title	date	author	documentclass	lang	papersize
Debuging with gdb	\today	Jasper Levin Spahl	scrartcl	en	a4

How to debug with gdb

Given we have as simple C program with a bug

/* main.c */
#include <stdio.h>
int main()
{
    int balance = 100;
    int target = 1000;
    float rate = 0.1;
    int yeat = 0;
    do
    {
        float interest = balance * rate;
        balance = balance + interest;
        year++
    } while ( balance >= target)
    printf("%d No. of years to achieve target balance.\n", year);
	return 0;
}

To find bug we first have to compile the program with gcc -g -o program main.c. The -g flag is used to compile the program with debug symbols. Now that we have compiled the program we can open int gdb

Because we don't want to type all these long commands everytime we compile we can use a Makefile to make our lifes easyer.

# Makefile
program: main.c
        gcc -g -o program main.c
debug: program
        gdb program

Now writing make debug in the terminal will compile the program and open it in gdb.

To debug the program we first have to set a breakpoint

(gdb) b main
Breakpoint 1 at 0x1141: file main.c, line4.

To run the program in gdb type run

(gdb) run
Starting program: program

Breakpoint 1, main () at main.c:4
4               int balance = 100;

To step with s

(gdb) s
5 int target=1000;
(gdb) s
6 float rate = 0.1; 
(gdb) s
7 int year = 0; 
(gdb) s
10 float interest = balance * rate; 
(gdb) s
11 balance = balance + interest; 
(gdb) s
12 year++; 

At line 12 we want to take a look at the variables. The gdb command for that is p

(gdb) p balance
$1 = 110
(gdb) p rate
$2 = 0.100000001
(gdb) p interest
$3 = 10