gdb-notes.md

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>