Lab 02: GDB and Pointers

Lab 02: GDB and Pointers
CSE-031-01

Overview
By now, you should have “mastered” the skills of coding and compiling your programs in a terminal. The
next step is to further explore the tools we can use in the terminal to enhance our “programming
experience”. The goal of this lab is to learn how to use GDB (or, optionally, LLDB in case you are using a
MAC computer) to debug and fix errors in your programs. Note that if you are using LLDB, it is your
responsibility to learn the We will practice the use of pointers and how they are related to memory. You can
refer to chapter 5 of K&R for references on pointers.
Getting Started
Before we begin any activities, create a directory (Lab_2) inside the CSE31 directory we created last week.
You will save all your works from this lab here. Note that all the files shown in green below are the
ones you will be submitting for this assignment.
You must have a clear idea of how to answer the lab activities before leaving lab to receive
participation score.
Tutorial in GDB and Setup
GDB (or GNU Debugger) is a debugger for C and C++ (and many other languages) that runs on Linux
systems.
TPS (Think-Pair-Share) activity 1: Perform the following tasks while paired with your classmates
assigned by your TA (you will be assigned to groups of 3-4 students):
1. Record your TPS partners’ names.
2. Independently search the internet for 3 online tutorials on how to setup and use GDB (or LLDB) in
your system.
3. Share your tutorials with your TPS partners.
4. Bookmark your results in the browser of your computer.
Your TA will “invite” one of you randomly after the activity to share what you have discussed.
Setting up GDB: Follow the tutorials you have found (or the tutorials that have been made available
through CatCourses) to setup GDB in your computer.
Running GDB
Copy your punishment.c file from Lab 1 into your Lab_2 directory.
TPS activity 2: Discuss questions 1 – 8 with your TPS partners in your assigned group (15 minutes) and
record your answers in a text file named tpsAnswers.txt under a section labelled “TPS 2” (you will
continue to use this file to record your answers to all the TPS questions that follow in the lab
handout):
1. How do you compile your punishment.c so that you can debug it using GDB? Try it with your code
and set the name of the executable to punish.
2. Once punishment.c is compiled, how do you load it in GDB? Try it with your program.
3. Once punish is loaded, how do you run it in GDB? Try to run your punish.
4. What are breakpoints? How do you set a breakpoint at a certain line of your program? Try to set a
breakpoint in punishment.c where the for loop begins.
5. Now run the program again. It should stop at the breakpoint you set in Q4. From here, how do you run
the program line by line? Try to run the next 3 lines with your program.
Lab 02: GDB and Pointers
CSE-031-01 Points: 20
6. While you are still running punish line by line, how can you see the value of a variable? Pick 3 variables
in your program and display them in the terminal one by one.
7. Now that you are tired of running line by line, how do you let the program finish its run? Try to finish
running your punish.
8. How do you exit from GDB?
Your TA will “invite” one of you randomly after the activity to share what you have discussed.
Create pointers.c
Use your favorite text editor to create a C program called pointers.c and copy the following code to
your file:
#include <stdio.h>
int main() {
int x, y, *px, *py;
int arr[10];
return 0;
}
TPS activity 3: Discuss questions 1 – 8 with your TPS partners in your assigned group (15 minutes) and
record your answers in tpsAnswers.txt under a section labelled “TPS 3”:
1. How many variables were declared in the first line of main()? How many of them are pointers (and
what are they)?
2. What will be the values of x, y, and arr[0] if you run the program? Validate your answer by running
the program. Why do you think it happens that way? You will need to insert printf statements to
display those values.
3. How do you prevent x, y, and the content of arr from having unexpected values? Try to fix them in
the program.
4. The moment you have declared a variable, the program will allocate a memory location for it. Each
memory location has an address. Now insert printf statements to display the addresses of x and y.
5. Now insert code so that px points to x and py points to y. Print out the values and addresses of those
pointers using only the pointer variables (yes, pointers have addresses too!). You should see that the value of
px is equal to the address of x, and the same is true for py and y.
6. As we have learned in lectures, an array name can be used as a pointer to access the content of the array.
Write a loop to output the contents of arr by using arr as a pointer (do not use [] in your loop).
7. Are array names really the same as pointers? Let us find out! An array name points to the first element
of an array, so arr should point to the address of arr[0]. Insert code to verify this.
8. Now print out the address of arr. Does the result make sense? Why?
Your TA will “invite” one of you randomly after the activity to share what you have discussed.
Individual Assignment 1: Segmentation Faults
Recall what causes segmentation fault and bus errors from lecture and the textbooks. Common cause is an
invalid pointer or address that is being dereferenced by the C program. Use the program average.c from
the assignment page for this exercise. The program is intended to find the average of all the numbers
inputted by the user. Currently, it has a bus error if you input more than one number.
Load average.c into GDB with all the appropriate information and run it. GDB will trap on the
segmentation fault and give you back the prompt. First, find where the program execution ended by using
backtrace (bt as shortcut) which will print out a stack trace. Find the exact line that caused the
segmentation fault. Answer the following questions in a text file named assignAnswers.txt (you will
continue to use this file to record your answers to all the assignment questions that follow in the lab
handout) under a section labelled “Assignment 1”:
1. What line caused the segmentation fault?
2. How do you fix the line so it works properly?
You can recompile the code and run the program again. The program now reads all the input values but the
average calculated is still incorrect. Use GDB to fix the program by looking at the output of
read_values. To do this, either set a breakpoint using the line number or set a breakpoint in the
read_values function. Then continue executing till the end of the function and view the values being
returned. Answer the following questions (in the text file assignAnswers.txt as mentioned above):
3. What is the bug here?
4. How do you fix it?
Individual Assignment 2: Fix appendTest.c
Compile appendTest.c from the assignment page and record your answers to the following questions in
assignAnswers.txt while running the program under a section labelled “Assignment 2”:
1. Run the program with the following input: “HELLO!” for str1 and “hello!” for str2. Is the output
expected?
2. Do not stop the program, enter “HI!” for str1 and “hi!” for str2. Is the output expected? What is
the bug here? Try to fix the program so it will print the output correctly.
3. Do not stop the program, enter “Hello! How are you?” for str1 and “I am fine, thank
you!” for str2. Is the output expected? Why do you think this happens? You do not need to fix
this.
You can now stop the program by pressing Ctrl-C.
Individual Assignment 3: Complete arrCopy.c
Study and complete arrCopy.c so that it outputs the following sample result in the same format. You
must only insert code in the segments labelled with //Your code here. Contents of all arrays must be
accessed through pointers, so you must not use any array notation ([]) in your code.
Hint: Use dynamic memory allocations (malloc)!
Your program must produce an output that exactly resembles the Sample Run, including identical
wording of prompts, spacing, input locations, etc.
Sample Run (user input shown in blue, with each run separated by a dashed line):
-------------------------------------------------------------------------------SAMPLE RUN 1
Enter the size of array you wish to create: 5
Enter array element #1: 1
Enter array element #2: 3
Enter array element #3: 5
Enter array element #4: 7
Enter array element #5: 9
Original array’s contents: 1 3 5 7 9
Copied array’s contents: 1 3 5 7 9
Collaboration
You must credit anyone you worked with in any of the following three different ways:
1. Given help to
2. Gotten help from
3. Collaborated with and worked together
What to hand in
When you are done with this lab assignment, submit all your work through CatCourses.
Before you submit, make sure you have done the following:
• Your code compiles and runs on a Linux machine (without the need for special libraries).
• Attached pointers.c, average.c, appendTest.c, arrCopy.c, assignAnswers.txt, and
tpsAnswers.txt.
• Filled in your collaborator’s name (if any) in the “Comments…” text-box at the submission page.
Also, remember to demonstrate your code to the TA or instructor before the end of the grace period.