Introduction to DBX
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Introduction to DBX Debugging is usually considered to be the most difficult, frustrating and longest step in programming. It is in this step that we come to fully understand the true nature of the programs we write. We need a tool to help us with this. That's why we have DBX. DBX is an interactive debugging tool in the UNIX environment that can help you to locate bugs quickly and easily. The current version of dbx that comes with IRIX 4 release 5.3 works with C/C++ (cc/CC), fortran77 (f77), assembler (as) and pascal (pc). DBX is easy to learn and use. (But even if it were difficult to learn, it would be well worth the trouble because of the potential troubles it may prevent.)
DBX is used after you finally got your code to compile, and it isn't doing what you want it to do (including the infamous core dump). At this point, there must be a logical problem or a dumb mistake in the code that the compiler cannot identify. Don't you wish that you could go thru your program with the CPU step by step with the ability to check the variables in your program on the way?? That is exactly what DBX enables you to do!! In this tutorial, we are going to describe release 5.3 of dbx that comes with IRIX 4.
Using DBX
run runs the program inputted into dbx stop at line_number sets a break point at line_number. A running program would automatically stop at the breakpoint and returns control to the debugger. stop in function_name sets a breakpoint at first line of function_name cont continues execution after a breakpoint print expression prints out the value of the expression which might be a variable step executes the current line of the program. If the current line is a function call, then go into the function and stop at the first line of the function next executes the current line of the program. If the current line is a function call, then execute the function as one single line and stop at the next line. list lists the C source code of the program with line numbers where prints out where you currently are in the program by printing out a list of active functions assign exp1 = exp2 assign the value of exp2 to the variable in exp1There are other commands that could be useful for you. If you are interested, type help in dbx for further details.delete event# delete the breakpoint specified by event#
delete all delete all breakpoints
trace variable_name prints out the value of the variable when the value changes
Examples Here's a very simple program that prints out a fahrenheit-to-centigrade table.
%(1) cat example1.c
#include <stdio.h>
main()
{ int fahr, celsius;
int lower, upper, step;
lower = 0;
step = 20;
fahr = lower;
while (fahr <= upper)
{ celsius = 5 * (fahr - 32) /9;
printf("%d\t%d\n", fahr, celsius);
fahr = fahr + step;
}
}
%(2) cc -g -o example1 example1.c
%(3) example1
0 -17
Somehow it only prints out one line of output. We will try to use dbx to
debug it.
%(4) dbx example1
dbx version 3.18 Jan 24 1995 12:01:24
Type 'help' for help.
Process 25121 (example1) started
[2] Process 25121 (example1) stopped at [main:4 ,0x4008a8]
4 lower = 0;
(dbx) list
5 step = 20;
6 fahr = lower;
7 while (fahr <= upper)
8 { celsius = 5 * (fahr - 32) /9;
9 printf("%d\t%d\n", fahr, celsius);
10 fahr = fahr + step;
11 }
12 return 0;
13 }
14
(dbx) list 7,11
7 while (fahr <= upper)
8 { celsius = 5 * (fahr - 32) /9;
9 printf("%d\t%d\n", fahr, celsius);
> 10 fahr = fahr + step;
11 }
(dbx)
The program seems to stop too soon. We'll set a breakpoint at the loop.
(dbx) stop at 7
Process 25121: [3] stop at "/home/ucc/disk003/poonc/work/example1.c":7
(dbx) run
Process 25121 (example1) terminated
Process 26430 (example1) started
[3] Process 26430 (example1) stopped at [main:7 ,0x4008c0]
7 while (fahr <= upper)
(dbx) next
Process 26430 (example1) stopped at [main:8 ,0x4008d8]
8 { celsius = 5 * (fahr - 32) /9;
(dbx) next
Process 26430 (example1) stopped at [main:9 ,0x4008fc]
9 printf("%d\t%d\n", fahr, celsius);
(dbx) next
0 -17
Process 26430 (example1) stopped at [main:10 ,0x40091c]
10 fahr = fahr + step;
(dbx) next
Process 26430 (example1) stopped at [main:12 ,0x400948]
12 return 0;
That's strange.. the program only ran for one loop then exit. Let's print
out the values of the variables and take a look..
(dbx) print fahr
20
(dbx) print upper
0
What? upper = 0! We forgot to set the upper value! We often have bugs that
are very simple but are overlooked. DBX gives us an advantage by providing
us more clues of the puzzle.
(dbx) quit
%(5)
Back to table of contents
The following program right justify an inputted line.
%(1) cat example2.c
#include <stdio.h>
#include <string.h>
int right_justify(char input[], int line);
void generate_blank_string(char blanks[], int linewidth, int word_length);
main()
{ int index, the_count, line_length = 30;
char in_string[50];
for (index = 0, the_count = 0; index < 20; index++)
{
(void) fgets(in_string,sizeof(in_string), stdin);
the_count += right_justify(in_string, line_length);
}
printf("\n\nThe number of times the article \"the\" occurs is %d\n",
the_count);
return 0;
}
int right_justify(char in_string[], int line_length)
{
char blank_string[40];
generate_blank_string(blank_string, line_length, strlen(in_string));
printf("%s%s", blank_string, in_string);
if (strcmp(in_string, "the\n") == 0) return 1;
else return 0;
}
void generate_blank_string(char blanks[], int linewidth, int word_length)
{
int counter, num_blanks;
num_blanks = linewidth - word_length;
for (counter = 1; counter <= num_blanks; counter++)
strcat(blanks, " ");
}
%(2) cc -g -o example2 example2.c
%(3) example2
I typed this line
y~` I typed this line
2nd line is output
y~` 2nd line is output
There are some garbage before the output. Let's use dbx to check it out.
%(4) dbx example2
dbx version 3.18 Jan 24 1995 12:01:24
Type 'help' for help.
Process 21048 (example2) started
[2] Process 21048 (example2) stopped at [main:10 ,0x4009c4]
10 for (index = 0, the_count = 0; index < 20; index++)
(dbx) list
11 {
12 (void) fgets(in_string,sizeof(in_string), stdin);
13 the_count += right_justify(in_string, line_length);
14
15 }
16 printf("\n\nThe number of times the article \"the\" occurs is %d\n
",
17 the_count);
18 return 0;
19 }
20
(dbx) list
21
22 int right_justify(char in_string[], int line_length)
23 {
24 char blank_string[40];
25 generate_blank_string(blank_string, line_length, strlen(in_string)
);
26 printf("%s%s", blank_string, in_string);
27 if (strcmp(in_string, "the\n") == 0) return 1;
28 else return 0;
29 }
30
(dbx) list
31 void generate_blank_string(char blanks[], int linewidth, int word_length
)
32 {
33 int counter, num_blanks;
34 num_blanks = linewidth - word_length;
35 for (counter = 1; counter <= num_blanks; counter++)
36 strcat(blanks, " ");
37 }
38
(dbx) stop in generate_blank_string
Process 21048: [3] stop in generate_blank_string
(dbx) run
Process 21048 (example2) terminated
Process 21405 (example2) started
I typed this line
[3] Process 21405 (example2) stopped at [generate_blank_string:34 ,0x400b4c]
34 num_blanks = linewidth - word_length;
(dbx) where
> 0 generate_blank_string(blanks = 0x7fffae58 = "\177\377\257\250", linewidth =
30, word_length = 18) ["/home/ucc/disk003/poonc/work/example2.c":34, 0x400b4c]
1 right_justify(in_string = 0x7fffaea8 = "I typed this line\n", line_length =
30) ["/home/ucc/disk003/poonc/work/example2.c":25, 0x400aac]
2 main() ["/home/ucc/disk003/poonc/work/example2.c":13, 0x4009f4]
The where command entered above shows us we are in a function call
"generate_blank_string()", which is called by "right_justify()", which is in
turned called by main().
(dbx) step
Process 21405 (example2) stopped at [generate_blank_string:35 ,0x400b60]
35 for (counter = 1; counter <= num_blanks; counter++)
(dbx) print num_blanks
12
(dbx) print blanks
0x7fffae58 = "\177\377\257\250"
There seems to be something in blanks.
(dbx) stop at 37
Process 21405: [4] stop at "/home/ucc/disk003/poonc/work/example2.c":37
(dbx) cont
[4] Process 21405 (example2) stopped at [generate_blank_string:37 ,0x400ba4]
37 }
(dbx) print blanks
0x7fffae58 = "\177\377\257\250 "
There are often some garbages in an uninitialized string. The spaces are being
appended to the garbages. I bet we can solve the problem by initializing
the string. Let's test out the theory.
(dbx) delete 4
The second breakpoint is removed here. Note the 4 in square brankets when
we set the breakpoint earlier. That's the event #, and is used as an
argument here for the delete command.
(dbx) run
Process 21405 (example2) terminated
Process 22123 (example2) started
I typed this line
[3] Process 22123 (example2) stopped at [generate_blank_string:34 ,0x400b4c]
34 num_blanks = linewidth - word_length;
(dbx) assign blanks[0]='\0'
'\000'
(dbx) cont
I typed this line
Interrupt
(dbx) quit
Process 22123 (example2) terminated
It seems to work great. Our problem is solved.
Core dumping is one of the most annoying thing in C programming in a UNIX environment. They give you little hints to the origin of the error. But what is core? It's a copy of the memory when your program crash. DBX can read this information and locate the error.
%(1) cat example3.c
#include <stdio.h>
#include <string.h>
int my_func(char in[]);
main()
{ char line[] = {'h', 'e', 'l', 'l', 'o', 'a', 'c', 'i', 'x' , 'u','\0'};
int num, count;
printf("num of vowels are %d\n", my_func(line));
return 0;
}
int my_func(char in[])
{ char in1[50];
int i;
for (i = 0; i < strlen(in); i++)
in1[i] = in[i+1];
switch (in[0])
{ case 'a' : return (1 + my_func(in1));
case 'e' : return (1 + my_func(in1));
case 'i' : return (1 + my_func(in1));
case 'o' : return (1 + my_func(in1));
case 'u' : return (1 + my_func(in1));
default : return my_func(in1);
}
}
%(2) cc -g -o example3 example3.c
%(3) example3
Segmentation fault (core dumped)
%(4) dbx example3
dbx version 3.18 Jan 24 1995 12:01:24
Type 'help' for help.
Process died at pc 0x400984 of signal : Segmentation fault
[using memory image in core]
(dbx)
DBX read the information provided by the core file and go back to the location
where the segmentation fault occur. Watch:
(dbx) where
> 0 my_func(in = (nil)) [0x400980]
1 my_func(in = 0x7c00009c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
2 my_func(in = 0x7c0000f4 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
3 my_func(in = 0x7c00014c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
4 my_func(in = 0x7c0001a4 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
5 my_func(in = 0x7c0001fc = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
6 my_func(in = 0x7c000254 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
7 my_func(in = 0x7c0002ac = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
8 my_func(in = 0x7c000304 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
9 my_func(in = 0x7c00035c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
10 my_func(in = 0x7c0003b4 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
11 my_func(in = 0x7c00040c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
12 my_func(in = 0x7c000464 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
13 my_func(in = 0x7c0004bc = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
14 my_func(in = 0x7c000514 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
15 my_func(in = 0x7c00056c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
16 my_func(in = 0x7c0005c4 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
17 my_func(in = 0x7c00061c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
18 my_func(in = 0x7c000674 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
19 my_func(in = 0x7c0006cc = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
20 my_func(in = 0x7c000724 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
21 my_func(in = 0x7c00077c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
22 my_func(in = 0x7c0007d4 = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
23 my_func(in = 0x7c00082c = "") ["/tmp/bruce/example3.c":25, 0x400ad4]
More (n if no)?n
(dbx)
Well.. it seems like we got a infinite recursion on our hands. It should not
recurse that many times.
(dbx) list
(dbx) stop in my_func
Process 0: [2] stop in my_func
(dbx) list 13
13 int my_func(char in[])
14 { char in1[50];
15 int i;
16
> 17 for (i = 0; i < strlen(in); i++)
18 in1[i] = in[i+1];
19 switch (in[0])
20 { case 'a' : return (1 + my_func(in1));
21 case 'e' : return (1 + my_func(in1));
22 case 'i' : return (1 + my_func(in1));
23 case 'o' : return (1 + my_func(in1));
(dbx) run
Process 25752 (example3) started
[2] Process 25752 (example3) stopped at [my_func:17 ,0x400990]
17 for (i = 0; i < strlen(in); i++)
(dbx) print in1
"e"
(dbx) print (in)
Note that we put 'in' in a () because it's a dbx keyword.
0x7fffaeec = "helloacixu"
(dbx) stop at 17 if (in)[0]='u'
Process 25752: [3] stop if in + 0 * 1 = 'u' at "/tmp/bruce/example3.c":17
Note above it is possible to add a conditional break point in dbx.
In the example above, the program returns control to the debugger when
in[0]=='u'.
(dbx) delete 3
(dbx) cont
[2] Process 25752 (example3) stopped at [my_func:17 ,0x400990]
17 for (i = 0; i < strlen(in); i++)
(dbx) print (in)
0x7fffabcc = "u"
(dbx) step
Process 25752 (example3) stopped at [my_func:18 ,0x4009b4]
18 in1[i] = in[i+1];
(dbx) step
Process 25752 (example3) stopped at [my_func:17 +0x40,0x4009d0]
17 for (i = 0; i < strlen(in); i++)
(dbx) print in1
""
(dbx)
We forgot to tell it to stop recursing when we finish processing the string!
All we need to do is add the following to solve the problem:
if (in[0] == '\0') return 0;
One very important note - depending on
the amount of memory your program consume at the time of error, the core is
often huge in size. The one created in the above example is about 67
megabyte. The home directories of undergrads on the AUXPEX (one of the main
storage system on the campus managed by computing servies) has a quota limit
of 1 megabyte. If you write your programs in your home directory and it is
on AUXPEX, the chance is that the core file can only be able to written
partially.
A Final Word Debuggers are the best thing since sliced bread. They are easy to use and help you find problems quickly. It is absolutely an indispensible tool for programmers in UNIX. C compiler packages such as turbo C and Borland C++, as well as other compiler packages for other languages, even have integrated debuggers in their editors. Most debuggers' functionalities are very similar. Some debuggers might have more fancies than the others, but the basic functions are identical.
Other References The Online help in dbx is good source of information. To use it, type help at the (dbx) prompt. You can also read the man page of dbx. To do this, type 'man dbx' at the unix prompt. (Man pages are also a very useful source of reference for C commands and other UNIX commands. Most C commands have their associated man pages. Try typing man printf at the unix prompt.)
