CSC 161	Grinnell College	Spring, 2009

Imperative Problem Solving and Data Structures

Gnu's DDD Debugger

Goals

This laboratory exercise walks you through the DDD tutorial provided by Gnu. DDD is a graphical debugger DDD (Data Display Debugger), a helpful tool for debugging C programs in a Unix environment.

Acknowledgement

This lab is an edited version of a DDD tutorial by Marge Coahran. Be sure you have reviewed today's reading before proceeding to the steps that follow.

Part A: Gnu's DDD Tutorial

In this exercise, I would like you to walk through the DDD tutorial provided by Gnu. Unfortunately, the sample code contains some C syntax that we have not studied yet, so you may not understand everything in the sample program. However, you will still learn a lot about the use of the debugger, and that is the point of this laboratory lesson.

Follow the instructions in the sample tutorial, except for the exceptions listed below. Stop at the end of the tutorial (i.e., when you run into the sample source code again), which is way before the end of the web page.
- In many places, the tutorial suggests further reading in the form "(see Topic Name)". I suggest that you skip these additional readings on your first pass through the tutorial. If you have extra time and inclination, you can follow these links later.
- On my system, the bug in the program does not show itself when using sample data suggested in the tutorial. However, it does appear if I multiply all of the suggested values by 100. This may be system (and moment) dependent, given the nature of the bug. I suggest you start by trying it my way, and if the bug does not appear, use even larger values.
- I also suggest that you add an & after the command when you invoke ddd. As usual, this allows you to run the program without disabling your terminal window.
```
  ddd sample &
```
Return to the top of Gnu's tutorial, and scan through it again, this time making a crib sheet of the various DDD features you learned. I suggest recording how the various features are invoked and also brief descriptions of what they do. You may want to keep this crib sheet handy for future use!

Part B: Various Shorts

Compile the program sample.c again, this time without using the option -gstabs, and try to run it in DDD. (The purpose of this exercise is so that you may recognize the error it produces should you happen to see it again in the future.)
Re-compile sample.c with the debugging information included. Run ddd again. Move the mouse over various icons on the toolbar, and press the F1 key over some of them to read about what they do.
Try selecting What Now? from the Help menu (in the far upper-right corner of the DDD window). You may also want to try the Tip of the Day from the same menu.

Part C: A Debugging Exercise

Copy or download the following bug-ridden C program: ~walker/c/debugger/debugme.c.
Use what you have learned to track down and fix the bugs in this program with the help of DDD.

You may find the Interrupt button on the DDD Command Tool useful for this exercise. It will suspend your program -- a useful feature for locating infinite loops.

At some point during this exercise, try moving a breakpoint from one line to another by dragging the stopsign icon.

For Those with Extra Time:Shell Sort

If you are interested in learning more about the Shell Sort algorithm used in Gnu's DDD tutorial, you can read about it on Wikipedia.

This document is available on the World Wide Web as

http://www.walker.cs.grinnell.edu/courses/161.sp09/labs/lab-debugger.shtml

created February 2007 by Marge Coahran restructured 4 May 2008 by Henry M. Walker last revised 24 January 2009 by Henry M. Walker
For more information, please contact Henry M. Walker at walker@cs.grinnell.edu.