lab06 : Debugging Tools: gdb and valgrind

num ready? description assigned due
lab06 true Debugging Tools: gdb and valgrind Mon 11/12 08:00AM Sun 11/18 11:59PM

Two restrictions: individual (not pair), and ON CSIL (not on your own machine)

THIS IS AN INDIVIDUAL LAB. Pair programming is not permitted.

Each individual must submit their own individual submission.

Note also, that with each submission, Gradescope will use the umail address associated with your Gradescope account to randomize the answers it expects for this lab. Therefore, throughout the lab, you need to pass in your umail address as the command line argument for the C++ programs you are running (see instructions below).

So, if you are tempted to copy answers from another student, please, please, please don’t (for obvious reasons). The expected answers for each student will be unique.

Background

Thus far you’ve learned about object oriented design and Test Driven Development. These techniques were designed to give structure to your code and your development process. This structure reduces the chance that a bug will sneak its way into your program. Even the most experienced programmer occasionally introduces bugs into his/her program. Given that a significant portion of your time will likely be spent debugging your code, we want to optimize the debugging process and make you more productive.

Step-by-Step

There are two sections of this lab.

In first section, Steps 1 and 2, you are guided through learning about gdb and valgrind. There is nothing to turn in from these steps, so there may be a temptation to skip over them. We encourage you, instead, to really take your time and work through those, to really learn about gdb and valgrind, because you’ll need those skills for step 3.

Step 3 requires you to work with gdb and valgrind, compute some answers, and put those answers in text files.

Those files will then be submitted to Gradescope for grading.

Step 0: Getting Started

This lab must be done individually. Pair programming is not allowed. As mentioned above, each individual student’s answers will be unique.

Step 1: gdb

The GNU debugger (a.k.a. gdb) is a very powerful tool for analyzing what your program is doing at run-time.

First read through the tutorial tools: gdb at the general CS32 course website (https://ucsb-cs32.github.io). The rest of these instructions assume that you already read through that. So if you continue without first looking over that, you are likely to not know how to proceed. If you then ask for help, we will first try to figure out: did you actually read through Tools: gdb? If you did not, we’ll tell you: ok, go read it.

Since you are already familiar with the code in lab03, you should use lab03 to practice using gdb. You will need to recompile lab03 with the -g option. Add -g to the CXXFLAGS in your Makefile.

The order of flags on the CXXFLAGS line does not matter. You can add it to the end or the beginning.

CXXFLAGS = -g -std=c++11 -Wall -Wextra -Wno-unused-parameter -Wno-unused-private-field

Every time you change your Makefile you should always do a make clean first. Rebuild your tests.

Now start gdb ./testStudent00 and set a breakpoint in Student::setName. (If you have no idea how to do that, it’s probably because you didn’t read though the tools: gdb tutorial first. Do that now.)

When you run the program (after setting the breakpoint, you type run), gdb will suspend the execution at the start of the setName method. You can then use various gdb commands to examine the state of the program, including what’s on the stack and what’s on the heap.

Before moving on, you should experiment with each of the following gdb commands:

There are examples for these commands on the [[tools: gdb](https://ucsb-cs32.github.io/topics/tools_gdb/ page. Since you might not always know what you are looking for when debugging, experimentation is very important. It helps to know your code, but sometimes you may be debugging code that you didn’t write.

Later steps in this lab will assume that you really did figure out how to use the commands above. So, if you are just skipping over this step, or skimming it, eventually you’ll make your way back here… Just sayin’.

Step 2: valgrind

After you are comfortable using gdb and you have stepped through a few routines in your program now it’s time to learn about valgrind. We talked a bit about valgrind, but there’s more to learn, so please now read about valgrind here at the tools: valgrind article on the https://ucsb-cs32.github.io website.

Run memcheck on your lab03 and ensure that you don’t have any memory leaks or errors that were detected by valgrind. You should fix any errors that valgrind reports in lab03. (Note that if your program doesn’t use new and/or delete, there probably won’t be any.)

Run each test with –leak-check=full

valgrind --leak-check=full ./testStudent00
valgrind --leak-check=full ./testStudentRoll00
etc...

Step 3: Debugging someone else’s code

It’s time to put your new skills to the test. You will be given an executable file, but not all the source code.

It is possible that you may have a program that calls a library and you won’t be given all the source code to the library or the library was built without the -g option. Here we are reversing this a little bit. You are given some code, but the code to the main function is not given to you. You will not be able to step through all of the code. In a sense, you can treat main function like a ‘black box’, which calls the code you are given. You don’t need to know what the main function is doing, but you will have to answer a few questions about how it calls the code you are given.

Download the 4 files from here:

Or copy from here:

Guidelines for Answering

To be sure that your answers pass the tests on Gradescope, be sure that you record them carefully. The tests on Gradescope are very picky about filenames, formatting, etc.

HINT: Suggestion for Creating the Answer Files

Use the echo command and redirect the output to a file. There is an example below. Note that the quotations in the echo command are not included in the file named ‘testfile’. The cat command is just printing the file to the terminal so you can see that the contents were in fact written.

-bash-4.2$ echo "100" > testFile
-bash-4.2$ cat testFile
100

The source code we’ve given you is poorly written code. Take your time and look through it, but note that these are examples of code that should be rewritten. However, that is not your job today. Instead, you will use debugging tools to understand why these programs are failing.

First run the segProgram, which is designed to segfault.

./segProgram cgaucho@umail.ucsb.edu
Segmentation fault (core dumped)

gdb was designed to make handling segfaults trivial. Use gdb to pry into segProgram and discover the reason why it is crashing. When using gdb, make sure you run gdb from the same directory as the program you are analyzing (gdb relies on the current working directory as a path to find the source code).

Question 1 (q1a, q1b)

Start up gdb and load in segProgram.

gdb ./segProgram

Now run the program with the run command followed by your umail address:

(gdb) run cgaucho@umail.ucsb.edu

It will stop as soon as it encounters the segfault. Note that when you use the command run [blah], it takes [blah] as the command line argument (i.e. the value of argv[1]). This is a necessary step in order to produce the answer Gradescope is expecting.

Question 2 (q2)

For the remainder of the questions use the debugProgram executable.

gdb ./debugProgram

Remember to pass in your umail address when running through gdb in order to obtain the correct values.

(gdb) run cgaucho@umail.ucsb.edu

q2: What is the value of the string ‘a’ when debugFunction1 is called?
Remember to omit the quotation marks in your answer. Write your answer to a file named q2.
Hint: use the break feature to stop the program

Question 3 (q3)

q3: What is the value of the string ‘a’ when debugFunction1 is called a second time?


Remember to omit the quotation marks in your answer. Write your answer to a file named q3.

Question 4 (q4)

q4: How many recursive calls to recursiveFunction are in the backtrace (also called a stack trace) when variable a == 100 in recursiveFunction? Put your answer in q4.

Hints:

Question 5 (q5a, q5b)

Hint: For this question, consider using the watch command.

Question 6 (q6)

q6: The function dataLost is leaking memory. How much data is lost in the dataLost function? Write your answer in a file called q6.

Hints:

valgrind --leak-check=full ./debugProgram cgaucho@umail.ucsb.edu

Step 4: Submitting via Gradescope

You will turn in your files for each question above (q1a, q1b, q2, q3, q4, q5a, q5b, q6).

The lab assignment “Lab06” should appear in your Gradescope dashboard in CMPSC 32. If you haven’t submitted anything for this assignment yet, Gradescope will prompt you to upload your files.

For this lab, you will need to upload your files. You either can navigate to your file, “drag-and-drop” them into the “Submit Programming Assignment” window, or even use your GitHub repo to submit your work.

If you already submitted something on Gradescope, it will take you to their “Autograder Results” page. There is a “Resubmit” button on the bottom right that will allow you to update the files for your submission.

If everything is correct, you’ll see a successful submission passing all of the autograder tests.