Why Do I Want A Debugger?

When you have finally got your program to compile with no errors, the next step is to execute it. If you are incredibly lucky or an astonishingly good programmer, then you can skip this chapter. Otherwise you will sooner or later need to use a debugger.

Once the program is executing, it may do any of a number of things. Typically, it will do what you expected, except for a few small details - taking the wrong branch, or calculating values which are too high or too low, or just plain ridiculous.

Most of these problems are head-slappers - you understand immediately what is wrong and run through the edit-compile-test cycle again. An example of this would be FIXME

The next stage you get to is the more subtle problems. This is where the program runs for a long time then inexplicably dies with a core dump, or one of the ouput values occasionally comes out negative when it shouldn't be able to. That's when you need a debugger.

A debugger allows you to take control of the running program. You can start it, stop it, make it run one line at a time, examine the values of variables at each step, and even change their values to see what would happen. Sometimes it may take 20 minutes for a program to run to the point where it all goes wrong - you can put a breakpoint in the code and the debugger will allow it to run normally until that point, then stop and give you full control.