Debuggers

Pencil and paper debugging is tedious and painful, but effective. Log-based debugging is less tedious (maybe) and less painful (no risk of poking yourself with a pencil when you’re not using a pencil), and while easy, don’t always work (for example, a printf in a C program that’s before a line that crashes won’t always print before the program crashes).

Despite being tedious, painful, and not always working, these strategies for debugging are approachable because they use tools that you have to know about to do programming in the first place, and they are universal: you can use log-based debugging in any language where you can print or produce output, and you can use pencil-and-paper debugging anywhere you’ve got a pencil, and paper.

Nevertheless, they are painful. Painful and tedious.

A debugger is a program that you can use to help you debug a program. Debuggers are usually very tightly coupled to the programming language or environment that you’re using (you use a Java debugger to debug Java programs, you use a Python debugger to debug Python programs, and you use a C debugger to debug C programs).

All debuggers give you generally the same tools for working with your program, and we generally use them in the same way. A debugger will let us run one line of code at a time and inspect the values that variables have when the line of code has completed running.

Almost all debuggers will use the same general terminology to describe actions or ideas:

• A break point is a way for you to tell the debugger where and when it should pause running your program. We can say things like “Start running from the beginning, then pause when you get to line 158.”
• We can step into a function or method call when the line we’re about to run is a function or method call. Stepping into the function or method call means that our debugger will “move” to the code for the function, then let us run lines inside that function.
• We can step over a line, running whatever the line was and then pausing on the next line of code.
• We can inspect values, either by printing them out, or by seeing their values in a window or pane.