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.

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

Example:

• 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:
   • 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/unixstartupguide). 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 its 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()
• To compile the program, use the following command.

$ g++ args.cpp -lreadline -o args