Mastering GDB: A Comprehensive Introduction to Debugging in C

1. Linux Programming Tutoring Introduction to fundamental GDB- More GDB function -

2. Get files Slides : http://140.112.28.132/gdb_leb Packages : $wgethttp://140.112.28.132/gdb.zip $unzip gdb.zip

3. Outline • Why debugger • Introduction • Running program • Stop and continue • Examining source and data

4. Why Debugger(1/3) • With print (printf in c) • Add lines to print values • Must comment debug lines when release Int main(int argc, char** argv) { int var1,var2, ... ; ... print(var1,var2, ...); ... //print(...); ... }

5. Why Debugger(2/3) • Use preprocessor • define, ifdef • Still not flexible enough #define debugmode Int main(int argc, char** argv) { int var1,var2, ... ; ... #ifdef debugmode print(...) #endif ... }

6. Why Debugger(3/3) • With debugger • We can concentrate on writing program easily • We can print values and stop anywhere without recompilation • We can make up some value or alter program flow

7. Outline • Why debugger • Introduction • Running program • Stop and continue • Examining source and data

8. GDB : The GNU Project Debugger • Official website: • http://www.gnu.org/software/gdb/ • We focus on how to use today • Documentation/User Manual

9. Invoking and quitting • gdb program • Debug program • gdb program pid • Debug running program with pid • quit[q] or <Ctrl> + d • Quit gdb

10. Something useful • shell command-strings • invoke shell to execute command-strings • $SHELL • <tab> • auto completion • blank line input (<ret>) • repeat last command

11. Outline • Why debugger • Introduction • Running program • Stop and continue • Examining source and data

12. Running program • gcc –g foo.c –o foo • compiling for debugging • gdbfoo • Debug program • run[r] • run the program under gdb

13. Arugments • Bash$ foo arg1 arg2 • set args arg1 arg2 • to set arguments • set args • clear arguments • show args • display current arguments

14. Enviroment • show environment • Show thecurrent environment • path directory • add directory to $PATH • set environmentvarname [=value] • assign value to varname set environment SHELL=/bin/bash

15. Working directory • Program inherits working directory from the gdb • cd directory • change working directory • pwd • display current workign directory

16. Outline • Why debugger • Introduction • Running program • Stopping and Continuing • Examining source and data

17. Stopping program • We use breakpoints to stop the program • info program • display status , why we stop

18. Stopping program • info breakpoints • print a table of all breakpoints

19. Breakpoints filename:linenum filename:function -/+offset • break[b] location • set breakpoint at location • clear location • clear breakpoint at location • enable/disable [breakpoints] [range...] • delete [breakpoints] [range...]

20. Continuing and Stepping • continue[c] • Resume program execution • step[s] [count] • Continue, step in function • next[n] [count] • Continue, execute function without stop

21. Schematic diagram of continue • Main • { • ins1 • foo(); • ins2 • ins3 • } foo { ins4 ins5 }

22. Schematic diagram of step • Main • { • ins1 • foo(); • ins2 • ins3 • } foo { ins4 ins5 }

23. Schematic diagram of next • Main • { • ins1 • foo(); • ins2 • ins3 • } foo { ins4 ins5 }

24. Outline • Why debugger • Introduction • Running program • Stopping and Continuing • Examining source and data

25. Examining source • list[l] location • print source line centered around specific location • list first, last • print lines between first and last  list 10,30

26. Examining source • list • print next lines • list - • print lines before the line last printed list - Last print list

27. Examining data • print[p] expression • print the value of expression • print function::variable • print static variable in function

28. Automic display • info display • print a list of all set up expression • display expr • print expr automatically • undisplay dnum

29. Automic display • delete display dnum • disable display dnum • enable display dnum

30. Outline • Breakpoint • Formating output • Examing memory • Print dynamic allocate array • Altering execution

31. Condition break(1/2) • info breakpoint • break[b] location • stop everytime • break location if expression • break foo break foo if argc == 1 break foo if argc >= 3

32. Condition break(2/2) • conditionbnum expression • add condition to breakpoint bnum • conditionbnum • clear condition • break foo • condition 1 argc == 1

33. One stop breakpoint • tbreak location • Set up as the same as break • But automatically remove after stop • tbreak foo

34. Outline • Breakpoint • Formating output • Examing memory • Print dynamic allocate array • Altering execution

35. Formating output(1/2) • We can format output like printf • command/format expression

36. Formating output(2/2) • p/format expression • print expression value in format • display/format expression • print expression valuein fromat automatically • p/x counter • display/x counter

37. Outline • Breakpoint • Formating output • Examing memory • Print dynamic allocate array • Altering execution

38. Examing memory • x address • examine address value • x/nfu address • n : number of elements • f : format as previously describe • u : unit of element x/4xb &variable

39. Print dynamic allocate array int *marray = (int*)malloc( len * sizeof(int) ); • pmarray • $1 = (int *) 0x601010 • p *marray • $2 = 0 • p *marray@len • $3 = {0, 0, 0, 0, 0}

40. Outline • Breakpoint • Formating output • Examing memory • Print dynamic allocate array • Altering execution

41. Altering execution • We can alter execution by • change value of variable • jump to some location • return makeup value

42. Altering execution • set var assignment_exp • print assignment_exp • gdb will  evaluate the expression • p foo=4

43. jump and return • jump location • jump to assigned location, and continue execution

44. jump and return • return expression • return from function call with expression value