Lecture #7: Debugging

Reading: none
Handouts: The Dynamic Debugger gdb; Guide to Faster, Less Frustrating Debugging
Homework: study for the midterm!

Greetings and felicitations
Debugging
1. programs have bugs; find and fix them
2. static debugging: insert debugging code into source, recompile and run
3. dynamic debugging: look at the program as it runs, observing (and maybe changing) variables, etc.
Static debugging
1. using printf to print variable values; mention %p (prints pointer value, usually as a hex integer)
2. using printf to print where you are (ie, on function entry printf("in function\n");
3. #ifdef DEBUG ... #endif around the printfs so you can leave them in the source if you need them again
4. assert(x) macro: assert(0 <= i && i <= n) causes program to exit with error message if (0 <= I && I <= n) is false; must include <assert.h>. To delete, say #define NDEBUG and they will not be in the compiled code.
Dynamic debugging
1. debugging tool instruments executable program so it can be stopped, examined, altered, and continued interactively
2. go through the handout
3. mention the "where" command which shows you the program stack

The Dynamic Debugger gdb

Introduction

This handout introduces the basics of using gdb, a very powerful dynamic debugging tool. No-one always writes programs that execute perfectly every time, and while reading the program source can help fiind bugs, some can only be discovered by running the program and seeing what happens. That's where a dynamic debugger comes in; it lets you stop execution during the run and look at variables.

Setting It Up

To use gdb, you must compile your program using gcc and give the -g flag:

gcc -ansi -g program.c -o program

The -g flag tells the compiler to add information for the debugger. To debug your program, simply say:

gdb program

If you omit program, gdb looks for an a.out file.

The rest of this handout contains several sample programs and how to use gdb to find the bugs. I recommend you use gdb to get used to it; in particular, make extensive use of its help command! Just type

help

to its prompt, and it will tell you what to do. Some of the features that I did not show which you will find particularly useful are the backtrace facility, which shows you what routines have been called and the values of the parameters.

Example 1

Here's a program that is supposed to add 2 to a variable j every time through the for loop:

#include <stdio.h>

main()
{
	int i, j = 0;
	for(i = 0; i < 100; i++);
		j += 2;
	printf("The value of j is: %d\n", j);
}

I compile and run it, and it does not work:

% gcc -ansi -g -o sample1 sample1.c
% sample1
The value of j is: 2

Oops! Let's get out gdb and see what happens. As usual, what I type is in boldface, what the computer prints is in plain face, and my comments are in italics.

% gdb sample1
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.10.pl1 (sparc-sun-sunos4.1), 
Copyright 1993 Free Software Foundation, Inc...
(gdb) l				list 10 lines
1	#include <stdio.h>
2	
3	main()
4	{
5		int i, j = 0;
6		for(i = 0; i < 100; i++);
7			j += 2;
8		printf("The value of j is: %d\n", j);
9	}
(gdb) b main			put in a breakpoint
				the program will stop at main
				when it is executed
Breakpoint 1 at 0x22b4: file sample1.c, line 4.
(gdb) r				run the program
Starting program: /usr/home/bishop/sample1 

Breakpoint 1, main () at sample1.c:5
5		int i, j = 0;
(gdb) n				do the next statement
6		for(i = 0; i < 100; i++);
(gdb) p i			print i's value
$1 = 100
(gdb) p j			print j's value
$2 = 0
				aha! why is it not 200?  It's not
				getting incremented right, so let's
				check the for loop ... and sure
				enough, that's where the problem is!
(gdb) q
The program is running.  Quit anyway (and kill it)? (y or n) y

Example 2

Now for a more complex example. Here's a program that's supposed to multiply s by 2 until s is greater than 100:

#include <stdio.h>

main()
{
	int i = 1, s;

	s = 3;
	while(i = 1){
		s += s;
		if (s > 100)
			i = 0;
	}
}

I compile and run it, and it hangs; I need to kill it with control-C:

% gcc -ansi -g -o sample2 sample2.c
% sample2
^C

Again, let's use gdb to figure out what happened:

% gdb sample2
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.10.pl1 (sparc-sun-sunos4.1), 
Copyright 1993 Free Software Foundation, Inc...
(gdb) l
1	#include <stdio.h>
2	
3	main()
4	{
5		int i = 1, s;
6	
7		s = 3;
8		while(i = 1){
9			s += s;
10			if (s > 100)
(gdb) b 9
Breakpoint 1 at 0x22b4: file sample2.c, line 9.
(gdb) r
Starting program: /usr/export/home/bishop/ECS-40/gdb/sample2 

Breakpoint 1, main () at sample2.c:9
9			s += s;

(gdb) p s
$1 = 3
(gdb) n
10			if (s > 100)
(gdb) c					continue the run
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
(gdb) p s
$2 = 6
					seems to be doubling s okay
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
(gdb) p s
$3 = 12
					I'm tired of always typing "p s"
					when the program stops, so 
					associate commands with that
					breakpoint
(gdb) commands 1 
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
p s
end
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
$4 = 24
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
$5 = 48
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
$6 = 96
(gdb) p i
$7 = 1
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
$8 = 192
					wait ... s is high here.  Let's
					watch the value of i
(gdb) watch i				break whenever i changes
Watchpoint 2: i
(gdb) c
Continuing.

Watchpoint 2, i				i just changed value
Old value = 1
New value = 0
main () at sample2.c:12
12		}
(gdb) c
Continuing.

Breakpoint 1, main () at sample2.c:9
9			s += s;
$9 = 384
(gdb) c
Continuing.

Watchpoint 2, i				why does i change -- the "if" is
					not executed!  Look in the while
					expression; we used =, not ==
Old value = 1
New value = 0
main () at sample2.c:12
12		}
(gdb) q
The program is running.  Quit anyway (and kill it)? (y or n) y

Example 3

This one is a character counter (with apologies to Kernighan and Ritchie, from whose book it was mangled):

#include <stdio.h>

main()	/* counts digits, white space, others */
{
	int c, i, num_digits[10], num_white, num_other;
	num_white = num_other = 0;
	for(i = 0; i < 10; i++)	/* initialize */
		num_digits[i] = 0;
	while(c = getchar() != EOF){
		switch(c){
		case '0': case '1': case '2': case '3':
		case '4': case '5': case '6': case '7':
		case '8': case '9':
			num_digits[c - '0']++;
			break;
		case ' ': case '\t': case '\n':
			num_white++;
			break;
		default:
			num_other++;
			break;
		}
	}
	printf("digits =");
	for(i = 0; i < 10; i++)
		printf(" %d", num_digits[i]);
	printf("white space = %d, other = %d\n",
		num_white, num_other);
}

Here's a data file for our testing and debugging pleasure:

7 4 88 6 6  3 4 7 87 8 2 34
  5 7  3 21 123q52

I compile and run it, and it hangs:

% gcc -ansi -g -o sample3 sample3.c
% sample3 < sample3.data
^C %

% gdb sample3
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.10.pl1 (sparc-sun-sunos4.1), 
Copyright 1993 Free Software Foundation, Inc...
(gdb) l
1	#include <stdio.h>
2	
3	main()	/* counts digits, white space, others */
4	{
5		int c, i, num_digits[10], num_white, num_other;
6		num_white = num_other = 0;
7		for(i = 0; i <= 10; i++)	/* initialize */
8			num_digits[i] = 0;
9		while(c = getchar() != EOF){
10			switch(c){
(gdb) b 8
Breakpoint 1 at 0x22f0: file sample3.c, line 8.
(gdb) comm 1
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
p i
end
(gdb) r
Starting program: /usr/export/home/bishop/ECS-40/gdb/sample3 

Breakpoint 1, main () at sample3.c:8
8			num_digits[i] = 0;
$1 = 0
(gdb) s				s means to skip to the next
				statement; if it calls a
				function, you'll stop in that
				function.  n means to skip to the
				next statement in this function;
				you skip over any functions that
				are called.
7		for(i = 0; i <= 10; i++)	/* initialize */
(gdb) n

Breakpoint 1, main () at sample3.c:8
8			num_digits[i] = 0;
$2 = 1
(gdb) cond 1 i>=9		this means to skip breakpoint 1
				until i >= 9 is true.
(gdb) c
Continuing.

Breakpoint 1, main () at sample3.c:8
8			num_digits[i] = 0;
$3 = 9
(gdb) c
Continuing.

Breakpoint 1, main () at sample3.c:8
8			num_digits[i] = 0;
$4 = 10				now we should leave the loop
(gdb) c
Continuing.

Breakpoint 1, main () at sample3.c:8
8			num_digits[i] = 0;
$5 = 9				huh?  This probably means we're
				overwriting i -- or our array has
				one less element than we think.
(gdb) q
The program is running.  Quit anyway (and kill it)? (y or n) y

We make the change, and run it:

% sample3a < sample3.data
digits = 0 0 0 0 0 0 0 0 0 0white space = 0, other = 48
% gdb sample3
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.10.pl1 (sparc-sun-sunos4.1), 
Copyright 1993 Free Software Foundation, Inc...
(gdb) l
1	#include <stdio.h>
2	
3	main()	/* counts digits, white space, others */
4	{
5		int c, i, num_digits[10], num_white, num_other;
6		num_white = num_other = 0;
7		for(i = 0; i < 10; i++)	/* initialize */
8			num_digits[i] = 0;
9		while(c = getchar() != EOF){
10			switch(c){
(gdb) b 10				since the for loop seems to
					work, it's probably in the while
Breakpoint 1 at 0x23ac: file sample3.c, line 10.
(gdb) r < sample3.data		note I can redirect standard
					input, output, and error as if
					I were in the shell
Starting program: /usr/export/home/bishop/ECS-40/gdb/sample3 < sample3.data

Breakpoint 1, main () at sample3.c:10
10			switch(c){
(gdb) p c
$1 = 1					oops ... there's no ^A in the file!
(gdb) c
Continuing.

Breakpoint 1, main () at sample3.c:10
10			switch(c){
(gdb) p c
$2 = 1					again!  looks like an assignment
					when I should be comparing ...
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y

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Send email to cs40@csif.cs.ucdavis.edu.

Department of Computer Science
University of California at Davis
Davis, CA 95616-8562

Page last modified on 11/11/97