CS 241: System Programming
CS 241

Utilities Unleashed Edit on Github

Learning Objectives

The learning objectives for Utilities Unleashed are:

  • Fork, Exec, Wait
  • Environment Variables
  • Writing a C Program
  • Using argv, argc
  • Introduction to core utils

Suggested Readings

We suggest you read the following from the wikibook before starting Utilities Unleashed:

Overview

In this lab, you will be implementing the following C utilities:

Notes:

  • Do not worry about flags or features that we do not mention.
  • Do not print any of your debug information out for your final submission.
  • All printing (except env vars) should be handled with format.h.
  • A common issue is double printouts. If this happens to you, try flushing stdout before you fork/exec. If this solves your issue, ask yourself why.

format.c and .h

Since this lab requires your programs to print messages to stdout and stderr, we have provided you with format.c and format.h. You should not be printing out to stdout and stderr at all, expect for the print feature of env; instead, you should be using the provided functions. You can find documentation for each function in format.h. This is our way of ensuring that you do not lose points for formatting issues, but it also means that you are responsible for handling any errors mentioned in format.c and format.h.

There is no provided format function for printing the envrionment variables. To do so, you should print each one on its own line (e.g. with printf("%s\n")).

Reference Executables

For this lab, we have provided the following reference executables:

  • time_reference
  • env_reference

These are correct implementations of the programs you are being asked to write. You can use these to determine how your programs should behave in edge cases. For example, if you are wondering what time should do if not given the correct number of arguments, then just try it out:

./time_reference

and you should see the following output:

./time <command> [args]

Setup

cd ~/cs241
svn up
cd utilities_unleashed

time

In this lab, you will be implementing time.

time – run a program and report how long it took

So if a user enters:

./time sleep 2

then time will run sleep with the argument 2 and record how long it took in seconds:

2.002345 seconds

Note that we only care about wall-clock time, and we recommend using clock_gettime with CLOCK_MONOTONIC.

Pro tip: 1 second == 1,000,000,000 nanoseconds.

Nota bene:

  • You may not use on the existing time program.
  • You must use fork, exec, and wait (no other solutions will be accepted).
  • If the program does not terminate successfully (where its exit status is non-zero), you should exit with status 1 and not print the time.
  • We will only run time with one program.
  • The commands we will run can take any number of arguments.
  • Do your time computations with double-precision floating pointer numbers (double) rather that single-precision (float).
  • We have provided functions in format.h that we expect you to use wherever appropriate.

Useful Resources

env

In this lab, you will be implementing env.

env – run a program in a modified environment

When run without arguments, it prints a list of all the current environment variables.

When run with arguments, it will be given at least two, and will be called like so:

./env <var-list> <command-name>
  • <var-list> is a comma-separated list of changes that are to be made to environment variables, and <command-name> is the name of a command that is to be run after making those changes.
  • Any additional arguments are to be passed as arguments to <command-name>.
  • Each of the environment variable changes in <var-list> is of the form <destvar>=<value>.
  • <destvar> is the name of the environment variable that is to be changed and <value> is the new value.
  • <value> may contain references to environment variables in the form %<srcvar>.
  • Each reference to <srcvar> should be replaced with the value of <srcvar>.
  • The names of the variables <destvar> and <srcvar> will contain only letters, numbers, or underscore characters.
  • For each environment variable change in <var-list>, your program will assign <value> to <destvar> in the current environment so when <command-name> is executed, it will inherit the new value of the <destvar> variable.

For example, if the user enters:

./env

then you should print out all the environment variables. Try ./env_reference in your terminal to see it in action.

If the user enters:

./env TZ=MST7MDT date

then it changes the TZ environment variable while running the date command.

And, if the user enters:

./env PATH=%HOME/bin:%PATH,IDIR=/%HOME/include,LIBDIR=/%HOME/lib make -j4

then it changes the PATH, IDIR, and LIBDIR variables while running make with the j4 option.

Nota bene:

  • You may not use the existing env program.
  • You may not replace % with $ or use wordexp(3).
  • All changes in enviroment variables and execution must happen only in the child process.
  • You must use fork/exec/wait.
  • If a variable doesn’t exist, interpret its value as a zero-length string.
  • Do not fork bomb the autograder! You will fail if you forkbomb the AG.

Useful Resources

Submission Instructions

Please fully read details on Academic Integrity . These are shared between all MPs in CS 241.

We will be using Subversion as our hand-in system this semester. Our grading system will checkout your most recent (pre-deadline) commit for grading. Therefore, to hand in your code, all you have to do is commit it to your Subversion repository.

To check out the provided code for utilities_unleashed from the class repository, go to your cs241 directory (the one you checked out for "know your tools") and run: 

svn up

If you run ls you will now see a utilities_unleashed folder, where you can find this assignment! To commit your changes (send them to us) type: 

svn ci -m "utilities_unleashed submission"

Your repository directory can be viewed from a web browser from the following URL: https://subversion.ews.illinois.edu/svn/fa16-cs241/NETID/utilities_unleashed where NETID is your University NetID. It is important to check that the files you expect to be graded are present and up to date in your SVN copy.