A Tutorial on Introduction to gdb

1. A Tutorial onIntroduction to gdb By Sasanka Madiraju Graduate Assistant Center for Computation and Technology

2. WISDOM • W - What do you learn from this • I - What is your Interest • S - Your level of Sophistication • D - Level of Detail • O - Ownership-“The Pragmatic Programmer” • By Andy Hunt and David Thomas • M - My Motivation

3. Topics discussed • Part 1: Introduction • Part 2: THE WHY: gdb? • Features • Part 3: THE HOW • Commands • Part 4: PRACTICE: Hands on debugging session • Part 4: Advanced concepts • Part 5: Conclusion • Q & A • References

4. Part 1 Introduction

5. Introduction • What is a debugger? • Why do we need one? • gdb is NOT • A shell • A text editor • A programming environment • Other types of debuggers • ddd, GDBTk, Insight, dbx, softice, etc.

6. What is a BUG? • Bug- program does not generate the expected output • BUG or Fault? • Errors or bugs • syntactic • Logical • Syntactic errors - caught by compiler • Logical errors - difficult to find and remove.

7. Part 2 Why do you need to learn GDB?

8. Advantages of gdb • Interactive • Responsive • Permissive • Capability to work with very large programs • Reported cases: file size close to 1 GB • Extremely portable • No other debugger works on so many configurations

9. Advantages continued • Can debug optimized code • Can debug macros • Multiple processor support • Capability to debug multi threaded applications • Remote debugging

10. Major Features • Set Breakpoints • Examine Data • Specify Files • Run Program • Examine Stack • Report Status • Trace Execution

11. Part 3: Working

12. Compiling the program • C-program - compiled using gcc compiler • Compilation command: • Example: “gcc -o testout -g test.c” • “-o” generates the output • “-g” enables gdb • C++ program - compiled using g++ • Compilation command: • Example: “g++ -o testout -g test.cpp” • “-o” generates the output • “-g” enables gdb

13. How to get in and get out • Command: gdb • Alternate Formats • gdb • gdb <testout> • gdb program pid • gdb program core • gdb a.out (not recommended) • gdb --args ./exe/<cactus_config> <.par file> • To quit gdb use the “quit” or “q” command

14. Modes of Operation • Has many modes some of which are: • Silent • Batch • useful for running GDB as a filter. • Example: Download and run a program on another computer • Shell • No windows • Set baud rate (for remote debugging) • Output statistics about time and memory

15. Logging output • set logging on • Enable logging. • set logging off • Disable logging. • set logging file file • Change the name of the current logfile. The default logfile is `gdb.txt'. • set logging overwrite [on|off] • By default, GDB will append to the logfile. Set overwrite if you want set logging on to overwrite the logfile instead. • set logging redirect [on|off] • By default, GDB output will go to both the terminal and the logfile. Set redirect if you want output to go only to the log file. • show logging • Show the current values of the logging settings.

16. Halting program execution • Break Points • Predefined points where program stops • Watch Points • Special breakpoint that stops the program when the value of an expression changes • Catch Points • Special breakpoint that stops the program on an event • Throwing of a C++ exception • Loading of a library

17. Break Points • break function • break +/-offset • break linenum • break filename:linenum • break filename:function • break *address • break • break ... if cond

18. Watch Points • watch expr • rwatch expr • awatch expr • info watchpoints • Hardware & Software watchpoints • set can-use-hw-watchpoints 0

19. Catch Points • Syntax: • catch event • Event can be any of the following • throw • The throwing of a C++ exception • catch • The catching of a C++ exception • exec • A call to exec. This is currently only available for HP-UX • fork • A call to fork. This is currently only available for HP-UX • vfork • A call to vfork. This is currently only available for HP-UX

20. Break point commands • info break • list • ignore <break point number> • Remove break points • “delete” <break point number> • “clear” <line number> or <function name>

21. Stepping • Stepping is the process of executing the program line by line • Commands • next – step over • step – step through • continue – jump between break points • To step over a few lines of code • step <Number of lines> • next <Number of lines>

22. Print command • Formats • o – octal • x – hexadecimal • d – decimal • u - unsigned decimal • t – binary • f – float • a – address • i – instruction • c – char • s - string

23. Useful commands • set • whatis • ptype • Call • Return • gdb has command completion • Hit TAB to complete a command • complete r • Lists all commands starting with r. • aprops

24. Most useful command help 

25. Examining Data • list linenum • list function • list • list - • list + • set listsize count • show listsize

26. Edit Data • edit • edit number • edit function • edit *address • edit filename:number • edit filename:function

27. Editors • Vi • Emacs • XEmacs • Choose an editor • EDITOR=/usr/bin/vi • export EDITOR • gdb …….

28. Search source files • forward-search regexp • search regexp • reverse-search regexp • SPECIFY SOURCE DIRECTORY • dir dirname ... • Adds the name of the directory to the beginning of the source directory path • show directories • Shows the source path • show directories • Resets the source path to empty.

29. Examine Stack • Very useful in debugging • Each function is placed on a stack • Function “main” is the only function • one stack frame • Each additional function has a stack frame • frame args: Move from one stack to another • select-frame: silent version of frame command

30. Frame Information • info f • This command prints a verbose description of the selected stack frame, including: • the address of the frame • the address of the next frame down (called by this frame) • the address of the next frame up (caller of this frame) • the language in which the source code corresponding to this frame is written • the address of the frame's arguments • the address of the frame's local variables • the program counter saved in it (the address of execution in the caller frame) • which registers were saved in the frame

31. Info command for frames • info frame addr • info args • info locals • info catch • SELECT A STACK FRAME • frame <number> • frame addr • up n • down n

32. Back Trace • bt (backtrace) • Stop by hitting control-C • backtrace n • backtrace full

33. D E M O Debugging Session Getting our hands dirty 

34. Q & A 10 Minutes