Pretty much everyone has had a program crash. A standard story is that you were typing in a paper when, all of a sudden, your word processor crashed. You had forgotten to save, and you had to start all over again. Old versions of Microsoft Windows used to crash more often than they should have, showing the dreaded “blue screen of death.” (Happily, they’ve gotten a lot better in the past several years.) Usually, your computer shows some kind of cryptic error message when a program crashes.
What happened in each case is that the people who wrote the program told the computer to do something it couldn’t do: open a file that didn’t exist, perhaps, or keep track of more information than the computer could handle, or maybe repeat a task with no way of stopping other than by rebooting the computer. (Programmers don’t mean to make these kinds of mistakes, they are just part of the programming process.)
Worse, some bugs don’t cause a crash; instead, they give incorrect information. (This is worse because at least with a crash you’ll notice that there’s a problem.) As a real-life example of this kind of bug, the calendar program that one of the authors uses contains an entry for a friend who was born in 1978. That friend, according to the calendar program, had his 5,875,542nd birthday this past February. Bugs can be entertaining, but they can also be tremendously frustrating.
Every piece of software that you can buy has bugs in it. Part of your job as a programmer is to minimize the number of bugs and to reduce their severity. In order to find a bug, you need to track down where you gave the wrong instructions, then you need to figure out the right instructions, and then you need to update the program without introducing other bugs.
Every time you get a software update for a program, it is for one of two reasons: new features were added to a program or bugs were fixed. It’s always a game of economics for the software company: are there few enough bugs, and are they minor enough or infrequent enough in order for people to pay for the software?
In this book, we’ll show you some fundamental techniques for finding and fixing bugs and also show you how to prevent them in the first place.