gdb tips - PART1

Some gdb ( GNU debugger) tips

It is very important to understand the debugging tools and their different functionalities to hack the system level programs easily.

How to compile a program to debug in gdb.

1. compile your C program with -g option

   $cc -g helloworld.c

2. Open the executable in gdb
   $gdb a.out

3. Run the program using below commands in gdb prompt using simply ‘r’.
   (gdb)run or (gdb)r

4. Continue run from breakpoint using ‘c’
   (gdb)c

Setting Breakpoint in gdb.

1.Setting up breakpoint in a line number
$b 10
2. breakpoint setting up in an address
$b *0x7c00

(gdb) b *0x7c00
Breakpoint 1 at 0x7c00
(gdb) c
Continuing.
Thread 1 hit Breakpoint 1, 0x00007c00 in ?? ()

3.Setting up breakpoint in a function
$b main

Switch to tui mode using ctrl+x 2

you can switch to tui ( text gui mode) mode using ctrl+x 2
to view the source file in the same screen.

you can ctrl+x 1 to see registers values

ctrl+x n to come back to gdb prompt.

stepping through line and instruction

$si \ steps through instruction
$s \ source level step through next line

Next step over through line

$n
$next

Printing variables

$p foo//print the value of foo
$p car // print the value of car variable

$p/x addr //printing hexa decimal values

information about frames and registers

Current function stack frame details can be displayed using the command ‘f’ or ‘frame’
$f
$frame

The registers details or values can be seen using the command ’ info r’

info r

backtrace of call stacks

How can we backtrace all call stacks in gdb?!. This would be very useful command especially when you are debugging big projects source codes.

$bt
shows the backtrace of all call stacks so far.

You can switch to different frames using below command

$f 1
to switch to the first frame
f 5 switch to 5th frame etc

Some of the helpful commands when you are in a function

$info locals
shows the local variable in a function

$info args
shows arguments of the function

getting help on gdb

(gdb) help

List of classes of commands:

aliases – Aliases of other commands
breakpoints – Making program stop at certain points
data – Examining data
files – Specifying and examining files
internals – Maintenance commands
obscure – Obscure features
running – Running the program

(gdb)h breakpoints //provide help on breakpoints

PART 2 -
https://naveendavisv.blogspot.com/2020/04/gdb-debugging-part-2.html