UNB/ CS/ David Bremner/ tags/ cs2023/ gdb

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The Fibonacci numbers are defined by F[0]=0, F[1]=1, and F[n]=F[n-1]+F[n-2]

The first few are as follows

0 1 2 3 4 5 6 7 8 9 10
0 1 1 2 3 5 8 13 21 34 55

Your task is to debug a program to compute factorials. You can either cut and paste from the browser, or copy from /fcs/courses/cs2023/tutorial3/fib.c. There are at least 3 bugs in this program.

Instructions.

In the following, "HAL9000> " indicates the bash command line, and "(gdb) " the gdb prompt. Follow along my script, filling in details where I left them out.

Initial run


 
HAL9000> gcc fib.c 
HAL9000> ./a.out 
Enter a positive integer 7 

What is the output?

First attempt at using the debugger


HAL9000> gdb a.out
GNU gdb (GDB) 6.8.50.20090628-cvs-debian
[snip]
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
(gdb) run
Starting program: /home/bremner/teaching/cs2023/tutorial/t3/a.out 
Enter a positive integer
7

Program received signal SIGSEGV, Segmentation fault.
0xb7ebb8f9 in _IO_vfscanf () from /lib/i686/cmov/libc.so.6
(gdb) bt
#0  0xb7ebb8f9 in _IO_vfscanf () from /lib/i686/cmov/libc.so.6
#1  0xb7ec0388 in scanf () from /lib/i686/cmov/libc.so.6
#2  0x08048472 in main ()
(gdb) list
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y

Compiling with debugging information.

Ok, that wasn't too helpful. Let us recompile with debugging information.


HAL9000> gcc -g fib.c
HAL9000> gdb ./a.out
GNU gdb (GDB) 6.8.50.20090628-cvs-debian
[snip]
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
(gdb) run
Starting program: /home/bremner/teaching/cs2023/tutorial/t3/a.out 
Enter a positive integer
7

Program received signal SIGSEGV, Segmentation fault.
0xb7ebb8f9 in _IO_vfscanf () from /lib/i686/cmov/libc.so.6
(gdb) bt
#0  0xb7ebb8f9 in _IO_vfscanf () from /lib/i686/cmov/libc.so.6
#1  0xb7ec0388 in scanf () from /lib/i686/cmov/libc.so.6
#2  0x08048472 in main () at fib.c:10
(gdb) list
1       #include <stdio.h>
2       
3       int main(void){
4       
5         int n;
6         int fib,last=1,before_last=0;
7         int i;
8       
9         printf("Enter a positive integer\n");
10        scanf("%d",n);
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y

OK, so the debugger showed us the source line with the error. Fix it, and put the new version in fib1.c

Next bug


HAL9000> gcc -g fib1.c
HAL9000> ./a.out
Enter a positive integer
3

Hmm, the output doesn't look right.


HAL9000> gdb ./a.out
GNU gdb (GDB) 6.8.50.20090628-cvs-debian
[snip]

(gdb) list
(gdb) list
(gdb) break 12
Breakpoint 1 at 0x8048472: file fib1.c, line 12.
(gdb) run
Starting program: /home/bremner/teaching/cs2023/tutorial/t3/a.out 
Enter a positive integer
3
Breakpoint 1, main () at fib1.c:12
12        for (i=2; i<n ; i++){
(gdb) n
13          fib=last+before_last;
(gdb) display fib
1: fib = 134518436
(gdb) n
14          before_last=last;
1: fib = 1
(gdb) n
15          last=fib;
1: fib = 1
(gdb) n
12        for (i=2; i<n ; i++){
1: fib = 1
(gdb) n
18        printf("F[%d]=%d\n",n,fib);
1: fib = 1
(gdb) n
F[3]=1
19      }
1: fib = 1
(gdb) n
0xb7e867a5 in __libc_start_main () from /lib/i686/cmov/libc.so.6
(gdb) q
The program is running.  Quit anyway (and kill it)? (y or n) y


There seems to be a problem with the loop bounds. Fix it, and put the version in fib2.c.

last bug?


HAL9000> gcc -g fib2.c
HAL9000> ./a.out
Enter a positive integer
3
F[3]=2
HAL9000> # yay
HAL9000> ./a.out
Enter a positive integer
1
F[1]=134518436
HAL9000> # boo!
HAL9000> gdb ./a.out
GNU gdb (GDB) 6.8.50.20090628-cvs-debian
[snip]
(gdb) list
1       #include <stdio.h>
2       
3       int main(void){
4       
5         int n;
6         int fib,last=1,before_last=0;
7         int i;
8       
9         printf("Enter a positive integer\n");
10        scanf("%d",&n);
(gdb) list
11      
12        for (i=1; i<n ; i++){
13          fib=last+before_last;
14          before_last=last;
15          last=fib;
16        }
17      
18        printf("F[%d]=%d\n",n,fib);
19      }
(gdb) break 12
Breakpoint 1 at 0x8048472: file fib2.c, line 12.
(gdb) run
Starting program: /home/bremner/teaching/cs2023/tutorial/t3/a.out 
Enter a positive integer
1

Breakpoint 1, main () at fib2.c:12
12        for (i=1; i<n ; i++){
(gdb) n
18        printf("F[%d]=%d\n",n,fib);
(gdb) n
F[1]=134518436
19      }
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y

Hmm. We didn't go through the loop at all. Actually, that is OK, but there is another problem to fix. Find it, and put the output in fib3.c


HAL9000> gcc -g fib3.c
HAL9000> ./a.out
Enter a positive integer
1
HAL9000> ./a.out
Enter a positive integer
10
Posted Tue 29 Sep 2009 08:30:00 AM ADT Tags: