Debugging Skill
Production-Grade Development Skill | C++ Debugging & Error Analysis
Master C++ debugging tools and systematic investigation techniques.
Debugging Workflow
┌─────────────┐ ┌──────────────┐ ┌───────────────┐
│ REPRODUCE │───▶│ ISOLATE │───▶│ INSTRUMENT │
│ (confirm) │ │ (minimize) │ │ (observe) │
└─────────────┘ └──────────────┘ └───────────────┘
│ │
▼ ▼
┌─────────────┐ ┌──────────────┐ ┌───────────────┐
│ VERIFY │◀───│ FIX │◀───│ ANALYZE │
│ (test) │ │ (correct) │ │ (root cause) │
└─────────────┘ └──────────────┘ └───────────────┘
GDB (GNU Debugger)
Essential Commands
# Compile with debug symbols
g++ -g -O0 program.cpp -o program
# Start GDB
gdb ./program
# Core dump analysis
gdb ./program core.dump
# Execution Control
run [args] # Start program
continue (c) # Continue execution
next (n) # Step over
step (s) # Step into
finish # Run until current function returns
until <line> # Run until line
# Breakpoints
break main # Break at function
break file.cpp:42 # Break at line
break *0x400520 # Break at address
break func if x > 5 # Conditional breakpoint
delete <num> # Remove breakpoint
disable/enable <num> # Toggle breakpoint
# Inspection
print variable # Print value
print/x variable # Print in hex
display variable # Print at each stop
info locals # Show local variables
info args # Show function arguments
ptype variable # Show type
# Stack Navigation
backtrace (bt) # Show call stack
frame <n> # Select frame
up/down # Move in stack
# Memory
x/10xw 0x400520 # Examine 10 words in hex
watch variable # Break when variable changes
rwatch variable # Break when variable is read
# Advanced
catch throw # Break on exception
set variable x = 5 # Modify variable
call func(args) # Call function
GDB Scripts
# .gdbinit - Auto-loaded configuration
set history save on
set print pretty on
set print array on
set pagination off
# Custom command
define pv
print *$arg0@$arg1
end
# Usage: pv array 10
LLDB (macOS/Clang)
# Start LLDB
lldb ./program
# Attach to process
lldb -p <pid>
# Breakpoints
breakpoint set --name main
breakpoint set --file main.cpp --line 42
breakpoint set --method MyClass::method
# Execution
run
continue
next
step
finish
# Inspection
frame variable # Show all locals
frame variable varname # Show specific variable
expression varname # Evaluate expression
expression -O -- object # Print object description
# Stack
thread backtrace
frame select <n>
up/down
# Watchpoints
watchpoint set variable varname
watchpoint set expression -- &array[5]
Sanitizers
AddressSanitizer (ASan)
# Compile with ASan
g++ -fsanitize=address -g -O1 program.cpp -o program
# Run (ASan is active)
./program
# Detects:
# - Heap buffer overflow
# - Stack buffer overflow
# - Global buffer overflow
# - Use after free
# - Use after return
# - Double free
# - Memory leaks (with ASAN_OPTIONS=detect_leaks=1)
UndefinedBehaviorSanitizer (UBSan)
# Compile with UBSan
g++ -fsanitize=undefined -g program.cpp -o program
# Detects:
# - Signed integer overflow
# - Division by zero
# - Null pointer dereference
# - Invalid shift
# - Out-of-bounds array access
# - Misaligned pointer access
ThreadSanitizer (TSan)
# Compile with TSan
g++ -fsanitize=thread -g program.cpp -o program
# Detects:
# - Data races
# - Deadlocks (with deadlock_detector=1)
# - Lock order violations
Combined Usage
# CMakeLists.txt
option(ENABLE_SANITIZERS "Enable sanitizers" OFF)
if(ENABLE_SANITIZERS)
add_compile_options(
-fsanitize=address,undefined
-fno-omit-frame-pointer
-g
)
add_link_options(
-fsanitize=address,undefined
)
endif()
Valgrind
Memory Leak Detection
# Check for memory leaks
valgrind --leak-check=full --show-leak-kinds=all ./program
# With line numbers
valgrind --leak-check=full --track-origins=yes ./program
# Generate suppressions for known issues
valgrind --gen-suppressions=all ./program
Memcheck Options
valgrind --tool=memcheck \
--leak-check=full \
--show-leak-kinds=all \
--track-origins=yes \
--verbose \
--log-file=valgrind.log \
./program
Cachegrind (Cache Profiling)
valgrind --tool=cachegrind ./program
cg_annotate cachegrind.out.*
# Output shows:
# - I1 cache read misses
# - D1 cache read/write misses
# - LL (last level) cache misses
Common Issue Patterns
Issue Diagnostic Table
| Issue | Symptoms | Primary Tool | Secondary Tool | |-------|----------|--------------|----------------| | Segfault | SIGSEGV, crash | GDB + bt | ASan | | Memory leak | Growing memory | Valgrind | ASan (leak check) | | Buffer overflow | Corruption, crash | ASan | Valgrind | | Use after free | Random crash | ASan | Valgrind | | Data race | Random behavior | TSan | Helgrind | | Deadlock | Hang | GDB + threads | TSan | | Integer overflow | Wrong results | UBSan | - | | Null deref | Crash at low address | ASan | GDB | | Stack overflow | Deep recursion crash | GDB | ulimit |
Quick Diagnosis Commands
# Check if crash is reproducible
for i in {1..100}; do ./program || echo "Crashed on run $i"; done
# Get core dump
ulimit -c unlimited
./program # Will generate core file
# Quick stack trace from core
gdb -batch -ex "bt" ./program core
# Check for memory issues quickly
MALLOC_CHECK_=3 ./program
Troubleshooting Decision Tree
Program crashed?
├── Segmentation fault (SIGSEGV)
│ ├── Get backtrace: gdb -batch -ex "bt" ./program core
│ ├── Check for NULL deref: look at crash address
│ ├── Check for buffer overflow: run with ASan
│ └── Check for use-after-free: run with ASan
├── Abort (SIGABRT)
│ ├── Check assert failures: look at stderr
│ ├── Check std::terminate: uncaught exception
│ └── Check double free: run with ASan
├── Hang (no progress)
│ ├── Attach debugger: gdb -p <pid>
│ ├── Check for deadlock: info threads, thread apply all bt
│ └── Check for infinite loop: break, bt
└── Wrong output
├── Check undefined behavior: run with UBSan
├── Add logging at key points
└── Use debugger to trace execution
Debug Build Configuration
# CMakeLists.txt
set(CMAKE_BUILD_TYPE Debug)
# Ensure debug symbols
set(CMAKE_CXX_FLAGS_DEBUG "-g -O0")
# Sanitizers for debug builds
if(CMAKE_BUILD_TYPE STREQUAL "Debug")
add_compile_options(-fsanitize=address,undefined)
add_link_options(-fsanitize=address,undefined)
endif()
# Export compile commands for IDE integration
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
Debug Logging Pattern
#include <iostream>
#include <source_location>
// Modern debug logging (C++20)
template<typename... Args>
void debug_log(Args&&... args,
std::source_location loc = std::source_location::current()) {
#ifndef NDEBUG
std::cerr << "[DEBUG] " << loc.file_name() << ":"
<< loc.line() << " (" << loc.function_name() << "): ";
(std::cerr << ... << std::forward<Args>(args)) << '\n';
#endif
}
// Usage
void process(int x) {
debug_log("Processing value: ", x);
// ...
}
Unit Test Template
#include <gtest/gtest.h>
#include <signal.h>
class DebuggingTest : public ::testing::Test {
protected:
void SetUp() override {
// Enable core dumps for this test
struct rlimit core_limit;
core_limit.rlim_cur = RLIM_INFINITY;
core_limit.rlim_max = RLIM_INFINITY;
setrlimit(RLIMIT_CORE, &core_limit);
}
};
// Test that sanitizers catch issues
TEST_F(DebuggingTest, DetectsBufferOverflow) {
// This should be caught by ASan if enabled
// Only run with sanitizers in CI
#ifdef __SANITIZE_ADDRESS__
GTEST_SKIP() << "Would trigger ASan, skipping";
#endif
}
TEST_F(DebuggingTest, DebugSymbolsPresent) {
// Verify debug build
#ifdef NDEBUG
FAIL() << "Tests should run in debug mode";
#endif
}
// Death test for expected crashes
TEST_F(DebuggingTest, CrashOnNullDeref) {
EXPECT_DEATH({
int* p = nullptr;
*p = 42;
}, "");
}
Integration Points
| Component | Interface |
|-----------|-----------|
| build-engineer | Debug build flags |
| memory-specialist | Memory issue patterns |
| performance-optimizer | Performance debugging |
| modern-cpp-expert | Smart pointer debugging |
C++ Plugin v3.0.0 - Production-Grade Development Skill