Exit GUI mode: press Escape

GUI Layout

┌─ Source ────────────────────────┬─ Variables ─────────┐

│ 3 int calculate_average(...) │ arr = 0x7fff... │

│ 4 int sum = 0; │ size = 5 │

│> 5 for (int i = 0; ... │ sum = 0 │

│ 6 sum += arr[i]; │ i = 0 │

├─ Threads ───────────────────────┴─────────────────────┤

│ thread #1: stop reason = breakpoint 1.1 │

└────────────────────────────────────────────────────────┘

Use Tab to switch between panes, arrow keys to navigate.

09Debugging Crashes

$ lldb ./crash_program

(lldb) run

Process launched...

Process stopped

* thread #1, stop reason = EXC_BAD_ACCESS (code=1, address=0x0)

# See where we crashed

(lldb) bt

* frame #0: crash`main at crash.c:5

frame #1: libdyld.dylib`start + 1

# Look at the code

(lldb) frame select 0

(lldb) source list

4 int *ptr = NULL;

-> 5 *ptr = 42;

# Check the pointer

(lldb) p ptr

(int *) $0 = 0x0000000000000000

# NULL pointer — found the bug!

10GDB to LLDB Translation

If you're coming from GDB, here's a quick translation guide:

GDB	LLDB (Canonical)	LLDB (Short)
run	process launch	r
break main	breakpoint set -n main	b main
break 25	breakpoint set -l 25	b 25
next	thread step-over	n
step	thread step-in	s
finish	thread step-out	finish
continue	process continue	c
print x	expression x	p x
info locals	frame variable	fr v
backtrace	thread backtrace	bt
list	source list	l
x/10xb addr	memory read -c 10 addr	x/10xb addr

11Quick Reference Card

Running

run — start
run arg1 — with args
c — continue
kill — stop
q — quit

Breakpoints

b main
b 25
br list
br del 1
watch var

Stepping

n — step over
s — step into
finish — step out
si — instruction

Examining

p x — print
fr v — locals
bt — backtrace
l — source
gui — visual

!Code Pitfalls: Common Mistakes & What to Watch For

copied code may work on paper but crash at runtime — LLDB is your essential tool for understanding why.

13Frequently Asked Questions

Q:How do I switch from GDB to LLDB?

A: LLDB supports most GDB shortcuts! Commands like b,r, n,s, p, and bt work the same. The main difference is in longer commands like breakpoint set.

Q:Why is LLDB the default on macOS?

A: LLDB is part of the LLVM project (same as Clang). Apple uses Clang as the default compiler, so LLDB integrates better with macOS and Xcode. It also supports Swift debugging natively.

Q:How do I print an array in LLDB?

A: Usep *arr@count to print count elements, or p (int[10])*arr to cast and print. For frame variable: fr v -Z 10 arr.

Q:Can I use LLDB on Linux?

A: Yes! Install withapt install lldb or similar. It works great, especially with Clang-compiled code. Many developers prefer it for its modern syntax and Python scripting.

13LLDB vs GDB: When to Use Which?

Use LLDB When:

✓Working on macOS
✓Using Xcode for iOS/macOS development
✓Compiling with Clang/LLVM
✓Debugging Swift or Objective-C
✓Need faster startup

Use GDB When:

✓Working on Linux servers
✓Debugging embedded systems
✓Using GCC toolchain
✓Need remote debugging
✓Working with legacy projects

Pro Tip

Learn both! The core concepts (breakpoints, stepping, examining) are identical. If you know GDB, you can use LLDB immediately with the GDB-compatible commands!

What You Will Learn

?Why Learn LLDB?

01What is LLDB?

LLDB Advantages

When to Use Which?

GDB → LLDB Command Cheat Sheet

02Installing LLDB

macOS Installation (Easiest!)

Linux Installation

Windows Installation

03Compiling for LLDB

Using Clang (Recommended)

Using GCC

Why -O0?

04Starting LLDB

LLDB Interface

05Essential LLDB Commands

Good News: GDB Commands Work!

06Complete Debugging Session

Step-by-Step Debugging

Bug Found!

07Watchpoints

08GUI Mode

GUI Layout

09Debugging Crashes

10GDB to LLDB Translation

11Quick Reference Card

Running

Breakpoints

Stepping

Examining

!Code Pitfalls: Common Mistakes & What to Watch For

copied code may work on paper but crash at runtime — LLDB is your essential tool for understanding why.

13Frequently Asked Questions

Q:How do I switch from GDB to LLDB?

Q:Why is LLDB the default on macOS?

Q:How do I print an array in LLDB?

Q:Can I use LLDB on Linux?

13LLDB vs GDB: When to Use Which?

Use LLDB When:

Use GDB When:

Pro Tip

Test Your Knowledge

Related Tutorials

Debugging with GDB

Header Files in C

Pointers in C

Have Feedback?

What You Will Learn

?Why Learn LLDB?

01What is LLDB?

LLDB Advantages

When to Use Which?

GDB → LLDB Command Cheat Sheet

02Installing LLDB

macOS Installation (Easiest!)

Linux Installation

Windows Installation

03Compiling for LLDB

Using Clang (Recommended)

Using GCC

Why -O0?

04Starting LLDB

LLDB Interface

05Essential LLDB Commands

Good News: GDB Commands Work!

06Complete Debugging Session

Step-by-Step Debugging

Bug Found!

07Watchpoints

08GUI Mode

GUI Layout

09Debugging Crashes

10GDB to LLDB Translation

11Quick Reference Card

Running

Breakpoints

Stepping

Examining

!Code Pitfalls: Common Mistakes & What to Watch For