Reading 10: Files and Processes

Readings

The readings for April 10, 2017 are:

  1. System Programming Wiki

  2. Linux Programming Interface

TL;DR

The focus of this reading is to introduce basic system calls dealing with files and processes in C.

Optional Resources

  1. System Calls Make the World Go Round

  2. Beej's Guide to Unix IPC

  3. Operating Systems: Three Easy Pieces

Activities

In your reading10/README.md, response to the following questions:

  1. Identify which system calls you would use to perform the following actions by briefly listing example C code:

    1. Print out the error message associated with a failed system call.

    2. Truncate a file.

    3. Move to the 10th byte of a file.

    4. Check if a path is a directory.

    5. Create a copy of the current process.

    6. Replace the code in the current process with another program.

    7. Tell a process to terminate.

    8. Receive the exit status of a child process.

  2. Write the C equivalent, walk.c, to the Python code below (walk.py):

    #!/usr/bin/env python2.7

import os

for name in os.listdir('.'):
    if os.path.isfile(name):
        print name, os.path.getsize(name)

    This should output something like this:

    $ ./walk.py
walk.c 728
walk 14496
Makefile 535
README.md 23
walk.o 8272
walk.log 688
walk.py 137

    Makefile

    Be sure to include a Makefile that builds the walk.c program:

    # Build walk program
$ make
Compiling walk.o...
Linking walk...

# Run walk program
$ ./walk
walk.c 728
walk 14496
Makefile 535
README.md 23
walk.o 8272
walk.log 688
walk.py 137

# Cleanup
$ make clean
Cleaning...

    Your program must:

    1. Compile with no warnings (be sure to have -Wall in your CFLAGS).

    2. Not contain any memory errors (as reported by Valgrind).

    3. Check if system calls fail (when appropriate).

    Hints

    Consider using the following system calls:

    • opendir: To open a directory.

    • readdir: To read one directory entry at a time.

    • stat: To read a file's metadata (including file size).

    • closedir: To close a directory.

    Testing

    To test your program, you can compare the output of your walk program to the output of the walk.py script:

    # Using the bash shell
$ diff <(./walk | sort) <(./walk.py | sort)

    Feel free to add a test rule to your Makefile to automate this.

    Note: Since the above command requires bash, you must set the SHELL variable in your Makefile to bash:

    SHELL=    bash

    Here is a possible trace:

    # Trace
$ make -n test
echo Compiling walk.o...
gcc -g -gdwarf-2 -Wall -std=gnu99 -c -o walk.o walk.c
echo Linking walk...
gcc -L.  -o walk walk.o
echo Testing walk...
diff <(./walk | sort) <(./walk.py | sort) > walk.log
[ `valgrind --leak-check=full ./walk 2>&1 | tee -a walk.log | grep ERROR | awk '{print $4}'` = 0 ] && ! grep -q 'failed' walk.log || cat walk.log

Feedback

If you have any questions, comments, or concerns regarding the course, please provide your feedback at the end of your response.

Submission

To submit your assignment, please commit your work to the reading10 folder in your assignments GitLab repository. Your reading10 folder should only contain the following files: