How to use GDB? (Example)

This section will demonstrate how to use GDB commands by going through example.

Here is program which is buggy in its behavior. 
This program is writtten to compute no. of years taken to reach target balance provided current balance and  yearly interest rate.
But it gives 1 year as result for any value of balance or target.
/* File: buggy.c */
#include <stdio.h>
int main()
{ 
	int balance=100;
	int target=1000;
	float rate = 0.1;
	int year = 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;
}
 
Here are steps to debug this program.
Step 1: Compile and Build program with debugging symbols $ gcc -g buggy.c
 
Step 2: Run program with GDB$ gdb a.out
 
Step 3: Set a breakpoint on main function.(gdb) b main
Breakpoint 1 at 0x400535: file buggy.c, line 5.
 
Step 4: Run program(gdb) run
Starting program: a.out
 
Breakpoint 1, main () at buggy.c:5
5 int balance=100;As breakpoing was set on main function, program will stop at main function and wait for gdb command.
 
Step 5: Step until we reach till line 13(gdb) s
6 int target=1000; 
(gdb) s
7 float rate = 0.1; 
(gdb) s
8 int year = 0; 
(gdb) s
11 float interest = balance * rate; 
(gdb) s
12 balance = balance + interest; 
(gdb) s
13 year++; 
 
Step 6: Print value of balance, rate, interest(gdb) p balance
$1 = 110
(gdb) p rate
$2 = 0.100000001
(gdb) p interest
$3 = 10
 
Step 7: Step until we reach to line 15(gdb) s
14 } while ( balance >= target );  
(gdb) s
15 printf("%d No. of years to achieve target balance.\n", year);
 
Step 8: Print value of target, year, balance(gdb) p target
$5 = 1000
(gdb) p year
$6 = 1
(gdb) p balance
$7 = 110Value of balance is 110 and target is 1000. That seems ok.
Codition in while loop i.e. balance >= target, will be flase. And loop will break.
So condition is wrong here. It should be balance < target.
Congratulation! You have debugged your first buggy program.

Comments

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Sun, 10/08/2017 - 20:58

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