Principles of Operating Systems

CMSC 421-04 - Spring 2016

Homework 3

Chapters 7-8

50 points total

Due by 11:59PM EDT on Thursday, April 14, 2016

Changelog

Sunday, April 3 @ 16:13 EDT — clarified that scripts are non-interactive, and that anything (logging, .421sh_profile) pertaining only to interactive mode does not apply when running a script from the command line.

Part I

20 points

Please provide written responses to the following questions. Each response should be about 5-7 sentences in length. Each question is worth 5 points.

1. There are four conditions necessary for a deadlock to occur, name and describe each.
2. ^[*] Consider the following snapshot of a system:

       Allocation      Max       Available
        A B C D      A B C D      A B C D
   P0   0 0 1 2      0 0 1 2      1 5 2 0
   P1   1 0 0 0      1 7 5 0
   P2   1 3 5 4      2 3 5 6
   P3   0 6 3 2      0 6 5 2
   P4   0 0 1 4      0 6 5 6

   • Is the system as described in a safe state? If so, give an order that the processes could complete successfully. If not, describe why the system is not in a safe state.
   • If a request from P1 for (0, 4, 2, 0) comes in, can it be granted immediately? Show why or why not.
3. In the context of memory management, describe what an ASID is and what it is used for.
4. Describe what a Translation Lookaside Buffer is. What is it's use in memory management? Why is it advantageous to use a TLB versus pure paging? Can a TLB be used with multilevel page tables? Why or why not?

You must submit a PDF file of the answers to these questions on the Homework 3 assignment on Blackboard. You may prepare your responses in any word processing application of your choosing, but you must submit a PDF file. Do not submit Microsoft Word/Apple Pages/LibreOffice/OpenOffice.org documents with your responses. You must convert to a PDF file before submitting or you will lose points.

[*] - Adapted from OSC 9e.

Part II

30 points

Complete the following programming assigment and submit your code using the GitHub repository that you will have created for this assignment. This assignment must be completed in the C programming language (you can choose to use C89/C90/ANSI-C, C99, or C11 as you see fit).

In this assignment, you will be augmenting your shell from Homeworks 1 and 2 with a few new features:

• Program path names may be provided that contain special characters, such as spaces, provided that they are properly escaped or enclosed in quote characters. The unescape() function should be helpful here.
• Arguments to programs should be unescaped. This will require a bit of a change to your parsing algorithm in all likelihood. A hint is provided below as to how you might accomplish this.
• Comments must be supported in your shell. Any line that begins with a # character shall be treated as a comment and the entire input line shall be ignored.
• Support for rudimentary scripts shall be provided. If the shell program is started with an argument, the argument shall be a text file that will contain commands to be executed. Each command shall be parsed in the same manner as if it were given in an interactive shell session. If the argument provided does not specify a filename, or specifies a file that cannot be read, then the shell shall immediately exit, printing an error stating why it is exiting (File not found, or similar). Script processing is considered to be a non-interactive mode of the shell — once the script is completed, the shell should exit. In addition, anything specified below for use in "interactive" shell sessions should not be done when processing a script. No support for running scripts from an interactive session need be provided.
• All commands entered in an interactive shell session (i.e, not from a script) should be logged to a file in the user's home directory called .421sh_history. This shall be done regardless of whether the command specified was executed succesfully. Each line of this file shall contain one command, and new commands shall be appended to the end of the file. The file must not be erased or cleared when the shell starts up (i.e, it should maintain a complete history of all sessions, space permitting).
• The shell shall look for a file in the user's home directory called .421sh_profile on startup of any interactive shell sessions. If this file exists, it shall be run as any other script would be run before giving the user a prompt to use in interactive mode.

As with homework 2, you are welcome to use the utility files that were provided in that assignment (here and the header file here). These files are unchanged from homework 2, so if you still have those versions in your repository, you should continue to use them.

The first_unquoted_space() function in the utils.c file can be used to aid in parsing commands without using functions like strtok_r() (which won't work for parsing because of requirements 1 and 2 of this assignment). Remember that C strings are just arrays, and you can easily overwrite characters at will. If you want to end a string at a given position, overwrite that character with a \0 character.

Remember, the first_unquoted_space() function returns -1 if there are no spaces left in the input string that are unquoted.

Your shell program is not allowed to use any external libraries other than the system's C library and the files provided in this assignment. Do not try to use libraries like Readline. You will lose points for using external libraries to implement shell functionality!

When submitting your shell program, please be sure to include the source code of the shell program (in one or more C source code files), as well as a Makefile that can be used to build the shell. Your shell must be able to be built and run on a VM as has been set up for this course in your projects. If you use my utility functions, make sure to include those files as well.

As this should be an extension of your earlier homework 1/2 shell, you should be using the same directory and git repository for it. To submit your code, you should do the following:

git add your_updated_and_added_files_go_here
git commit
git push -u origin master

git tag hw3
git push origin --tags

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