GCC Debugging/gdb

Preping for the debugger

program must first be compiled for debugging:

gcc -g <filname.c> -o <output filename>

eg: gcc -g debug_me.c -o debug_me

the "-g" flag is important (To create symbol table)

Starting the debugger

start the debugger with:

gdb <program name>

eg: gdb debug_me

common commands

run, print, where, break, cond, step, next, set, ignore

run

runs the program

to pass arguments to the program on run:

run <arg1> <arg2>

eg: run 12 35

eg: run "hello world" "goodbye world"

break

sets a "breakpoint" at a certain area \ function

break <function name>

eg: break main

or

eg: b main

break <filename>:<line of code to stop in>

eg: break debug_me.c:5

b

shorthand for break

step

next line of code, stepping into functions

eg: step

continue

go to next breakpoint or end of program

cont

shorthand for continue

print

print out a variables \ expressions contents

print <variable name>

eg: print x

disp

prints out a variable \ expression value every step

eg: disp x

undisplay

cancel a display command

eg: undisplay 3

would stop the var at pos 3 from displaying every time

where

move within the call stack

...

up and down

up: previous step inside call stack

down: next step inside call stack

list

shows code at a certain location

list <no args>

code at current location and then at next location if typed again

eg: list

list -

shows code at previous location and then at the location before that if typed again

list <function name>

shows code in function definition

eg: list func1

cond

conditional

cond <breakpoint> <condition>

eg: cond 1 val>0

stop at breakpoint "1" when "val" becomes greater than 0

set

change a value, eg

set variable x = 12

will change x's value to 12

ignore

ignore a breakpoint for a user specified number of times.

usage: ignore <id> <count>

ignore 2 20 --> Breakpoint 2 will not be hit for 20 times.

quit

exits the gdb

q

shorthand for quit