GDB Refresher

1 minute read
notes
,

GDB Refresher

This cheatsheet provides a concise summary of the key GDB commands and concepts discussed in Debugging Refresher tutorial. It covers basic commands, memory examination, variable manipulation, disassembly, breakpoint management, scripting, configuration, and tips for using GDB.

Basic Commands

  • gdb <program>: Start GDB with a program
  • run [args]: Run the program (with optional arguments)
  • break <function/line>: Set a breakpoint
  • continue or c: Continue execution
  • next or n: Step over
  • step or s: Step into
  • finish: Run until the current function returns
  • print <expr> or p <expr>: Print value of expression
  • display <expr>: Display expression value after each step
  • info break: List breakpoints
  • delete <breakpoint-num>: Delete a breakpoint
  • quit or q: Exit GDB

Examining Memory and Registers

  • info registers or info reg: Show register values
  • print $<register>: Print specific register value (e.g., print $rax)
  • x/<format> <address>: Examine memory
    • Formats: x (hex), d (decimal), u (unsigned decimal), o (octal), t (binary), a (address), i (instruction), c (char), s (string)
    • Example: x/10i $rip (examine 10 instructions at current instruction pointer)

Variables and Memory Manipulation

  • set <variable> = <value>: Set a variable’s value
  • set $<register> = <value>: Set a register’s value
  • set {<type>}<address> = <value>: Set memory at address

Disassembly

  • disassemble <function>: Disassemble a function
  • disassemble <address>: Disassemble at an address

Breakpoint Commands

  • commands <breakpoint-num>: Specify commands to run when breakpoint is hit
  • silent: Make breakpoint silent (use within commands)
  • end: End list of commands for a breakpoint

Scripting

  • Create a file with GDB commands (e.g., myscript.gdb)
  • Run with: gdb -x myscript.gdb <program>

Configuration

  • Create ~/.gdbinit for persistent configurations
  • set disassembly-flavor intel: Set to Intel syntax (add to .gdbinit for persistence)

Plugins

  • GEF (GDB Enhanced Features): Provides richer output and additional commands
  • Enable in .gdbinit with: source /path/to/gef.py

Tips

  • Use $_ to reference the last value printed
  • Variables printed are stored as $1, $2, etc., for later reference
  • Use set pagination off to disable paging for long outputs

Reference