The document discusses debugging techniques for C programs using gcc and gdb. It begins with debugging concepts like program state and state predicates. It then provides a small example program to average numbers and shows how to debug it using gdb. Key gdb commands are explained like breakpoints, print, and stepping. The document also introduces the ddd graphical frontend for gdb and provides an example of debugging a pointer bug in an inorder tree traversal function.

1. Debugging Techniques for C Programs

2. CS3411 Debugging 2
Debugging Basics
• Will focus on the gcc/gdb combination.
• Will also talk about the ddd gui for gdb (lots of value added to
gdb).
• First, debugging in the abstract:
– Program state is a snapshot of all variables, PC, etc.
– A statement in your program transforms one program state
into another.
– You should be able (at some level) to express what you
expect the state of your program to be after every statement.
– Often state predicates on program state; i.e., “if control is
here, I expect the following to be true.”
• Map into a toy example.

3. CS3411 Debugging 3
Small Example: ave.c
#
inc
lude <s
td
io
.h>
i
n
tsum=0, va
l
,num=0;
doub
le ave;
main(
)
{
whi
le(scanf
(
"%dn"
,&val
)!= EOF){
sum += va
l
;
num++;
}
i
f(num > 0){
ave = sum/num;
p
r
in
t
f
(
"Averageis %f
n"
, ave)
;
}
}
sum should be 0 and num
should be 0.
sum should be the total of
the num input values and
there is no more input.
sum should be the total of
the num input values
processed.
ave should be the floating
point mean of the num input
data values.

4. CS3411 Debugging 4
Small Example: ave.c
% a.out
1
Average i
s 1
.000000
% a.out
1
2
3
Average i
s 2
.000000
% a.out
1
2
3
4
Average i
s 2
.000000
Experienced programmer can probably “eyeball debug” the
program from this output

5. CS3411 Debugging 5
Using gdb
• Make sure to compile source with the -g switch asserted.
• In our case, gcc -gave.c
• Breakpoint: line in source code at which debugger will pause
execution. At breakpoint, can examine values of relevant
components of program state. break command sets a
breakpoint; c
l
ear removes the breakpoint.
• Diagnostic pr
in
t
f
(
)crude, but effective way of getting a snapshot
of program state at a given point.
• Once paused at a breakpoint, use gdb pr
in
t, or displ
ay to show
variable or expression values. disp
lay will automatically print
values when execution halts at breakpoint.
• From a breakpoint, may step or nextto single step the program.
s
tep stops after next source line is executed. nextsimilar, but
executes functions without stopping.

6. CS3411 Debugging 6
Using gdb
• May find out where execution is, in terms of function call chain,
with the where command; also shows function argument values.
• Apply some of this in context of bogus averaging program.
• To make things easier, put the problematic data set in a file
named data.
% a.ou
t < da
ta
Averagei
s 2
.000000

7. CS3411 Debugging 7
Using gdb (ave.c)
% gdb a.ou
t
GNU gdb 6
.1
Copyr
ight 2004 Free Software Foundat
ion,Inc
.
GDB isf
ree so
f
t
ware, covered by the GNU Genera
l Pub
l
i
c
L
icense, and you a
re we
l
come to change i
tand/or d
is
t
r
ibu
t
e
cop
ies ofi
tundercer
ta
in cond
i
t
ions.
Type "show copy
ing"tosee the cond
i
t
ions.
Thereis abso
lu
t
e
ly no war
ran
tyf
or GDB. Type "
show war
ran
ty
”
fo
r de
ta
i
l
s
.
This GDB was conf
igured as "
i586-suse-
l
inux"
.
.
.
Using host
l
i
b
th
read_dbl
ib
rary"
/
l
i
b
/
t
l
s
/
l
ib
th
read_db.so.1"
.
(gdb)

8. CS3411 Debugging 8
Using gdb (ave.c)
(gdb)l
1 #
incl
ude <s
td
i
o.h>
2
3 in
tsum=0, va
l
,num=0;
4 doub
le ave
;
5
6 ma
i
n(
)
7 {
8 whi
l
e(scan
f
(
"%dn"
,&va
l
)!= EOF) {
9 s
um += va
l
;
10 num++;
(gdb)l
11 }
12 i
f(num > 0){
13 ave = sum/num;
14 pr
in
t
f
(
"Averageis%fn"
,ave)
;
15 }
16 }
17
(gdb)
Interesting point: top of main loop.
Another interesting point: just
before ave is computed.

9. CS3411 Debugging 9
Using gdb (ave.c)
(gdb) b
reak 8
Breakpo
i
nt 1 a
t 0x80483dc:f
i
l
e ave
.c
,l
i
ne 8
.
(gdb) b
reak 13
Breakpo
i
nt 2 a
t 0x8048414:f
i
l
e ave
.c,l
i
ne 13
.
(gdb) d
isp
lay num
(gdb) d
isp
lay va
l
(gdb) d
isp
lay sum
(gdb)r <data
Star
t
ing program:/
home/
jmayo/courses
.d …
Breakpo
i
nt 1
, ma
in (
)a
tave
.
c
:8
8 whi
l
e(scanf
(
"%dn"
,&va
l
)!= EOF) {
3: sum = 0
2: va
l = 0
1: num = 0
(gdb) c
Cont
inu
ing.
Breakpo
i
nt 1
, ma
in (
)a
tave
.
c
:8
8 whi
l
e(scanf
(
"%dn"
,&va
l
)!= EOF) {
3: sum = 1
2: va
l = 1
1: num = 1

10. CS3411 Debugging 10
Using gdb (ave.c)
(gdb) c
Cont
inu
i
ng.
Breakpoi
n
t1
, ma
in(
)a
tave.c
:8
8 wh
i
le(scanf
(
"%dn"
,&va
l
)!
= EOF) {
3: sum = 3
2: va
l =2
1: num = 2
(gdb) c
Cont
inu
i
ng.
Breakpoi
n
t1
, ma
in(
)a
tave.c
:8
8 wh
i
le(scanf
(
"%dn"
,&va
l
)!
= EOF) {
3: sum = 6
2: va
l =3
1: num = 3

11. CS3411 Debugging 11
Using gdb (ave.c)
(gdb) c
Cont
inu
i
ng.
Breakpoi
n
t1
, ma
in(
)a
tave.c
:8
8 wh
i
le(scanf
(
"%dn"
,&va
l
)!
= EOF) {
3: sum = 10
2: va
l =4
1: num = 4
(gdb) c
Cont
inu
i
ng.
Breakpoi
n
t2
, ma
in(
)a
tave.c
:13
13 ave = sum/num;
3: sum = 10
2: va
l =4
1: num = 4

12. CS3411 Debugging 12
Using gdb (ave.c)
(gdb) n
14 p
r
in
t
f
(
"
Average i
s %f
n",ave)
;
3: sum = 10
2: va
l =4
1: num = 4
(gdb) pave
$1 = 2
(gdb) p(
doub
le)sum/(doub
le)num
$2 = 2
.5
(gdb) c
Cont
inu
i
ng.
Average i
s 2
.000000
Program ex
i
ted wi
th code 024.
(gdb) q
%
Everything fine until ave is
computed. Integer division the
problem.
Evaluate expression inside
gdb to validate our reasoning.

13. CS3411 Debugging 13
A GUI for gdb: ddd
• The ddd program is just a GUI front-end for gdb.
• Value added three main ways:
– Can mouse left on source line, then mouse left on Break at
(
)
to set a breakpoint. Or mouse right on a source line and set
a breakpoint in the menu that pops up.
– Can mouse left on variable, then mouse left on Pr
in
t
(
)or
Disp
lay(
)to examine data values. Or get value displayed at
bottom of ddd window by ``mouse hovering'' over a variable
name.
– Displayed values graphically displayed. Click on a pointer
value, graphically display thing pointed to. Visualize complex
linked data structures.
• Play with inorder tree traversal program.

14. CS3411 Debugging 14
Using ddd (inorder.c)
Introduce a pointer-related bug into the program by modifying
the inorder() function:
.....
vo
idinorder
(
r
)
s
t
ruc
t node *
r
;
{
inorder
(
r
->
le
f
t
)
;
p
r
in
t
f
(
"%c"
,
r
->data)
;
inorder
(
r
->r
ight
)
;
}
Formerly:
i
f(
r!= NILNODE){
inorder
(
r
->
le
f
t
)
;
p
r
in
t
f
(
"%c"
,
r
->data)
;
inorder
(
r
->r
ight
)
;
}

15. CS3411 Debugging 15
Quickie Post Mortem Debugging (inorder
.
c)
% a.out
Segmenta
t
ionfau
l
t(core dumped)
% gdb a.ou
tcore
GNU gdb 6
.1
..........
Core was generated by `
.
/a
.ou
tcore
'
.
Program te
rminated w
i
ths
igna
l 11, Segmenta
t
ion fau
l
t
.
..........
Read
ing symbo
l
sf
rom /
l
i
b/
t
l
s
/
l
ibc.
so
.6
.
.
.done.
Loaded symbolsfo
r/
l
i
b
/
t
l
s
/
l
ibc
.so.6
Read
ing symbo
l
sf
rom /
l
i
b/
ld
-
l
inux
.so
.2
.
.
.
done.
Loaded symbolsfo
r/
l
i
b
/
l
d-
l
inux
.so.2
#0 0x080484d5i
ninorder(
r=0x0) a
tbuggy_
inorder
.c
:38
38 inorder
(
r
->
le
f
t
)
;
(gdb)
Function in which segfaulted.
Arguments to function.
Line of source where segfaulted.

16. CS3411 Debugging 16
Quickie Post Mortem Debugging (inorder
.
c)
(gdb) where
#0 0x080484d5i
ninorder(
r=0x0) a
tbuggy_
inorder
.c
:38
#1 0x080484ddi
ninorder(
r=0x804a008) a
tbuggy_
inorder
.c
:38
#2 0x080484ddi
ninorder(
r=0x804a028) a
tbuggy_
inorder
.c
:38
#3 0x080484ddi
ninorder(
r=0x804a048) a
tbuggy_
inorder
.c
:38
#4 0x080484ddi
ninorder(
r=0x804a068) a
tbuggy_
inorder
.c
:38
#5 0x08048479i
n main(
)a
t buggy_
inorder
.c
:21
The above listing walks back the call chain as it was at the moment
of the segfault.
Clear that we dereferenced a null pointer in a call to i
norder
(
)at
a leaf node of the binary tree.

17. CS3411 Debugging 17

18. CS3411 Debugging 18

19. CS3411 Debugging 19

20. CS3411 Debugging 20
Debugging Tips
• Examine the most recent change
• Debug it now, not later
• Read before typing
• Make the bug reproducible
• Display output to localize your search
• Write a log file
• Use tools
• Keep records