My GDB Cheat Sheet

Posted on In ,

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.
1
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 number 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.
1
gdb ./a.out --tui -x debug.py -q

The example debug.py:

1
2
3
4
5
6
7
8
9
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
  • 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
  • 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
  • tbreak or tb: set a temporary breakpoint
  • break ... if ...: set a conditional breakpoint
  • watch a: stop when the value of variable a changed
  • 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
  • info breakpoint or i break: show all breakpoints
  • 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
  • save breakpoints file: save breakpoints to file
  • source file: load breakpoints from file
  • 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)
  • 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)
  • 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
  • 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.
    • 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

Resources