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gdb-notes.md

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Prerequisites

  • Make sure to install the gdb package
  • Make sure to compile with -g -gdb3 and -O0

Starting

Load the binary with gdb ./hellband

Setting breakpoints and watches

  • break <filename>:<line>
  • watch <variable> (breaks on write access)
  • rwatch <variable> (breaks on read access)
  • awatch <variable> (breaks on read and write access)

Querying variables

  • info variables to list "All global and static variable names".
  • info locals to list "Local variables of current stack frame" (names and values)
  • info args to list "Arguments of the current stack frame" (names and values).
  • print <variable> to show a variable variable
  • explore <variable> to inspect a variable variable

Call stack

  • bt to show all frames aka the the call stack
  • select-frame <n> to select frame number n from the call stack

Program flow

  • continue to run the program till the end or the next breakpoint
  • step to execute the whole line at once
  • next to execute the next atomic part of the line
  • finish to return to previous frame

Getting fancy

  • Consider installing https://github.com/cyrus-and/gdb-dashboard
  • Having 2 ssh sessions with one running in gdb dashboard -output <tty> can be gold
  • c is shorthand for continue
  • frame is shorthand for select-frame
  • p is shorthand for print

Debugging a stuck game

  • Open two sessions
  • Get one session stuck within a gdb run
  • Find the hellband/gdb session with ps -al
  • Send from the other session a SIGSEGV with kill -s SIGSEGV <pid>