lab_debug

Disastrous Debugging

Due: Jan 31, 23:59 PM

Preface: How Did We Get Here?

Your lab partner is writing an image recognition program. They are working on the tracing algorithm, which turns the image into a trace of the outlines in the image. After going through all the compiler errors (sketchify.cpp:33, etc), the program finally compiles! Overjoyed to have a program, you both decide to test it on a couple images.

Segmentation Fault

Ouch. What do we do now?

Checking Out the Code

All assignments will be distributed via our release repo on github this semester. You will need to have set up your git directory to have our release as a remote repo as described in our git set up.

You can merge the assignments as they are released into your personal repo with

git pull --no-edit --no-rebase release main --allow-unrelated-histories
git push

The first git command will fetch and merge changes from the main branch on your remote repository named release into your personal. The --no-edit flag automatically generates a commit message for you, and the--no-rebase flag will merge the upstream branch into the current branch. Generally, these two flags shouldn’t be used, but are included for ease of merging assignments into your repo.

The second command will push to origin (your personal), which will allow it to track the new changes from release.

You will need to run these commands for every assignment that is released.

All the files for this lab are in the lab_debug directory.

Preparing Your Code

This semester for MPs we are using CMake rather than just make. This allows for us to use libraries such as Catch2 that can be installed in your system rather than providing them with each assignment. This change does mean that for each assignment you need to use CMake to build your own custom makefiles. To do this you need to run the following in the base directory of the assignment. Which in this assignment is the lab_debug directory.

mkdir build
cd build

This first makes a new directory in your assignment directory called build this is where you will actually build the assignment and then moves to that directory. This is not included in the provided code since we are following industry standard practices and you would normally exclude the build directory from any source control system.

Now you need to actually run CMake as follows.

cmake ..

This runs CMake to initialize the current directory which is the the build directory you just made as the location to build the assignment. The one argument to CMake here is .. which referes to the parent of the current directory which in this case is top of the assignment. This directory has the files CMake needs to setup your assignment to be build.

At this point you can in the build directory run make as described to build the various programs for the MP.

Make this Yours!

Determining What’s Going Wrong

You could start and open sketchify.cpp and try to figure out what’s happening. This is good for logical bugs – when you only rotate half of your image, for example, or the image doesn’t rotate at all. Walking through what your code does to yourself or your partner is a good exercise in debugging bugs in your algorithm. However, this is often a poor choice for debugging runtime errors or general code bugs. In this case, you should attempt to use the following workflow to debug your code.

Basic Instrumentation: Print (std::cout) Statements!

The easiest way to debug your code is to add print statements. To do this, you can add comments at various points in your code, such as:

std::cout << "line " << __LINE__ << ": x = " << x << std::endl;

The above line prints out the current line number as well as the value of the variable x when that line number executes, for example:

line 32: x = 3

__LINE__ is a special compiler macro containing the current line number of the file.

If you’re getting compiler errors after trying to use std::cout statements, then you need to #include the iostream library like this:

#include <iostream>

Print statements work for debugging in (almost) any language and make repeated debug testing easy – to repeat debug testing with a new change, all you need to do is compile and run the program again. They also require nothing new to learn (smile).

Debugging Your Code

To make and run the code, first ensure you’re in the build directory as mentioned above. Then, type the following into your terminal:

make
cp ../tests/in_01.png in.png
./sketch

Your First Bug

As you can see, your code caused a Segmentation Fault, or segfault. This happens when you access memory that doesn’t belong to you – such as dereferencing a NULL or uninitialized pointer.

Try adding print statements to lines 40 and 44, before and after the calls to original->readFromFile(), width(), and height().

Use the std::cout statement below on lines 39 and 43.

std::cout << "Reached line " << __LINE__ << std::endl;

Now run main again. You’ll see line 40 print out, but not line 44. This means the segfault occurred sometime between executing lines 40 and 44.

Work on getting your program to run to Line 44!

Bug 2

Once you’ve fixed the first bug, you’ll get another segfault. You’ll want to narrow down the line it’s occurring on and its cause by printing more information. Try putting std::cout statements at the beginning and end of the inner for loop.

More debugging!

Like almost most all code written, the code isn’t perfect. But fixing it is as simple as repeating the above to learn more about what the program is actually doing at runtime so that you can solve the issues. Good luck!

Checking Your Output

Once you think sketchify is working, you can compare your output (out.png) to the expected output by opening each image up using a graphical viewer.

If the outputs differ, you’ve still got a bit more debugging to do – go back and add some print statements to figure out why the outputs differ!

The command diff can also be used to compare two different files.

diff out.png ../tests/out_01.png

If the files differ, the output of this command will either say they differ, or provide specific differences between the two files. If there is no output, then the files are identical. This can be used on any file extension (e.g. .txt), and not just images.

You can also use the compare utility to generate a visual comparison of the images. For this to work, you’ll have to temporarily change your color’s hue to 280, since compare will expect the images to match exactly. Once you’ve recompiled and rerun with that new hue, run the following command:

compare out.png ../tests/out_01.png comparison.png

Differences between your image and the expected image will be highlighted in red in comparison.png.

Autograder Testing

You can run a subset of the test cases that will be used in the autograder with the following commands:

make test
./test

Submitting Your Work

The following files are used in grading:

  • sketchify.cpp

To submit your assignment you upload these file to the lab_debug question on PrairieLearn

Additional Resources