C++ Debugging Basics | GDB and LLDB — Breakpoints and Watchpoints in Minutes

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C++ Debugging Basics | GDB and LLDB — Breakpoints and Watchpoints in Minutes

이 글의 핵심

GDB/LLDB essentials: -g -O0 builds, break, watch, next/step, bt, thread apply all bt, and production core-file analysis.

Introduction: Limits of printf debugging

“I added 100 prints and still can’t find it”

Segfaults with confusing logs, buffering, and slow rebuild cycles—debuggers stop at the line, show variables, stack, and memory live.

Breakpoint: stop at a line. Watchpoint: stop when memory changes. Backtrace: see call chain.

Requirements: build with -g (symbols). -O0 often easiest for inspecting variables.

Environment: GDB on Linux/WSL; LLDB on macOS (Xcode CLI tools).

After reading:

  • Core GDB/LLDB commands
  • Breakpoints including conditions
  • Watchpoints for corruption hunts
  • Inspect vars/stack; multi-threaded deadlock introspection

Table of contents

  1. Scenarios
  2. Starting the debugger
  3. Breakpoints
  4. Watchpoints
  5. Stepping
  6. Inspecting variables
  7. Examples
  8. Common errors
  9. Best practices
  10. Production patterns

1. Scenarios

  • Segfault → run under debugger → bt at crash
  • Rare iteration → conditional breakpoint break main.cpp:10 if i == 500
  • Mystery mutation → watch variable
  • Infinite loop → Ctrl+C → bt, inspect loop vars
  • Deadlock → thread apply all backtrace
  • Production → core dump + gdb ./app core
flowchart TD
    A[Bug] --> B{Type?}
    B -->|Crash| C[GDB run → bt]
    B -->|Conditional| D[Conditional breakpoint]
    B -->|Corruption| E[Watchpoint]
    B -->|Hang| F[Ctrl+C → bt]
    B -->|Deadlock| G[thread apply all bt]
    B -->|Prod| H[Core file]

2. Starting

g++ -g -O0 main.cpp -o myapp
gdb ./myapp
(gdb) run

LLDB:

lldb ./myapp
(lldb) run

3. Breakpoints

GDB:

break main
break file.cpp:15
break file.cpp:20 if i == 50
info breakpoints
delete 1

LLDB:

breakpoint set --name main
breakpoint set -f file.cpp -l 15
breakpoint set -f file.cpp -l 20 -c 'i == 50'

4. Watchpoints

GDB (often set after break main / run when variable exists):

watch global_counter
watch *ptr

LLDB:

watchpoint set variable global_counter

5. Stepping

CommandGDBLLDB
Next line (no step into)next / nnext
Step intostep / sstep
Until function returnsfinishfinish
Continuecontinue / ccontinue

6. Inspecting

  • print x, print *p, print arr[0]@10
  • backtrace / bt, frame N, info locals, info args
  • x/ (examine memory) in GDB

7. Examples

Array bounds off-by-one

for (int i = 0; i <= size; ++i) {  // should be i < size
    arr[i] = i * 2;
}

At SIGSEGV: bt, print i, print size.

Null pointer

print head → 0x0.

Deadlock

thread apply all backtrace — see each thread blocked on locks.

8. Common errors

  • No symbols → missing -g or stripped binary
  • optimized out → compile -O0 or use volatile sparingly for debug
  • Hardware watchpoint limit → reduce or use software watchpoints (slower)

9. Best practices

  • Debug builds: -g -O0
  • On crash: bt full first
  • Use .gdbinit for set print pretty on

10. Production patterns

ulimit -c unlimited
./myapp   # crash
gdb ./myapp core
(gdb) bt full

gdbserver for remote attach (freeze-aware—use during maintenance windows).

Split debug info: objcopy --only-keep-debug + --add-gnu-debuglink for smaller release binaries with separate symbols.

GDB vs LLDB quick map

TaskGDBLLDB
Runrunrun
Breakbreak mainbreakpoint set -n main
Stepnext / stepsame
Stackbtbt
Watchwatch varwatchpoint set variable var

Summary

ToolPlatform
GDBLinux, typical for servers
LLDBmacOS, also Linux
VSWindows GUI debugging

Principles: prefer debugger over printf for control flow; bt on crashes; watch for mystery writes; thread bt for concurrency.

Next: Sanitizers #16-2

Previous: Compile-time optimization #15-3

FAQ

When is this useful?

A. Any non-trivial C++ bug—especially crashes, concurrency, and state you can’t print cleanly.

Read first?

A. Series index and build basics.

  • Sanitizers
  • Logging and assertions
  • Segfault debugging

Keywords

C++ debugging, GDB, LLDB, breakpoint, watchpoint, backtrace, segfault, core dump

Practical tips

Debugging

  • Warnings first; minimal repro

Performance

  • Profile before optimizing
  • GDB/LLDB deep dive
  • GDB basics
  • LLDB basics