Debugging Techniques for C Programs Debugging Basics Will focus - - PDF document

Debugging Techniques for C Programs

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.

CS3411 Debugging 3

Small Example: ave .c

# inc lude <s td i

• .h>

i n tsum=0 , va l , n um=0 ; doub le ave ; main ( ) { wh i l e ( s can f ( "%d \n " ,&val ) ! = EOF) { sum += va l ; num++ ; } i f ( num > ) { ave = sum/num; p r i n t f ( "Ave ragei s % 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.

CS3411 Debugging 4

Small Example: ave .c

% a .ou t 1 Average i s 1 .000000 % a .ou t 1 2 3 Average i s 2 .000000 % a .ou t 1 2 3 4 Average i s 2 .000000

Experienced programmer can probably “eyeball debug” the program from this output

CS3411 Debugging 5

Using gdb

• Make sure to compile source with the -g switch asserted.
• In our case, gcc -g

ave .c

• Breakpoint: line in source code at which debugger will pause
• execution. At breakpoint, can examine values of relevant

components of program state. br eak command sets a breakpoint; c l ea r removes the breakpoint.

• Diagnostic pr

i n t f ( )crude, but effective way of getting a snapshot

• f program state at a given point.
• Once paused at a breakpoint, use gdb pr

i n t, or d ispl a y to show variable or expression values. d i sp lay will automatically print values when execution halts at breakpoint.

• From a breakpoint, may st

ep or nex tto single step the program. s tep stops after next source line is executed. nex tsimilar, but executes functions without stopping.

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 Average i s 2 .000000

CS3411 Debugging 7

Using gdb (ave .c)

% gdb a.

• u

t GNU gdb 6 .1 Copy r i g h t 2004 Free So f twa re Founda t i

• n

, I nc . GDB i s f r ee so f t ware , cove red by t he GNU Gene r a l Pub l i c L i cense , and you a re we l come to change i t and /o r d i s t r i bu t e cop ies

• f

i t under ce r t a i n cond i t i

• ns

. Type " s how copy ing " t

• see

t he cond i t i

• ns

. There i s abso lu t e l y no wa r ran t y f

• r

GDB. Type " show wa r ran t y ” f

• r

de ta i l s . Th i s GDB was con f i gu red as " i 5 86

• suse-

l i nux " . . . Us ing hos t l i b t h read_dbl i b rar y " / l i b / t l s / l i b t h read_db .so .1 " . ( gdb )

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Using gdb (ave .c)

( gdb ) l 1 # i ncl ude <s td i

• .h>

2 3 i n ts um=0 , va l , num=0 ; 4 doub le ave ; 5 6 ma i n ( ) 7 { 8 whi l e ( scan f ( "%d \n " ,&va l ) ! = EOF) { 9 s um += va l ; 10 num++ ; ( gdb ) l 11 } 12 i f ( num > ) { 13 ave = sum/num; 14 p r i n t f ( "Ave ragei s %f \n " , ave ) ; 15 } 16 } 17 ( gdb )

Interesting point: top of main loop. Another interesting point: just before ave is computed.

CS3411 Debugging 9

Using gdb (ave .c)

( gdb ) b reak 8 Breakpo i n t 1 a t 0x80483dc : f i l e ave . c , l i ne 8 . ( gdb ) b reak 13 Breakpo i n t 2 a t 0x8048414 : f i l e ave . c , l i ne 13 . ( gdb ) d i s p lay num ( gdb ) d i s p lay va l ( gdb ) d i s p lay sum ( gdb ) r < da ta Sta r t i ng pr

• g

ram: / home/ jmayo /cou rses .d … Breakpo i n t 1 , ma in ( ) a t ave . c :8 8 whi l e ( scan f ( "%d \n " ,&va l ) ! = EOF) { 3 : sum = 2 : va l = 1 : num = ( gdb ) c Con t i nu ing . Breakpo i n t 1 , ma in ( ) a t ave . c :8 8 whi l e ( scan f ( "%d \n " ,&va l ) ! = EOF) { 3 : sum = 1 2 : va l = 1 1 : num = 1

CS3411 Debugging 10

Using gdb (ave .c)

( gdb ) c Con t i nu i ng . Breakpoi n t 1 , ma in ( ) a t ave .c :8 8 wh i l e ( scan f ( "%d \n" ,&va l ) ! = EOF) { 3 : sum = 3 2 : va l = 2 1 : num = 2 ( gdb ) c Con t i nu i ng . Breakpoi n t 1 , ma in ( ) a t ave .c :8 8 wh i l e ( scan f ( "%d \n" ,&va l ) ! = EOF) { 3 : sum = 6 2 : va l = 3 1 : num = 3

CS3411 Debugging 11

Using gdb (ave .c)

( gdb ) c Con t i nu i ng . Breakpoi n t 1 , ma in ( ) a t ave .c :8 8 wh i l e ( scan f ( "%d \n" ,&va l ) ! = EOF) { 3 : sum = 10 2 : va l = 4 1 : num = 4 ( gdb ) c Con t i nu i ng . Breakpoi n t 2 , ma in ( ) a t ave .c :13 13 ave = s um/num; 3 : sum = 10 2 : va l = 4 1 : num = 4

CS3411 Debugging 12

Using gdb (ave .c)

( gdb ) n 14 p r i n t f ( " Ave rage i s % f \n " , ave ) ; 3 : sum = 10 2 : va l = 4 1 : num = 4 ( gdb ) p ave $1 = 2 ( gdb ) p ( doub le ) s um/ (doub le )num $2 = 2 .5 ( gdb ) c Con t i nu i ng . Average i s 2 .000000 Prog ram ex i t ed wi th code 024 . ( gdb ) q %

Everything fine until ave is

• computed. Integer division the

problem. Evaluate expression inside gdb to validate our reasoning.

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 a t ( ) 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.

CS3411 Debugging 14

Using ddd (inorder.c)

Introduce a pointer-related bug into the program by modifying the inorder() function:

. . . . . vo id i no r de r ( r ) s t ruc t n

• de

* r ; { i no rde r ( r

• >

l e f t ) ; p r i n t f ( "%c" , r

• >da

ta ) ; i no rde r ( r

• >

r i g h t ) ; }

Formerly:

i f ( r ! = NI LNODE) { i no rde r ( r

• >

l e f t ) ; p r i n t f ( "%c" , r

• >da

ta ) ; i no rde r ( r

• >

r i g h t ) ; }

CS3411 Debugging 15

Quickie Post Mortem Debugging ( i norder . c)

% a .ou t Segment a t i

• n

f a u l t ( co re dumped ) % gdb a.

• u

t co r e GNU gdb 6 .1 . . . . . . . . . . Core was gene r a ted by ` . / a .ou t co re ' . Prog ram t e rm inat ed w i t h s igna l 1 1 , Segmen ta t i

• n

f au l t . . . . . . . . . . . Read ing s ymbo l s f r

• m

/ l i b / t l s / l i b c . so .6 . . . d

• ne

. Loaded symbo l s f

• r

/ l i b / t l s / l i bc . so. 6 Read ing s ymbo l s f r

• m

/ l i b / l d

• l

i nux . so .2 . . . done . Loaded symbo l s f

• r

/ l i b / l d

• l

i nux . s

• .2

#0 0x080484d5 i n i no rder( r=0x0 ) a t buggy_ ino rder . c : 38 38 i no rde r ( r

• >

l e f t ) ; ( gdb ) Function in which segfaulted. Arguments to function. Line of source where segfaulted.

CS3411 Debugging 16

Quickie Post Mortem Debugging ( i norder . c)

( gdb ) whe re #0 0x080484d5 i n i no rder( r=0x0 ) a t buggy_ ino rder . c : 38 #1 0x080484dd i n i no rder( r=0x804a008 ) a t buggy_ ino rder . c : 38 #2 0x080484dd i n i no rder( r=0x804a028 ) a t buggy_ ino rder . c : 38 #3 0x080484dd i n i no rder( r=0x804a048 ) a t buggy_ ino rder . c : 38 #4 0x080484dd i n i no rder( r=0x804a068 ) a t buggy_ ino rder . c : 38 #5 0x08048479 i n ma in ( ) a t buggy_ ino rde r . c :21

The above listing walks back the call chain as it was at the moment

• f the segfault.

Clear that we dereferenced a null pointer in a call to i no rder ( )at a leaf node of the binary tree.

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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