Debugging Gdb Fundamentals

GDB Fundamentals

Last Updated: 2025-10-26

Overview

GDB (GNU Debugger) is the standard debugger for C, C++, Rust, Go, and other compiled languages. This guide covers fundamental debugging workflows, advanced features, and best practices for effective debugging.

Core Concepts

GDB Architecture

Program (debuggee)
  ↓
ptrace() system call → GDB (debugger)
  ↓
Debug symbols (DWARF) → Source mapping
  ↓
User commands → GDB commands → ptrace() actions

Debug Symbols

Compile with debug symbols:

# C/C++
gcc -g program.c -o program
g++ -g -O0 program.cpp -o program  # -O0 disables optimization

# Rust
cargo build  # Debug build includes symbols
cargo build --release  # No debug symbols

# Zig
zig build-exe program.zig -O Debug  # Debug symbols

Check if symbols present:

file program
# program: ELF 64-bit LSB executable, x86-64, with debug_info, not stripped

nm program | grep -i debug
objdump -h program | grep debug

Starting GDB

Basic Launch

# Start GDB with program
gdb ./program

# Start with arguments
gdb --args ./program arg1 arg2

# Attach to running process
gdb -p <pid>
gdb attach <pid>

# Load core dump
gdb ./program core.12345

# Quiet mode (no banner)
gdb -q ./program

Batch Mode

# Run commands from file
gdb -batch -x commands.gdb ./program

# Example commands.gdb:
# break main
# run
# backtrace
# quit

Breakpoints

Setting Breakpoints

Function breakpoints:

# Break at function entry
(gdb) break main
(gdb) b calculate_sum

# Break at C++ method
(gdb) break MyClass::method

# Break at Rust function
(gdb) break my_crate::my_function

# Conditional breakpoint
(gdb) break main if argc > 1
(gdb) b process_data if count > 100

# Temporary breakpoint (auto-delete after hit)
(gdb) tbreak main

Line breakpoints:

# Break at line number
(gdb) break file.c:42
(gdb) b 42  # Current file

# Break at all matching functions
(gdb) rbreak ^calculate.*  # Regex: all functions starting with "calculate"

Address breakpoints:

# Break at memory address
(gdb) break *0x400567

Managing Breakpoints

# List all breakpoints
(gdb) info breakpoints
(gdb) i b

# Disable/enable breakpoint
(gdb) disable 1
(gdb) enable 1

# Delete breakpoint
(gdb) delete 1
(gdb) d 1

# Delete all breakpoints
(gdb) delete

# Ignore breakpoint N times
(gdb) ignore 1 10  # Ignore breakpoint 1 for 10 hits

Watchpoints

Monitor variable changes:

# Watch variable (break when value changes)
(gdb) watch my_variable

# Watch memory location
(gdb) watch *0x7fffffffe000

# Watch expression
(gdb) watch count > 100

# Read watchpoint (break on read)
(gdb) rwatch my_variable

# Access watchpoint (break on read or write)
(gdb) awatch my_variable

Catchpoints

Break on events:

# Catch C++ exceptions
(gdb) catch throw
(gdb) catch catch

# Catch system calls
(gdb) catch syscall open
(gdb) catch syscall write read

# Catch signals
(gdb) catch signal SIGSEGV

# Catch library loads
(gdb) catch load libcrypto.so

Program Control

Running Programs

# Start program
(gdb) run
(gdb) r

# Run with arguments
(gdb) run arg1 arg2
(gdb) set args arg1 arg2
(gdb) run

# Run with input redirection
(gdb) run < input.txt

# Run with environment variables
(gdb) set environment VAR=value
(gdb) run

Stepping Through Code

# Step into (enter functions)
(gdb) step
(gdb) s

# Step over (don't enter functions)
(gdb) next
(gdb) n

# Step one instruction
(gdb) stepi
(gdb) si

# Continue until function returns
(gdb) finish

# Continue until line
(gdb) until 50

# Continue execution
(gdb) continue
(gdb) c

Advanced Control

# Skip function during stepping
(gdb) skip function std::string::operator=

# List skipped functions
(gdb) info skip

# Return from function early
(gdb) return
(gdb) return 42  # Return with value

Stack Inspection

Backtraces

# Full backtrace
(gdb) backtrace
(gdb) bt

# Backtrace with local variables
(gdb) bt full

# Limited backtrace
(gdb) bt 10  # Show 10 frames

# Backtrace all threads
(gdb) thread apply all bt

Frame Navigation

# Show current frame
(gdb) frame
(gdb) f

# Select frame by number
(gdb) frame 2
(gdb) f 2

# Move up/down stack
(gdb) up
(gdb) down
(gdb) up 3

# Show frame info
(gdb) info frame
(gdb) info locals
(gdb) info args

Variable Inspection

Print Variables

# Print variable
(gdb) print my_variable
(gdb) p my_variable

# Print with format
(gdb) p/x my_variable  # Hexadecimal
(gdb) p/d my_variable  # Decimal
(gdb) p/t my_variable  # Binary
(gdb) p/c my_variable  # Character
(gdb) p/f my_variable  # Float

# Print pointer
(gdb) p *ptr
(gdb) p ptr[0]@10  # Print array of 10 elements

# Print structure
(gdb) p my_struct
(gdb) p my_struct.field

Display (Auto-print)

# Display variable every stop
(gdb) display count
(gdb) display expr

# List displays
(gdb) info display

# Delete display
(gdb) delete display 1

Memory Examination

# Examine memory
(gdb) x/10xw 0x7fffffffe000  # 10 words in hex
(gdb) x/10i $pc              # 10 instructions at PC
(gdb) x/s 0x400678           # String at address

# Format: x/[count][format][size] address
# Formats: x(hex) d(decimal) u(unsigned) o(octal) t(binary) a(address) c(char) s(string) i(instruction)
# Sizes: b(byte) h(halfword) w(word) g(giant, 8 bytes)

Type Inspection

# Show type
(gdb) ptype my_variable
(gdb) whatis my_variable

# Show type with size
(gdb) p sizeof(my_struct)

TUI Mode (Text User Interface)

Activation

# Start GDB in TUI mode
gdb -tui ./program

# Or within GDB:
(gdb) tui enable
(gdb) Ctrl-x Ctrl-a  # Toggle TUI mode

TUI Layouts

# Source + command
(gdb) layout src

# Assembly + command
(gdb) layout asm

# Source + assembly
(gdb) layout split

# Registers + source
(gdb) layout regs

# Cycle layouts
(gdb) Ctrl-x 2

TUI Navigation

Ctrl-x o      # Switch active window
Ctrl-x 1      # Single window
Ctrl-x 2      # Split window
Ctrl-l        # Refresh screen
Ctrl-p/n      # Command history
PgUp/PgDn     # Scroll source window

Advanced Features

.gdbinit Customization

~/.gdbinit (global) or ./.gdbinit (project):

# Pretty printing
set print pretty on
set print array on
set print array-indexes on

# History
set history save on
set history size 10000
set history filename ~/.gdb_history

# Disable pagination
set pagination off

# Auto-load local .gdbinit
set auto-load safe-path /

# Custom commands
define hook-stop
    info registers
    x/24wx $sp
end

# Aliases
alias -a bp = break
alias -a c = continue
alias -a s = step
alias -a n = next

Python Scripting

Load Python scripts:

(gdb) source script.py

script.py:

import gdb

class HelloCommand(gdb.Command):
    """Print hello message."""

    def __init__(self):
        super(HelloCommand, self).__init__("hello", gdb.COMMAND_USER)

    def invoke(self, arg, from_tty):
        print(f"Hello, {arg}!")

HelloCommand()

# Custom breakpoint
class CountBreakpoint(gdb.Breakpoint):
    def __init__(self, spec):
        super(CountBreakpoint, self).__init__(spec)
        self.count = 0

    def stop(self):
        self.count += 1
        print(f"Hit count: {self.count}")
        return True  # Stop execution

CountBreakpoint("main")

Pretty printers:

import gdb

class MyStructPrinter:
    def __init__(self, val):
        self.val = val

    def to_string(self):
        return f"MyStruct(x={self.val['x']}, y={self.val['y']})"

def my_struct_printer(val):
    if str(val.type) == 'struct MyStruct':
        return MyStructPrinter(val)
    return None

gdb.pretty_printers.append(my_struct_printer)

Remote Debugging

Start gdbserver on target:

# Remote machine
gdbserver :2345 ./program
gdbserver :2345 --attach <pid>

Connect from GDB:

# Local machine
(gdb) target remote 192.168.1.100:2345
(gdb) continue

SSH tunnel:

# Local machine
ssh -L 2345:localhost:2345 user@remote
gdb ./program
(gdb) target remote localhost:2345

Core Dump Analysis

Enable core dumps:

# Check current limit
ulimit -c

# Enable unlimited core dumps
ulimit -c unlimited

# Set core pattern (Linux)
sudo sysctl -w kernel.core_pattern=/tmp/core.%e.%p

Load core dump:

gdb ./program core.12345

# Or within GDB:
(gdb) core-file core.12345

# Analyze crash
(gdb) bt full
(gdb) info registers
(gdb) x/10i $pc

Multi-threaded Debugging

Thread Commands

# List threads
(gdb) info threads

# Switch to thread
(gdb) thread 2

# Apply command to all threads
(gdb) thread apply all bt

# Thread-specific breakpoint
(gdb) break main thread 2

# Lock scheduler (single-step one thread)
(gdb) set scheduler-locking on
(gdb) set scheduler-locking off

Debugging Optimized Code

Challenges

Optimized code problems:

• Variables optimized out
• Inlined functions
• Reordered instructions
• Registers used instead of stack

Strategies

# Compile with minimal optimization
gcc -g -O1 program.c -o program  # -O1 safer than -O2/-O3

# Use -Og (optimize for debugging)
gcc -g -Og program.c -o program

# Disable specific optimizations
gcc -g -O2 -fno-inline program.c -o program

Handling optimized variables:

# Variable optimized out
(gdb) p my_var
$1 = <optimized out>

# Try different frame
(gdb) up
(gdb) p my_var

# Use disassembly to find value in register
(gdb) disassemble
(gdb) info registers

Common Workflows

Debugging Segfaults

# Run until crash
gdb ./program
(gdb) run

# Examine crash
(gdb) bt full
(gdb) info registers
(gdb) x/10i $pc
(gdb) frame 0
(gdb) list
(gdb) info locals

Debugging Memory Leaks

Use Valgrind instead of GDB:

valgrind --leak-check=full --show-leak-kinds=all ./program

Debugging Race Conditions

# Break on thread creation
(gdb) catch syscall clone

# Enable scheduler debugging
(gdb) set scheduler-locking step

# Record execution for reverse debugging
(gdb) record
(gdb) reverse-continue
(gdb) reverse-step

GDB with Rust

Rust-specific Commands

# Break at Rust function
(gdb) break my_crate::my_module::my_function

# Pretty-print Rust types
(gdb) set language rust
(gdb) p my_vec

# Load Rust pretty printers
(gdb) source /path/to/rust-gdb

Use rust-gdb wrapper:

rust-gdb ./target/debug/my_program

GDB with C++

C++-specific Features

# Break at method
(gdb) break MyClass::myMethod

# Print STL containers
(gdb) p my_vector
(gdb) p my_map

# Catch exceptions
(gdb) catch throw
(gdb) catch catch

# Demangle symbols
(gdb) set print asm-demangle on

Performance Tips

Faster Debugging

# Disable pagination for scripts
set pagination off

# Disable confirmation prompts
set confirm off

# Reduce output
set print elements 10  # Limit array elements

# Use hardware breakpoints (limited, but faster)
(gdb) hbreak main

Logging

# Enable logging
(gdb) set logging on
(gdb) set logging file gdb.log

# Log overwrite vs append
(gdb) set logging overwrite on

Anti-Patterns

Common Mistakes

❌ NEVER: Debug optimized code without trying -Og first
   → Variables optimized out, inaccurate stepping

❌ NEVER: Forget to compile with -g
   → No source mapping, only assembly

❌ NEVER: Set too many breakpoints in hot loops
   → Massive slowdown, unusable debugging

❌ NEVER: Use GDB for memory leak detection
   → Use Valgrind, AddressSanitizer instead

❌ NEVER: Attach to production process without understanding impact
   → Process paused = downtime

❌ NEVER: Modify variables in production debugging
   → Undefined behavior, data corruption

Best Practices

✅ ALWAYS: Compile with -g -O0 for debug builds
✅ ALWAYS: Use conditional breakpoints in hot paths
✅ ALWAYS: Save .gdbinit for project-specific commands
✅ ALWAYS: Use TUI mode for visual debugging
✅ ALWAYS: Test core dump analysis before production issues
✅ ALWAYS: Use gdbserver for remote/embedded debugging

Related Skills

• lldb-macos-debugging.md: LLDB debugger for macOS/iOS
• python-debugging.md: Python-specific debugging tools
• browser-devtools.md: Browser debugging for web applications
• remote-debugging.md: Remote debugging techniques
• test-debugging-strategies.md: Debugging test failures

Summary

GDB is the standard debugger for compiled languages. Key capabilities:

1. Breakpoints: Function, line, conditional, watchpoints, catchpoints
2. Stack inspection: Backtraces, frame navigation, variable inspection
3. TUI mode: Visual interface for source/assembly viewing
4. Python scripting: Custom commands, pretty printers, automation
5. Remote debugging: gdbserver for remote/embedded targets
6. Core dumps: Post-mortem analysis of crashes
7. Multi-threading: Thread-aware debugging, scheduler control

Quick start:

gcc -g -O0 program.c -o program
gdb -tui ./program
(gdb) break main
(gdb) run
(gdb) step
(gdb) print my_variable
(gdb) backtrace

Master GDB for efficient low-level debugging across C, C++, Rust, and Go.