The GNU debugger is a powerful tool for the debugging of (mainly) C/C++ programs. In this post I document the common operations of gdb and some useful learning resources.

Before Debugging

Tips of Compiling

• The -g flag tells gcc to generate source-level debugging information.
• The -ggdb flag tells gcc to generate more debugging information for gdb.
• The -Og flag tells gcc to optimize your code without affecting debugging.

gcc hello.c -g -ggdb -Og

Tips of Starting GDB

• The --tui flag tells gdb to start in the TUI (Text User Interface) mode.
• The -q flag tells gdb not to print version info on startup.
• The -x option tells gdb to execute some commands (.gdb files only in the past, now even .py files are supported) on startup.

gdb ./a.out --tui -x debug.py -q

The example debug.py:

import gdb

def on_quit():
  gdb.execute('kill')

gdb.events.exited.connect(on_quit)

gdb.execute('...')
gdb.Breakpoint('...')

GDB Commands

Basic Commands

• set args arg1 arg2 arg3: set command line arguments of the program to be executed
• set args: remove all command line arguments set before
• show args: show the current command line arguments

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• run or r: start to run the program (run to complete unless met a breakpoint)
  • You can also put args after run if you don’t use set args to set them before, e.g. run arg1 arg2 arg3
• kill: stop running the program

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• file program: load program and start to debug it
• quit: exit the debugger
• set print pretty on: make the outputs prettier

Stopping and Rerunning Commands

• break or b: set a breakpoint
  • break 10: set a breakpoint to stop at line 10 of the current file
  • break hello.c:10: set a breakpoint to stop at line 10 of hello.c
  • break main: set a breakpoint to stop at the beginning of the main function
  • break: stop at the current line
  • break *0x400522: stop at a specific address (0x400522 here)
• tbreak or tb: set a temporary breakpoint
• break ... if ...: set a conditional breakpoint

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• watch a: stop when the value of variable a changed

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• clear: clear a breakpoint
  • clear main: remove the breakpoint for the main function
  • clear hello.c:10: remove the breakpoint at line 10 of hello.c

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• info breakpoint or i break: show all breakpoints

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• disable 2: don’t stopPrints n memory values of length unit u starting from addr in f format: at breakpoint #2 but keep it there
• enable 2: stop at breakpoint #2 again
• delete 2: remove breakpoint #2

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• save breakpoints file: save breakpoints to file
• source file: load breakpoints from file

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• step or s: step forward one line of code (goes into functions)
• step 2: step forward two line of code (goes into functions)
• stepi or si: step a single assembly instruction forward (goes into functions)

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• next or n: step forward one line of code (does not go into functions)
• next 2: step forward two line of code (does not go into functions)
• nexti or ni: step a single assembly instruction forward (does not go into functions)

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• finish or fin: continue running until the current function finished
• return: stop running the current function and return immediately
  • return expression: stop running and return the value of expression as the function’s return value
• continue or c: continue running until the next breakpoint

Inspecting Variable Values

• print a or p a: print value of variable a (which must be in the current function)

  • print/x a: print value of a as a hexadecimal number
  • print/o a: print value of a as a octal number
  • print/t a: print value of a as a binary number (show all bits)
  • print/s a: print value of a as a string even if it is not one
  • print *arr@10: print the first ten elements of array arr

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• x/nfu addr: prints n memory values of length unit u starting from addr in f format, where
  • f: x for hex output and o for oct output, etc. In particular, i is for instruction output and s asks gdb to print a string.
  • u: b for byte, h for half word (two bytes), w for word (four bytes), and g for giant word (eight bytes)

Tracing

• backtrace or bt: show the backtrace

Assembly and Registers

• layout src: switch to standard layout (of TUI mode) — source on top, and command window on the bottom

• layout asm: assembly on top, and command on the bottom

• layout split: source on top, assembly in the middle, and command at the bottom

• layout reg: open the register window on top of either source or assembly

• tui reg general: show the general registers

• tui reg float: show the floating point registers

• tui reg system: show the “system” registers

• tui reg next: show the next page of registers

• set disassembly-flavor [intel / att]: set the look-and-feel of the disassembly

Resources

Modern UI

• Pwndbg
• cyrus-and/gdb-dashboard

Tutorial and Tips

• Beej’s Quick Guide to GDB
• CSCI 2021 Quick Guide to gdb: The GNU Debugger
• hellogcc/100-gdb-tips