Lab 5

2020_S19/wk5/lab5.cpp
2020_S19/wk5/lab5.txt

The purpose of this lab is to use the GNU Debugger (GDB) and practice debugging programs that contain compilation warnings and runtime errors. Also, practice polymorphic behavior, an abstract base class with pure virtual member functions.

Follow the steps provided and create a lab5.txt file that contains your responses to the given lab.
wget http://www.cs.csubak.edu/~derrick/cs2020/examples/shape_withErrors.cpp .
Copy the given shape_withErrors.cpp and compile the source code using the -g flag to enable debugging information and -Wall to show all warnings.
g++ -g shape_withErrors.cpp -Wall

  1. What is the warning that g++ gives us?
  2. What is the solution to fix the warning? (hint: destructor)


    3. Your program right now is able to execute, however, warnings are still errors!
    Recompile the program after fixing the warning, compile, and run the program.

  3. A segmentation fault should occur. In your own words (with out debugging), why might you think a segmentation fault occurred?


    4. gdb executable
    Load the executable into gdb (a.out or however you might haved named your executable if given the -o flag with g++) and set a break point at function main (break main). Next, run the program by typing run and step through the program, and enter a choice when prompted.

  4. What is the GDB output when the Operating System receives a signal SIGSEGV?
  5. Which function invokes the segmentation fault?


    6. Re-run the program again in the GDB environment, and step though again. Upon the first instance of GDB notifying us that a SIGSEGV signal has been received, use the info locals command.

  6. What is the output of 'info locals'?
  7. What is the clue that suggests why a segmentation fault occerred?


    8. Exit GDB by typing quit. Edit the code and fix the segmentation fault by allocating the derived object that corresponds to the menu options. Compile and run the program.

    Open GDB again, and type info functions. Set a break point to main and Shape::inputShape. Step through the program, enter option 1 for Square, and step until you reach the second breakpoint at 'inputShape'. Use command print this and ptype this.

  8. What is the output for print this and ptype this?
  9. Which version of inputShape and printShape is called, compared to earlier?


    10. This happens because we are using a base class pointer to reference a derived object, however the functions that are invoked are only the base class's due to static linking/binding.

    Add the necessary keyword to the base class member function prototypes to allow for polymorphic behavior and dynamic binding. Open GDB again, and type info functions. Set a break point to main and Square::inputShape. Step through the program, enter option 1 for Square, and step until you reach the second breakpoint at 'inputShape'. Use command print this and ptype this.

  10. What is the output for print this and ptype this for option 1?


    11. Re-run the pogram with run and 'y' to start at th beginning. Type info breakpoints and delete the breakpoint for Square::inputShape() with delete 2 and set a new breakpoint for Rectangle::inputShape(). Repeat the process with option 2 and use command print this and ptype this. Take notice to the output and datatype this is referencing.

  11. What is the output for print this and ptype this for option 2?


    12. Continue the program if you used option 1 or 2 to enter a Shape object, then chose option 0. Another segmentation fault should occur. This segmentation fault occurs whenever you attempt to deallocate already freed memory (double free error).

    Add an if statement at the end of our menu based loop that checks to see if your Shape pointer is not null, if true, deallocate the memory and set your Shape pointer to null.

    In addition, add the same if statment outside of the loop before we close our program. It's always good practice to check, deallocate, and handle dangling pointers. Recompile and run again.

  12. By now your source code should be error free and memory leak free and allows for polymorphic behavior. Use the unix copy command and copy your source code and call it 'lab5.cpp'
    cp ./shape_withErrors.cpp ./lab5.cpp
  13. Edit your lab5.cpp so that your Shape Class is an Abstract Base Class by making inputShape and printShape pure virtual member functions (=0). Remove the Shape::inputShape and Shape::printShape definitions. Compile lab5.cpp and run the program.


Have your completed lab5.cpp source code and lab5.txt answers:
2020_S19/wk5/lab5.cpp
2020_S19/wk5/lab5.txt