Utilities Unleashed
Content
Overview
WARNING!
format.c and .h
time
env
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
Overview
WARNING!
format.c and .h
Entire Assignment due 2021-09-15 23�59
Graded files:
env.c
time.c
In this lab, you will be implementing the following C utilities:
time
env
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.
If you fork bomb on any autograder run, you will receive a zero on this
assignment.
To prevent you from fork bombing your own VM, we recommend looking into
ulimit
(https://ss64.com/bash/ulimit.html). This will allow you to set a limit for how many times you can fork.
https://ss64.com/bash/ulimit.html
time
Since this lab requires your programs to print messages to
stdout
(https://linux.die.net/man/3/st and
stderr
(https://linux.die.net/man/3/stderr), we have provided you with format.c and format.h . You should not
be printing out to stdout and stderr at all. Instead, you should be using the
provided functions. You can find documentation for each function in format.h .
Please read the documentation in format.h multiple times to determine when
each function should be used. 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 .
It is common for students to fail certain test cases on this assignment with
seemingly functional code, it is almost always because of improper usage of
format.h .
In this lab, you will be implementing
time
(https://linux.die.net/man/3/time).
time – run a program and report how long it took
So if a user enters:
./time sleep 2
then time will run
sleep
(https://linux.die.net/man/3/sleep) with the argument 2 and print how long it took in
seconds:
sleep 2 took 2.002345 seconds
For more examples, you can play with Linux s̓ builtin
time
(https://linux.die.net/man/3/time) command by typing
time YOURCOMMAND ( time ls -l , for example) in your terminal. Be sure to
add ./ to the beginning (or use the full path to your
time
(https://linux.die.net/man/3/time) executable file if
you are in another directory), otherwise the builtin
time
(https://linux.die.net/man/3/time) will be called.
Weʼve also provided a test executable to run basic tests on your time
implementation. Note that although these tests are similar to those that will be
run on the autograder they are not identical, so passing locally does not
guarantee you will receive full credit. It is still your responsibility to ensure you
have functional code.
Note that we only care about
wall-clock time
(https://en.wikipedia.org/wiki/Wall-
clock_time), and we recommend using
clock_gettime
(http://linux.die.net/man/3/clock_gettime) with CLOCK_MONOTONIC .
Pro tip: 1 second == 1,000,000,000 nanoseconds.
Nota bene:
You may not use the existing
time
(https://linux.die.net/man/3/time) program.
https://linux.die.net/man/3/stdout
https://linux.die.net/man/3/stderr
https://linux.die.net/man/3/time
https://linux.die.net/man/3/sleep
https://linux.die.net/man/3/time
https://linux.die.net/man/3/time
https://linux.die.net/man/3/time
https://en.wikipedia.org/wiki/Wall-clock_time
http://linux.die.net/man/3/clock_gettime
https://linux.die.net/man/3/time
env
You must use
fork
(https://linux.die.net/man/3/fork),
exec
(https://linux.die.net/man/3/exec), and
wait
(https://linux.die.net/man/3/wait) (no other solutions will be
accepted).
If the child process does not terminate successfully (where its exit status
is non-zero), you should exit with status 1 without printing the time.
We will only run
time
(https://linux.die.net/man/3/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
Program arguments: argc & argv
(http://cs-
education.github.io/sys/#chapter/2/section/0/activity/0)
fork, exec, wait
(http://cs241.cs.illinois.edu/coursebook/Processes#the-
fork-
exec-
wait-
pattern)
fork and waitpid
(http://cs-
education.github.io/sys/#chapter/5/section/1/activity/0)
In this lab, you will be implementing a special version of
env
(https://linux.die.net/man/3/env).
env – run a program in modified environments
Usage:
./env [key=val1] [key2=val1] … — cmd [args] ..
Please re-read this section multiple times before starting:
Each variable is in the form of NAME=v1 , separated by spaces.
Values may contain references to environment variables in the form
%NAME , including variables that were set earlier. As a result, variables
should be processed from left to right.
Each reference should be replaced with its value.
The names of variables (both in
key
(https://linux.die.net/man/3/key) and in value ) only contain letters,
numbers, or underscore characters.
For each environment variable
key
(https://linux.die.net/man/3/key)/ value pair,
env
(https://linux.die.net/man/3/env) will assign value to
key
(https://linux.die.net/man/3/key) in the child environment.
Each execution must be done with
fork
(https://linux.die.net/man/3/fork), exec, and
wait
(https://linux.die.net/man/3/wai.
The last variable/value(s) pairing is followed by a — .
Everything following the — is the command and any arguments that will
be executed by env.
https://linux.die.net/man/3/fork
https://linux.die.net/man/3/exec
https://linux.die.net/man/3/wait
https://linux.die.net/man/3/time
http://cs-education.github.io/sys/#chapter/2/section/0/activity/0
http://cs241.cs.illinois.edu/coursebook/Processes#the-fork-exec-wait-pattern
http://cs-education.github.io/sys/#chapter/5/section/1/activity/0
https://linux.die.net/man/3/env
https://linux.die.net/man/3/key
https://linux.die.net/man/3/key
https://linux.die.net/man/3/env
https://linux.die.net/man/3/key
https://linux.die.net/man/3/fork
https://linux.die.net/man/3/wait
Invalid input should result in the usage being printed. Invalid usage
includes:
Cannot find — in arguments
Cannot find = in an variable argument
Cannot find cmd after —
This is the canonical example and a practical use case:
$ ./env TZ=EST5EDT — date
Sat Sep 9 19:19:42 EDT 2017
$
Example of using references to other variables:
$ ./env TEMP=EST5EDT TZ=%TEMP — date
Sat Sep 9 19:19:42 EDT 2017
$
This has the exact same behavior as before, because TEMP is first set to
EST5EDT , and then when TZ is set to %TEMP , the value of EST5EDT is
retrieved and then TZ is set to that. Notice that the variables are set
sequentially, or else it wouldnʼt work.
We have provided you with a reference executable env-reference for you to
test your understanding of
env
(https://linux.die.net/man/3/env)s̓ expected behavior. You can also use it to see
if your
env
(https://linux.die.net/man/3/env)s̓ output matches the expected output.
Again like
time
(https://linux.die.net/man/3/time), you can play with Linux s̓ builtin
env
(https://linux.die.net/man/3/env) command by typing env
your terminal. Again, remember to add ./ to the beginning (or the full path to
your
env
(https://linux.die.net/man/3/env) executable file if you are in another directory), otherwise the builtin
env
(https://linux.die.net/man/3/env) will be called. Do not use the built-in env, or you will immediately fail
the assignment
In addition, keep in mind that the builtin
env
(https://linux.die.net/man/3/env) uses $ instead of % to denote
environment variables. In practice, it can be very useful to change some
environment variables when running certain commands.
Extra: Why Env?
For example, you may notice people write #!/usr/bin/env python on the first
line of their Python script. This line ensures the Python interpreter used is the
first one on user s̓ environment $PATH . However, users may want to use
another version of Python, and it may not be the first one on $PATH . Say, your
desired location is /usr/local/bin for instance.
One way to solve this is by exporting $PATH to the correct position in your
terminal, however, this may mess up other commands or executable under the
same session.
An alternative and better way is to use our
env
(https://linux.die.net/man/3/env), and enter:
./env PATH=/usr/local/bin — ./XXX.py
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
https://linux.die.net/man/3/time
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
then it runs the script with the desired Python interpreter.
Nota bene
You may not use the existing
env
(https://linux.die.net/man/3/env) program. (Our specification is different
than the existing
env
(https://linux.die.net/man/3/env) program.)
You may not replace % with $ or use wordexp(3) .
You may not use
execvpe
(https://linux.die.net/man/3/execvpe),
execve
(https://linux.die.net/man/3/execve), or
execle
(https://linux.die.net/man/3/execle).
All changes in environment variables and execution must happen only in
the child process.
You must use
fork
(https://linux.die.net/man/3/fork)/
exec
(https://linux.die.net/man/3/exec)/
wait
(https://linux.die.net/man/3/wait).
If a variable doesnʼt exist, interpret its value as a zero-length string.
Do not fork bomb the autograder! You will fail if you forkbomb the AG.
(See the warning.)
Useful Resources
Environment variables
(http://cs-
education.github.io/sys/#chapter/2/section/1/activity/0)
Environment variable functions
(http://www.gnu.org/software/libc/manual/html_node/Environment-
Variables.html)
string.h
(http://man7.org/linux/man-
pages/man3/string.3.html)
Split a string by a delimiter
(https://www.quora.com/How-
do-
you-
write-
a-
C-
program-
to-
split-
a-
string-
by-
a-
delimiter)
https://linux.die.net/man/3/env
https://linux.die.net/man/3/env
https://linux.die.net/man/3/execvpe
https://linux.die.net/man/3/execve
https://linux.die.net/man/3/execle
https://linux.die.net/man/3/fork
https://linux.die.net/man/3/exec
https://linux.die.net/man/3/wait
http://cs-education.github.io/sys/#chapter/2/section/1/activity/0
http://www.gnu.org/software/libc/manual/html_node/Environment-Variables.html
http://man7.org/linux/man-pages/man3/string.3.html
https://www.quora.com/How-do-you-write-a-C-program-to-split-a-string-by-a-delimiter