CSE031-Lab 2 GDB and Pointers Solved

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 (or LLDB for MAC)? 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 (or LLDB for MAC)? Try it with your program.

3.       Once punish is loaded, how do you run it in GDB (or LLDB for MAC)? 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.

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 (or LLDB for MAC)?

 

 

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?

 

 

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.

 

Make a copy of average.c as you will be using the original version of this file to demonstrate the steps for debugging during the demo. Name the copy average_original.c.

 

Load average.c into GDB (or LLDB for MAC) with all the appropriate information and run it. GDB/LLDB 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?

 

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.

 

 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: 9 7 5 3 1