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

---

# How to debug with `gdb`

Given we have as simple C program with a bug

```c
/* 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
# 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
```