DebuggingJohn Graham-CummingSeptember 2013

Ada Lovelace (1840s)

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.

Writing code is hardBut 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 goodYou will spend many hours

debugging

ProgrammerIf a person who debugs code is a

debugger than a person who writes the bugs must be a... bugger?

Always have a fighter pilot in your presentation

Colonel John Boyd

OODA Loop

THE GREATEST DEBUGGER YOU WILL EVER USE IS IN YOUR HEAD

Ignore your tools!

ESSENTIAL SKILL: RUN CODE IN YOUR HEAD

RISEReproduceIsolateTargetExcise

Reproduce

IsolateTighten the OODA loop

◦The smallest◦Fastest◦Most easily repeated

And automate itThen turn it into a test case

Target

ESSENTIAL SKILL: ALWAYS BE ASKING WHY

Debugging Christmas Lights

ESSENTIAL SKILL: DOUBTING

ExciseFix the bugDon’t introduce new bugs!

You can check this with the test case that you made earlier

Talk to the teddy

Look Around DebuggingWhere there’s smoke...... research shows that bugs tend

to cluster

When you fix a bug look for more in the same spot.

5 bugs in a single linedir.permissions = 666; // Make the file writable

Bug 1: updates permissions without asking

Bug 2: doesn’t make the file writable

Bug 3: creates security holeBug 4: used decimal not octalBug 5: comment is incorrect

Remember This One Thing!The most effective debugging

tools are: your brain, a unit test, and the print statement.