Source: Jon Bentley

Every programmer knows that debugging is hard. Great debuggers, though, can make the job look simple. Distraught programmers describe a bug that they've been chasing for hours, the master asks a few questions, and minutes later the programmers are staring at the faulty code. The expert debugger never forgets that there has to be a logical explanation, no matter how mysterious the system's behavior may seem when first observed.

That attitude is illustrated in an anecdote from IBM's Yorktown Heights Research Center. A programmer had recently installed a new workstation. All was fine when he was sitting down, but he couldn't log in to the system when he was standing up. That behavior was one hundred percent repeatable: he could always log in when sitting and never when standing.

Most of us just sit back and marvel at such a story. How could that workstation know whether the poor guy was sitting or standing? Good debuggers, though, know that there has to be a reason. Electrical theories are the easiest to hypothesize. Was there a loose wire under the carpet, or problems with static electricity? But electrical problems are rarely one-hundred-percent consistent. An alert colleague finally asked the right question: how did the programmer log in when he was sitting and when he was standing? Hold your hands out and try it yourself.

The problem was in the keyboard: the tops of two keys were switched. When the programmer was seated he was a touch typist and the problem went unnoticed, but when he stood he was led astray by hunting and pecking. With this hint and a convenient screwdriver, the expert debugger swapped the two wandering keytops and all was well.

A banking system built in Chicago had worked correctly for many months, but unexpectedly quit the first time it was used on international data. Programmers spent days scouring the code, but they couldn't find any stray command that would quit the program. When they observed the behavior more closely, they found that the program quit as they entered data for the country of Ecuador. Closer inspection showed that when the user typed the name of the capital city (Quito), the program interpreted that as a request to quit the run!

Bob Martin once watched a system ``work once twice''. It handled the first transaction correctly, then exhibited a minor flaw in all later transactions. When the system was rebooted, it once again correctly processed the first transaction, and failed on all subsequent transactions. When Martin characterized the behavior as having ``worked once twice'', the developers immediately knew to look for a variable that was initialized correctly when the program was loaded, but was not reset properly after the first transaction.

In all cases the right questions guided wise programmers to nasty bugs in short order: ``What do you do differently sitting and standing? May I watch you logging in each way?'' ``Precisely what did you type before the program quit?'' ``Did the program ever work correctly before it started failing? How many times?''

Rick Lemons said that the best lesson he ever had in debugging was watching a magic show. The magician did a half-dozen impossible tricks, and Lemons found himself tempted to believe them. He then reminded himself that the impossible isn't possible, and probed each stunt to resolve its apparent inconsistency. He started with what he knew to be bedrock truth -- the laws of physics -- and worked from there to find a simple explanation for each trick. This attitude makes Lemons one of the best debuggers I've ever seen.

The best book I've seen on debugging is The Medical Detectives by Berton Roueche', published by Penguin in 1991. The heroes in the book debug complex systems, ranging from mildly sick people to very sick towns. The problem-solving methods they use are directly applicable to debugging computer systems. These true stories are as spellbinding as any fiction.

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