A talk I gave at the UKHAS 2013 conference on software debugging.

1. Debugging
John Graham-Cumming
September 2013

2. Ada Lovelace (1840s)

3. Maurice Wilkes (1950s)
 As soon as we started programming,
we found to our surprise that it wasn't
as easy to get programs right as we
had thought. Debugging had to be
discovered. I can remember the
exact instant when I realized that a
large part of my life from then on
was going to be spent in finding
mistakes in my own programs.

4. Writing code is hard
 But writing bugs is easy!
 Everyone does it all the time without
even thinking about it.
 Estimates in GOOD code there are 2
to 4 defects per 1,000 lines
 You’re not that good
 You will spend many hours debugging

5. Programmer
 If a person who debugs code is a
debugger than a person who writes
the bugs must be a... bugger?

6. Always have a fighter pilot in
your presentation

7. Colonel John Boyd

8. OODA Loop

9. THE GREATEST
DEBUGGER YOU WILL
EVER USE IS IN YOUR
HEAD
Ignore your tools!

10. ESSENTIAL SKILL: RUN
CODE IN YOUR HEAD

11. RISE
 Reproduce
 Isolate
 Target
 Excise

12. Reproduce

13. Isolate
 Tighten the OODA loop
◦ The smallest
◦ Fastest
◦ Most easily repeated
 And automate it
 Then turn it into a test case

14. Target

15. ESSENTIAL SKILL:
ALWAYS BE ASKING
WHY

16. Debugging Christmas Lights

17. ESSENTIAL SKILL:
DOUBTING

18. Excise
 Fix the bug
 Don’t introduce new bugs!
 You can check this with the test case
that you made earlier

19. Talk to the teddy

20. Look Around Debugging
 Where there’s smoke...
 ... research shows that bugs tend to
cluster
 When you fix a bug look for more in
the same spot.

21. 5 bugs in a single line
dir.permissions = 666; // Make the file writable
 Bug 1: updates permissions without
asking
 Bug 2: doesn’t make the file writable
 Bug 3: creates security hole
 Bug 4: used decimal not octal
 Bug 5: comment is incorrect

22. Remember This One Thing!
 The most effective debugging tools
are: your brain, a unit test, and the
print statement.