Example of use of debugging tools:

I was given the enclosed version of the program for Problem I of
Homework 2.  I was told that the program fails with a bus error in the
form given below, but works correctly if the strings are read in from
the keyboard (scanf() or cin) instead of from the command line.  The
person who wrote the program asked me what the problem was.

As you know, my answer to questions about program bugs is "Use a
debugger; use a debugger; use a debugger;..."  Here is how to use a
debugger in this case.  (I used gdb in order to have a record in this
text file, but would otherwise have used DDD for greater convenience.)

Recall that a bus error occurs if we attempt a word access with an
address not divisible by 4.  So, I had to find out why that was
happening.

First, I ran the program under gdb without any breakpoints, in order to
determine WHERE the bus error was occurring:

(gdb) r abcd abce 4
Starting program: /home/matloff/c1 abcd abce 4
warning: Unable to find __d_pid symbol in object file.
warning: Suggest linking with /usr/lib/end.o.
warning: GDB will be unable to track shl_load/shl_unload calls

General comparison
result is -1
String S1 'abcd' less than string S2 'abce'

strcmp comparison result is -1

Big Endian comparison

Program received signal SIGBUS, Bus error.
0x3430 in bestrcmp (S1=0x7b03a011 "abcd", S2=0x7b03a016 "abce", N=4)
    at compare.c:49
49                      if (*ptr1 - *ptr2 < 0)

Aha!  Since we got a bus error, one of these two pointers was not
divisible by 4.  Not only that, but we can see it was probably ptr1,
because we see above that S1 = 0x7b03a011.  Let's check:

(gdb) p ptr1
$1 = (int *) 0x7b03a011

Yep, sure enough!

Now, why did that occur?  Well, S1 came from argv[1], as seen in the
source code:

        S1=argv[1];

So, the problem arose was caused by argv[1] not starting on a word boundary.

SO, WE'VE FOUND THE SOURCE OF OUR BUG IN JUST 2 MINUTES, USING THE
DEBUGGER.  Now, why did argv[1] not start at a word boundary?  Well, why
should it?  It's just a string, not an integer, and the operating system
put it in the first spot the OS found to be free.  This happened to be
at 0x7b03a011.

To fix the problem, the person who wrote the program would have to copy
the string to a place which is guaranteed to start at a word boundary.
The most convenient way to ensure this is to copy the string to an int
array.

NM

Script started on Wed Jan 30 12:55:01 2002
hp16% cat compare.c
#include <stdio.h>
#include<string.h>

int genstrcmp(char *S1, char *S2, int N)
{
        int count=1;            /* when count = N then the comparison stops */
        char *ptr1,*ptr2;

        /* make ptr1 and ptr2 point to the first character in S1 and S2 respectively */
        ptr1=(char *)S1;
        ptr2=(char *)S2;
        
        /* do <= N comparisons */
        while (count<=N)
        {
                if (*ptr1 - *ptr2 < 0)
                { 
                        /* S1 less than S2 */
                        return -1;
                }
                else if (*ptr1 - *ptr2 > 0)
                {
                        /* S1 greater than S2 */
                        return 1;
                }
                else
                { 
                        /* count character of both S1 and S2 are equal */
                        count++;
                        ptr1++;
                        ptr2++;
                }       
        }
        
        /* the strings are equal */
        return 0;
}

int bestrcmp(char *S1, char *S2, int N)
{
        int *ptr1=(int *)S1;
        int *ptr2=(int *)S2;
        int wcount,i=1;

        wcount = N/4;  /* count the number of words in N . Strings are supposed to start and end on word boundaries */

        while (i<=wcount)
        {
                if (*ptr1 - *ptr2 < 0)
                {
                        /* S1 less than S2 */
                        return -1;
                }
                else if (*ptr1 - *ptr2 > 0)
                {
                        /* S1 greater than S2 */
                        return 1;
                }
                else
                {
                         /* count character of both S1 and S2 are equal */
                        i++;
                        ptr1++;
                        ptr2++;
                }
        }

        /* the strings are equal */
        return 0;               
}       

int main(int argc, char *argv[])
{
        char *S1, *S2;
        int N,result;

        /* get N, S1 and S2 from the user */
        
        S1=argv[1];
        S2=argv[2];
        N=atoi(argv[3]);

        /* call GenStrCmp */

        printf("\nGeneral comparison \n");
        
        result=genstrcmp(S1,S2,N);

        printf("result is %d \n",result);

        if (result == -1)
        {
                printf("String S1 '%s' less than string S2 '%s' \n",S1,S2);
        }
        else if (result == 1)
        {
                printf("String S2 '%s' greater than string S1 '%s' \n",S2,S1);
        }
        else
                printf("String S1 '%s' and string S2 '%s' are the same \n",S1,S2);
        
        /* strcmp */

        result=strcmp(S1,S2);
        
        printf("\nstrcmp comparison result is %d\n",result);

        /* call BEStrCmp */

        printf("\nBig Endian comparison \n");
        
        result=bestrcmp(S1,S2,N);

        printf("result is %d \n",result);
        
        if (result == -1)
        {
                printf("String S1 '%s' less than string S2 '%s' \n",S1,S2);
        }
        else if (result == 1)
        {
                printf("String S2 '%s' greater than string S1 '%s' \n",S2,S1);
        }
        else
                printf("String S1 '%s' and string S2 '%s' are the same \n",S1,S2);

        printf("\n");
        
        return 0;

}

hp16% gcc -g -o c1 compare.c
hp16% c1 abcd abce 4

General comparison 
result is -1 
String S1 'abcd' less than string S2 'abce' 

strcmp comparison result is -1

Big Endian comparison 
Bus error
hp16% gdb c1
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 4.16 (hppa1.1-hp-hpux10.10), 
Copyright 1996 Free Software Foundation, Inc...
(gdb) r abcd abce 4
Starting program: /home/matloff/c1 abcd abce 4
warning: Unable to find __d_pid symbol in object file.
warning: Suggest linking with /usr/lib/end.o.
warning: GDB will be unable to track shl_load/shl_unload calls

General comparison 
result is -1 
String S1 'abcd' less than string S2 'abce' 

strcmp comparison result is -1

Big Endian comparison 

Program received signal SIGBUS, Bus error.
0x3430 in bestrcmp (S1=0x7b03a011 "abcd", S2=0x7b03a016 "abce", N=4)
    at compare.c:49
49                      if (*ptr1 - *ptr2 < 0)
(gdb) p ptr1
$1 = (int *) 0x7b03a011
(gdb) q
The program is running.  Quit anyway (and kill it)? (y or n) y
hp16% e
exit

script done on Wed Jan 30 13:00:10 2002