Slides from a lecture on the debugging process and using the GDB.

1. GNU Debugger
goakley123@gmail.com
Glen Oakley

2. What is a debugger?

3. The Four Things
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Start your program
Make your program stop
Examine what has happened
Change things in your program

4. Your Things
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Start your program
Break / Watch
Checkpoint
Next / Step / Continue
Backtrace
Print

5. ZEROTH...

6. https://sourceware.org/gdb/current/onlinedocs/gdb/

8. Debugging Flow
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Testing Brute-forcing
Identify symptom - what happened?
Identify problem - why?
Locate problem
Blame someone (else)
Fix problem
Repeat with the new problems

9. GDB Execution
● Run program in ‘virtual’ environment
● Run until
○ Termination
○ Breakpoint
● Flow control
● State Information

10. Commands
● Run
○ Execute program with args
○ Redirection
● Backtrace
○ Stack trace of your location in the program
○ Identify where crash happened

11. Example
verbose_array.c

12. Commands
● Break
○ Location at which to halt the program
○ Breaks before execution of statement(s)
● Watch
○ Monitors a variable for changes
○ Requires variable to be in scope
● Clear
○ Remove breakpoints

13. Commands
● Checkpoint
○ Store the state of the program
○ Programs can be restarted at checkpoints
● Step
○ Step through the program line by line
● Next
○ Execute the next line in the current frame
○ Does not step through sub-functions

14. Example
osh.c

15. Commands
● Print
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Show the state of memory/variables
Works with array and struct/union notation
Will assume c-strings when applicable
@ can be used on arrays for grouping prints
● Set Var
○ Change memory/variables
● Continue
○ Run until another break/watch point is triggered

16. Example
sudoku.c

17. Recap
● Debugging process is unavoidable
● GDB provides assistance
○ If I see one more stray printf()...
● Basic flow control commands
○ Allow tracing a stopped program
○ Set breakpoints to stop the program
○ Enable querying/modifying the program state

18. Thanks!
http://glenoakley.com/
@goakley123
goakley123@gmail.com