Due: 5/24 11:59pm
Please read the instructions carefully
This lab may only be done on CSIL and it is an INDIVIDUAL ASSIGNMENT. Pair programming is not permitted.
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.
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 their 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.
There are two sections of this lab.
In first section, Steps 1 (gdb) and 2 (valgrind), 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.
2.1. Getting Started
This lab must be done individually. Pair programming is not allowed. As mentioned above, each individual student's answers will be unique.
2.2. 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 previous labs, you should use them to
practice using gdb. You will need to recompile your lab 3 code with the
option to use gdb on it. Add
-g to the
CXXFLAGS in your
Makefile. Note that your code for
that lab does not need to work completely for this step, you can debug a buggy
program and that is the point of debugging.
The order of flags on the
CXXFLAGS line does not matter. You can add
it to the end or the beginning. For example, you can have
CXXFLAGS = -g -std=c++17 -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.
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
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 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 saying.
2.3. 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.
memcheck on your lab 3 and ensure that you don't have
any memory leaks or errors that were detected by valgrind. Ideally, you should
fix any errors that valgrind reports in lab 3 as an exercise. (Note that
if your program doesn't use
delete, there probably won't
Run each test with –leak-check=full
valgrind --leak-check=full ./testStudent00 valgrind --leak-check=full ./testStudentRoll00 etc...
2.4. 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.
If you visit the following link, you can browse the 4 files:
You should see a listing of several C++ programs. We are going to copy
those into your
~/cs32/lab8 github repo all
at once with the following command:
cp ~emre/public_html/cs32/code/lab8/* ~/cs32/lab8
2.4.1. 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.
- Do not add extra text to answers (e.g. reduce answers like
The answer is: x. to only
- If a question asks for you to write a string "my string", do not
include the quotation marks (e.g. "my string" =>
- All questions should be placed in separate files matching the question
name (e.g. your answer to q1a should be placed in a file called
q1a; DO NOT create q1a.txt, etc )
- If a question asks you to enter a number 5,000 only include the
digits without the comma (e.g. 5,000 =>
2.4.2. HINT: Suggestion for Creating the Answer Files
You can 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.
$ echo "100" > testFile $ cat testFile 100
The source code we've given you is deliberately poorly written. 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 firstname.lastname@example.org Segmentation fault (core dumped)
gdb was designed to make handling segfaults trivial. Use
gdb to pry
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).
2.5. Question 1 (q1a, q1b)
gdb and load in
Now run the program with the
run command followed by your umail
(gdb) run email@example.com
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). This is a necessary step in
order to produce the answer Gradescope is expecting.
- q1a: What index of the array is the program trying to access? Write your answer in q1a. That's q-one-a, not q-L-a.
- q1b: What value is the program trying to store in the array when the fault occurred? Write your answer in file q1b
2.6. Question 2 (q2)
For the remainder of the questions use the debugProgram executable.
Remember to pass in your umail address when running through gdb in order to obtain the correct values.
(gdb) run firstname.lastname@example.org
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
2.7. 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.
2.8. 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: * Use
bt), along with conditional
breakpoints: break someplace if condition * What is the number of stack
frames when the last instance of recursiveFunction is created? You may
need to page through them. * Then think about how many of those stack
frames are actually for the recursive function, as opposed to other
functions that have been put on stack before or after.
2.9. Question 5 (q5a, q5b)
Hint: For this question, consider using the watch command.
- q5a: The main function assigns values to watchdata[ ]. What is watchdata's new value after being assigned a new value by main. Put the new value of watchdata (after it is modified by main) into the file q5a
- q5b: In addition, what is the value of the variable watchsomevalue at the instant that watchdata is updated? Please put the value of watchsomevalue at that moment into the file q5b.
2.10. 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.
- Run your program using
valgrind --leak-check=full ./debugProgram email@example.com
- Don't include data lost elsewhere.
- Valgrind formats its numerical output with commas. Don't include commas in the answer.
- Only submit the number. Don't use any units like Bytes.
3. Submitting via Gradescope
You will turn in your files for each question above (q1a, q1b, q2, q3, q4, q5a, q5b, q6).
The lab assignment "Lab8" should appear in your Gradescope dashboard in CS 32. If you haven't submitted anything for this assignment yet, Gradescope will prompt you to upload your files. For this lab, you are required to submit your files with your github repo.
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.