Operating Systems Programming Assignment 1 Solution

Goals

By completing this assignment, you will know how to develop a program in the Linux platform and to make system calls related to process management.

Introduction

You need to implement a C/C++ program called BP that allows user to run multiple processes (up to three at the same time) at the background. When three processes are currently running, further execution request will be pended (the process state is changed to “stopped”) and wait until another process is stopped or terminated. While processes are running at the background, the user can input command to display information of the background processes, stop or kill a background process.

Requirements

    1. Your BP needs to show a prompt for user input as follows. $ ./BP

BP >

    2. BP accepts the following commands from the user and takes the corresponding action.



BP >bg [name of executable file] [a list of arguments]

Action: BP runs the executable file with a list of arguments at the background and continues to accept input from the user. If there are already 3 running processes, the process is stopped.

Example: BP runs the executable file demo1 with a list of [arguments]: running 2 5 at the background and continues to accept input from the user.

BP >bg demo1 running 2 5

BP >


BP >bglist

Action: Display the process id(s), name(s) and the state(s) of ALL background processes.

Example:

BP >bglist
16529: demo1(running)
16605: demo2(stopped)
16613: demo3(terminated)


BP >bgstop [pid]

Action: Stop the process with process id pid and display a message. If there exists another stopped process (e.g. stopped earlier because there’re already 4 running processes), the earliest process in stopped state (creation order, not runtime order) should be automatically restarted.

Example:

BP >bgstop 16529
16529 stopped
16624 automatically restarted
BP >bgkill [pid]

Action: Terminate the process with process id pid and display a message. Similar to bgstop, if there exists another stopped process, the earliest process (creation order, not runtime order) in stopped state should be automatically restarted. Example:

BP >bgkill 16529
16529 killed


BP >exit

Action: BP executes bgkill to terminate all background processes, if any, and exits.
Example:

BP >exit
16605 killed
16607 killed
$



    3. BP should display a message after a background process has completed. Example:
16529 completed

    4. You may assume that the syntax of the input commands and pids are valid, but BP needs to handle redundant commands (e.g. bgkill a process which is already terminated) by displaying a message. Examples:

16529 already stopped

16529 already terminated
16529 does not exist

Hints

Use fork() and execvp() so that the parent process accepts user input and the child process executes the background process.

When you use fork(), it is important that you do not create a fork bomb, which easily eats up all the resources allocated to you. If this happens, you can try to use the command “kill” to terminate your processes (http://cslab.cs.cityu.edu.hk/supports/unix-startup-guide). However, if you cannot log into your account any more, you need to ask CSLab for help to kill your processes.

Use waitpid() with an option WNOHANG to check if a background process has completed.

Use kill()to send a signal to a process, e.g., a SIGTERM signal to terminate a process. Do note that kill()can also be used to stop / resume a process, regardless of it’s name.

Study the man pages of the system calls used in your program. For example, the following command displays the man pages of kill() in Section 2.

$ man 2 kill

Helper programs
demo.cpp

This demo program can be used to act as a background process for testing your BP as its execution can be visualized by displaying a word every few seconds a number of times.

This program takes three arguments, word, interval and times. The first argument word is a single word to be displayed repeatedly.
The second argument interval is the number of seconds between two consecutive displays of the word.

The third argument times is the number of times the word to be displayed.

For example, the following command displays the word “running” 5 times in 2-second interval. $ demo running 2 5

args.cpp

This example program shows how to read a line from terminal, as well as parsing (cutting) the string using the strtok() function.

To compile the program, use the following command. $ g++ args.cpp -lreadline -o args

Marking

You program will be tested on our CSLab Linux servers (cs3103-01, cs3103-02, cs3103-03). You should describe clearly how to compile and run your program in the text file. If an executable file cannot be generated and running successfully on our Linux servers, it will be considered as unsuccessful.

Marking scheme (total: 100%):

o
bg
20%
o
bglist
10%
o
bgstop
10%
o
bgkill
10%
o
exit
10%
o  Correctly having 3 processes running at the same time (not 2 or 4…)
10%
o  Correctly restart another process upon stop/termination
10%
o  Handling of redundant calls (e.g. stop a stopped process)
10%
o  programming style and in-program comments
10%

Submission

This assignment is to be done individually or by a group of two students. You are encouraged to discuss the high-level design of your solution with your classmates but you must implement the program on your own. Academic dishonesty such as copying another student’s work or allowing another student to copy your work, is regarded as a serious academic offence.

Each submission consists of two files: a source program file (.cpp file) and a text file containing user guide, if necessary, and all possible outputs produced by your program (.txt file).

Write down your name(s), eid(s) and student ID(s) in the first few lines of your program as comment. Use your student ID(s) to name your submitted files, such as 5xxxxxxx.cpp, 5xxxxxxx.txt for individual submission, or 5xxxxxxx_5yyyyyyy.cpp, 5xxxxxxx_5yyyyyyyy.txt for group submission. Only ONE

submission is required for each group. Submit the files to Canvas.

The deadline is 11:00am, 28-FEB-2020 (Friday). No late submission will be accepted.

Questions?
This is not a programming course.  You are encouraged to debug the program on your own first.

If you have any questions, please submit your questions to Dr. Kenneth Lee via the Discussion board "Programming Assignment #1".

To avoid possible plagiarism, do not post your source code on the Discussion board. If necessary, you may also contact Dr. Kenneth Lee at kenkclee@cityu.edu.hk.