Debugging

GDB

Execution

full	short	desc
run ARG1	r	run program
attach PID		attach gdb to already running process
next	n	step to next source line (ie don’t step in to subroutine)
step	s	step through program (steps into subroutines)
stepi	si	step one instruction exactly
continue	c	continue running program
break LOC	b	set breakpoint at function/line
tbreak LOC		set breakpoint that breaks only once
watch EXPR		stop execution when a variable changes
rwatch EXPR		stop execution when a variable is read
info breakpoints	i b	list of breakpoints
disable NUM	dis	disable a breakpoint
kill		stop execution of program

Stack info

full	short	desc
backtrace	bt	backtrace
backtrace full	bt full	backtrace (with locals for each frame)
list	l	list surrounding source
info locals	i lo	show local variables
info args	i ar	show arguments
info frame	i f	show info about stack frame
finish	fin	return from function
call		call function linked into program
up N		go up n frames in the stack
down N		go down n frames in the stack

Examine memory:

cmd	desc
disassemble	disassemble section of memory
p/x VAR	print variable in hexadecimal
x ADDR	Examine memory at addr
whatis VAR	show variable type
set var VAR=VAL	Set variable to value
info registers	show registers (excluding floating point)
show env	Show environment
set env FOO = bar	Set environment variable FOO to bar
set env VAR=VAL	Set environment variable to value

cmd	desc
set print pretty on	pretty print structures
set print array on	readable array output
set print array-indexes on	show array indices
set print demangle on	demangle C++ names
macro expand MACRO	expands a macro (requires -gdwarf-2 -g3 compiler flags)
source FILE	source a GDB macro file

code	print type
x	hexadecimal
u	unsigned int
d	signed int
o	octal
u	unsigned decimal
t	binary
f	floating point
a	address
c	char
s	string

Examine a core dump:

gdb executable coredump

Extensions

http://rr-project.org/

LLDB

LLDB <-> GDB Cheatsheet

command	description
r ARGS	run program (with arguments)
s	step
env FOO=BAR	set environment variable
frame variable	list local variables
bt	backtrace
disassemble -f	disassemble frame

Bryan Cantrill

Debugging Under Fire (YouTube)

Debugging is the process by which we understand pathological behavior in a software system

The essence of debugging is asking and answering questions - and the craft of writing debuggable software is allowing the software to be able to answer questions about itself.

Recovery in lieu of understanding normalizes broken software.

Mike Perham

The Three Best Debugging Tools

1. Your coworker
2. Your ‘creative side’
3. Your Mental Model of the System

I agree a lot with #3 - often when I’m helping someone debugging, it’s helping them establish this mental model.