Lab 2 Assignment Solution

Instructions This lab assignment explores the data shared problem and process synchronization using Peterson’s solution.
Objectives of this assignment:
    • to work on a Unix based system
    • to “dust off” your programming skills in C
    • to understand the fork() function to create a”child” process
    • to understand the relationship (or lack of) between parent and child process
    • to experience the data shared problem
    • to deploy the Peterson’s solution to address the data shared problem

IMPORTANT: 
    1) Your code will be tested and graded REMOTELY on the Engineering Unix (Tux) machines. If the code does not work on those machines, you will not get any credit even if your code works on any other machine.
    2) A late submission will get a 50% penalty if submitted right after the deadline. The next day, you cannot submit the lab.
    3) One submission per group.
    4) Writing and presentation of your report are considered to grade your lab (30%). Your conclusions must be supported by the data/measurements you collect.
    5) The quality of your code will be evaluated (80%).
    6) Questions about this lab must be posted on Piazza if you need a timely answer benefiting all students.

Use this file to answer the questions. Highlight your answers and do NOT remove anything from this file. Just Insert your answers.

Part I: Programming on Tux machines
(10 points) Program Exercise 1: 
Exercise 1: Download the program lab2-1.c. Compile it and execute it. Observe the code and observe the output. This program has a parent and child processes sharing a variable. This program is intended to increment the shared (common) variable counter *countptr. The parent process is supposed to increment *countptr by increments of 20 while the child increments by 2s.  A satisfactory execution of this program may be: the child increments the counter *countptr twice (reaching 4), then the parent increments the counter *countptr thrice to reach finally 64. Answer the following questions:
1) Does the program really execute as supposed (or intended)? Justify/Explain No, since the final number generated by the output is not 64 – it is in fact 60 (or 50, depending on if you’re looking at the child or parent process). The child process increments the shared variable 25 times instead of 2 times, but the parent process does increment the shared variable 3 times as desired.
2) Is the variable *countptr really a shared (common) variable?  In other words, are the changes made to *countptr by the child visible by the parent, and vice versa?  Explain. No, since both the parent and child processes increment their own versions of the variable. No matter what the value of the shared variable is for one process, the other process will have its own value for it. This means that any changes made to the shared variable are not visible to the other process, as they both have their own local copies of the variable.

(90 points) Program Exercise 2: 
The program lab2-2.c creates a genuine shared variable *countptr. Download, compile, and execute this program.
1)       Based on the execution, show that *countptr is now a genuine shared variable (countptr points to a zone shared by the parent and the child). Now, are the changes  to *countptr made by the child visible by the parent? No, since the variable has different values for the child process and the parent process. The child process increments to 37, then increments to 41 after the parent process suddenly increments from 0 to 40.
2)       Does the program really execute as supposed (or intended), i.e, the counter increases exclusively in increments of 2 or 20? Explain what is happening. No, the child process increments by 4 and the parent process doesn’t seem to follow a pattern with incrementing the shared variable. Also, the shared variable starts off with a value of 1 and is incremented by the child process 9 times before the parent process increments the variable. The parent process increments the starting value of the variable to 40, and then to 66 the second time it executes.
3)       Without modifying the routine add_n(), use the Peterson’s solution to correct the program lab2-2.c. to execute as intended: the variable should increase by 2’s or twenty’s.
Hint: Besides the pointer countptr used to point to the shared memory zone, you need to map three other integers Interested[2] and Turn (Peterson’s variables); These variables may be shared exactly the way  that the zone pointed by countptr is shared.



What to turn in?
Electronic copy
    Turn in separate files:
    1) THIS file with INSERTED answers
    2) Program lab2-2..c (corrected)
 A penalty of 10 points will be applied if these instructions are not followed.

    1) Your report must:
        a. state whether your code works. If is does work, state any issues you are aware of.
        b. Good writing and presentation are expected.



Our code works as you can see in the above screenshot. I ssh into the school computer, cd to my working directory, compile the code, and ran it. The parent increments by 20 and the child increments by 2, as desired. Therefore, the code does work as specified in the problem statement.