CSCI 2021 Lab06: GDB Basics

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                            LAB 06 QUESTIONS
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Lab Instructions
================

  Follow the instructions below to experiment with topics related to
  this lab.
  - For sections marked QUIZ, fill in an (X) for the appropriate
    response in this file. Use the command `make test-quiz' to see if
    all of your answers are correct.
  - For sections marked CODE, complete the code indicated. Use the
    command `make test-code' to check if your code is complete.
  - DO NOT CHANGE any parts of this file except the QUIZ sections as it
    may interfere with the tests otherwise.
  - If your `QUESTIONS.txt' file seems corrupted, restore it by copying
    over the `QUESTIONS.txt.bk' backup file.
  - When you complete the exercises, check your answers with `make test'
    and if all is well, create a zip file with `make zip' and upload it
    to Gradescope. Ensure that the Autograder there reflects your local
    results.
  - IF YOU WORK IN A GROUP only one member needs to submit and then add
    the names of their group.


QUIZ gdb basics and phase03.c
=============================

Starting Up
~~~~~~~~~~~

  After starting up GDB with the `phase03' program, the `run' command
  will yield the following output
  ,----
  | > gdb ./phase03
  | ...
  | 
  | (gdb) run
  | Starting program: ./phase03 
  | usage: ./phase03 <infile>
  | [Inferior 1 (process 1943881) exited with code 01]
  `----
  which means that the `phase03' program wants a command line
  argument. How does one accomplish this in GDB?
  - ( ) Use the command `set args input.txt' once and then each `run'
    will use those arguments
  - ( ) Use the command `run input.txt' each time you run
  - ( ) Either of the above will work
  - ( ) Start GDB as `gdb ./phase03 input.txt'


Setting a Breakpoint
~~~~~~~~~~~~~~~~~~~~

  Which of the following commands will set a breakpoint in GDB at the
  line that reads `int hit = shot ^ targ;'?
  - ( ) `break int hit = shot ^ targ'
  - ( ) `break phase03'
  - ( ) `break phase03.c:32'
  - ( ) `break next hit'

  After hitting the specified breakpoint, which of the following GDB
  commands will advance to the next **breakpoint** that is set?
  - ( ) `next'
  - ( ) `continue'
  - ( ) `run'
  - ( ) `advance'


next command in gdb
~~~~~~~~~~~~~~~~~~~

  During a debugging, the debugger has `phase03' stopped at the line
  indicated below via a `>' character.
  ,----
  | /// INITIAL ///
  |   void phase02(){
  |     int a = atoi(next_input());
  | >   int b = atoi(next_input());
  |     int c = atoi(next_input());
  | ....
  | 
  |   char *next_input(){
  |     input_idx++;
  |     int ret = fscanf(input_fh, "%s", inputs[input_idx]);
  | ...
  | 
  `----

  When the `next' command is executed by GDB, then the debugger executes
  some code and leaves the state of the running program in one of the
  following positions:

  ,----
  | /// OPTION A ///
  |   void phase02(){
  |     int a = atoi(next_input());
  |     int b = atoi(next_input());
  | >   int c = atoi(next_input());
  | ....
  | 
  |   char *next_input(){
  |     input_idx++;
  |     int ret = fscanf(input_fh, "%s", inputs[input_idx]);
  | ...
  | 
  | /// OPTION B ///
  |   void phase02(){
  |     int a = atoi(next_input());
  | >   int b = atoi(next_input());
  |     int c = atoi(next_input());
  | ....
  | 
  |   char *next_input(){
  |     input_idx++;
  |     int ret = fscanf(input_fh, "%s", inputs[input_idx]);
  | ...
  | 
  | /// OPTION C ///
  |   void phase02(){
  |     int a = atoi(next_input());
  |     int b = atoi(next_input());
  |     int c = atoi(next_input());
  | ....
  | 
  |   char *next_input(){
  | >   input_idx++;
  |     int ret = fscanf(input_fh, "%s", inputs[input_idx]);
  | ...
  `----

  After executing `next', which option correctly indicates where GDB
  will get control back?
  - ( ) OPTION A because `next' moves to the next line of code executing
    all functions and operations on a given line.
  - ( ) OPTION B because `next' executes the next operation and
    completes it which will be the `next_input()' function but the
    `atoi()' function will not be run
  - ( ) OPTION C because `next' steps into functions and executes the
    next operation which is to start running the `next_input()'
    function.


step command in gdb
~~~~~~~~~~~~~~~~~~~

  Instead of using the `next' command in the above example, what if the
  `step' command was used instead from the `INITIAL' position? Which
  `OPTION' represents the code position GDB would be at after `step'?

  - ( ) OPTION A because `step' moves to the next line of code executing
    all functions and operations on a given line.
  - ( ) OPTION B because `step' executes the next operation and
    completes it which will be the `next_input()' function but the
    `atoi()' function will not be run
  - ( ) OPTION C because `step' steps into functions and executes the
    next operation which is to start running the `next_input()'
    function.


CODE input.txt for phase03
==========================

  The mock `phase03' program can be compiled via `make' and run via
  ,----
  | > ./phase03 input.txt
  `----


  Use `gdb' and analyze what inputs are required in the file `input.txt'
  in order to "pass" the phase. A correct run yields the output:
  ,----
  | > ./phase03 input.txt
  | Running mock Phase03
  | Right on target: nice shootin' bitslinger!
  | Phase complete
  `----
  which is checked for in the CODE tests.

  Test that the code behaves correctly via the command
  ,----
  | make test-code
  `----

  and verify that both code/quiz are correct via
  ,----
  | make test
  `----

  before using
  ,----
  | make zip
  `----

  to create a zip to submit.


Hints to Determine the Input
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  - There are 4 input numbers required for the phase and these are
    extracted from the input.txt file
  - There is a variable called `targ' which is important to examine; you
    may wish to examine its value carefully using gdb's `print' command;
    set a breakpoint after `targ' has its value set and try the
    following commands
    ,----
    |   gdb> print targ     # print as decimal int
    |   gdb> print/x targ   # print as hexadecimal
    |   gdb> print/t targ   # print in binary
    `----