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日期:2021-02-19 11:34

CS3103: Operating Systems

Spring 2021

Programming Assignment 1

1 Introduction

The purpose of this assignment is to help you better understand process management and system calls in Linux

environment. To achieve this goal, you will use C/C++ to implement a mini shell that supports running

programs not only in the foreground but also in the background: if a program is designated to run in the

foreground, then your shell should load and run the program, and wait for the program to terminate before

available for the next command line; in contrast, the shell does not wait for the program to terminate before

awaiting the next command line if the program runs in the background.

2 Requirements

2.1 Prompt for User Input

Your mini shell should display the prompt for user input. For example, if running your mini shell in the Linux

terminal,

$ . / s h e l l

it should prompt

sh >

indicating being ready for user input.

2.2 Foreground Execution of a Program

Your mini shell should allow a program to run in the foreground, which has the following form:

sh >fg [ path o f e x e c u t a bl e f i l e ] [ a l i s t o f arguments ]

For example, if you want to run the test program mentioned in Section 4 in the foreground, you can type:

sh >fg . / test h e l l o 2 5

where ./test is the path of the test program, and hello 2 5 are the arguments. When running a process in

the foreground, the shell is unable to accept the next command from the user until the foreground process

terminates. At any time, at most ONE process can run in the foreground.

2.3 Background Execution of Programs

The shell should also allow programs to run in the background:

sh >bg [ path o f e x e c u t a bl e f i l e ] [ a l i s t o f arguments ]

Unlike the fg command, the shell is able to accept the next command immediately after executing the bg

command. Below is an example:

sh >bg . / test h e l l o 2 5

sh >

Also note that the shell should allow ANY number of processes to exist in the background simultaneously.

1

2.4 Completion of Programs

When a foreground process or a background process completes and exits normally, your shell should display a

message. For example:

sh >fg . / test h e l l o 2 5

h e l l o

h e l l o

h e l l o

h e l l o

h e l l o

P ro c e s s 4096 completed

sh >

2.5 Ctrl-C and Ctrl-Z

When your mini shell program is running in the Linux terminal, it will receive a SIGINT (SIGTSTP) signal if

typing Ctrl-C (Ctrl-Z) on the keyboard. Your shell program is expected to handle the signal and terminate

(stop) the foreground process. If there is no foreground process running in the shell, then the signal should

have no effect.

For each process that is terminated (stopped) by Ctrl-C (Ctrl-Z), the shell should display the corresponding

message. For example, if a process with PID = 4096 is terminated by Ctrl-C, then the shell is expected to

display the following message:

sh >P ro c e s s 4096 t e rmina t e d

Similarly, if a process with PID = 4096 is stopped by Ctrl-Z, then the shell is expected to display the following

information:

sh >P ro c e s s 4096 s topped

2.6 List All Processes

The list command should list the information of all processes that are running in the background as well as all

the stopped processes. The information should include PID, state, path, and arguments. Note that terminated

processes are NOT supposed to appear in the list. Therefore, you need to reap the process if it completes and

exits normally, or it is terminated by Ctrl-C. Otherwise, the list command may behave unexpectedly. Here is

an example with correct behavior:

sh >bg . / test t e s t 1 2 5

sh >bg . / test t e s t 2 5 10

sh > l i s t

4096: running . / test t e s t 1 2 5

4097: running . / test t e s t 2 5 10

sh >fg . / test t e s t 3 2 5

P ro c e s s 4098 te rmi na t e d

sh >fg . / test t e s t 4 5 10

P ro c e s s 4099 s topped

sh > l i s t

4096: running . / test t e s t 1 2 5

4097: running . / test t e s t 2 5 10

4099: s topped . / test t e s t 4 5 10

sh >

2.7 Exit the Shell

The exit command should cause the shell program to exit. But before that, make sure that all processes have

been terminated and reaped. Here is an example where there are two processes in the background before

executing exit:

2

sh >ex it

P ro c e s s 4096 te rmi na t e d

P ro c e s s 4097 te rmi na t e d

$

2.8 Handling Unexpected Inputs

You can never make any assumption about what users will input in your shell. For example, a user may input

an empty line, an invalid command, or a wrong path of the executable file. You are encouraged to come up

with these unexpected inputs as many as possible and handle them properly.

3 Hints

? Study the man pages of the system calls used in your program. For example, the following command

displays the man pages of kill():

$ man 2 k i l l

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

foreground/background processes.

? If a process is going to run in the foreground, use sigsuspend() or sleep() in a busy loop until the

foreground process terminates.

? Some signals are useful in this assignment. For example, typing Ctrl-C will trigger a SIGINT signal

automatically, and typing Ctrl-Z will trigger a SIGTSTP signal automatically. Besides, sending a

SIGTERM signal can terminate a process. Another signal that you might be interested in is the

SIGCHLD sigal, which will be sent to the parent process by the kernel whenever a child process stops or

terminates. Your program needs to handle these signals correctly. To handle signals, use signal() to

bind a signal to the corresponding handler that you implement. By the way, kill() is a useful system call

for sending a signal (don’t get confused by its name).

? When a process terminates for any reason, the process becomes a “zombie” until it is reaped by its

parent. To reap a terminated child process (if any), use waitpid(-1, &status, WNOHANG |

WUNTRACED). This system call will return immediately, with a return value 0 if none of the children

has stopped or terminated, or with a return value equal to the PID of one of the stopped or terminated

children.When the parent reaps the terminated child, the kernel passes the child’s exit status to the

parent and then discards the terminated process, at which point it ceases to exist. Also note that the exit

status may be useful when handling the SIGCHLD signal.

? When you run your shell from the standard Linux shell, your shell is running in the foreground process

group of the standard Linux shell. If your shell then creates a child process, by default that child will

also be a member of the foreground process group. Since typing Ctrl-C sends a SIGINT to every process

in the foreground group, typing Ctrl-C will send a SIGINT to your shell, as well as to every process

that your shell created. This is obviously not the desired behavior, since we hope only our shell can

receive this signal and handle the signal properly. Here is the workaround: After the fork(), but before

the execvp(), the child process should call setpgid(0, 0), which puts the child in a new process group

whose group ID is identical to the child’s PID. This ensures that there will be only one process, your

shell, in the foreground process group of the standard Linux shell. When you type Ctrl-C, your own shell

should catch the resulting SIGINT and handle it properly.

? 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.

3

4 Helper Programs

4.1 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++ a rg s . cpp ?l r e a d l i n e ?o a rg s

4.2 test.cpp

This program can be used to test your shell. It takes three arguments: the first argument is a single word to be

displayed repeatedly; the second argument is the number of seconds between two consecutive displays of the

word; the last argument 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:

$ . / test running 2 5

5 Marking

Important Note: 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.

? fg: 20%

? bg: 20%

? Ctrl-C: 10%

? Ctrl-Z: 10%

? list: 10%

? exit: 10%

? Reaping terminated processes correctly: 10%

? Handling unexpected inputs: 5%

? Programming style and in-program comments: 5%

6 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, 18-FEB-2021 (Thursday). No late submission will be accepted.

4

7 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 Mr ZHOU Zikang 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 Mr ZHOU Zikang at zikanzhou2-c@my.cityu.edu.hk.

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