Next week, we will discuss system calls involving networking. We will explore using sockets to communicate between clients and servers and eventually create our own HTTP client in the form of thor.


The focus of this reading is to explore system calls related to network sockets in C.


The readings for this week are:

  1. Beej's Guide to Network Programming

    You can skip the parts about datagram sockets and slightly advanced techniques.

  2. What is a URL?

Optional References

  1. System Programming Wiki

    This has basic coverage of networking, along with some information about remote procedure calls (RPC) that you can skip.


This week, the reading is split into two sections: the first part is a dredd quiz, while the second part involves one C program: ncat.c.

To test the C program, you will need to download the Makefile and test scripts:

$ git checkout master                 # Make sure we are in master branch
$ git pull --rebase                   # Make sure we are up-to-date with GitHub

$ git checkout -b reading12           # Create reading12 branch and check it out

$ mkdir reading12                     # Create reading12 folder

$ cd reading12                        # Go into reading12 folder

# Download Reading 12 Makefile
$ curl -LO https://raw.githubusercontent.com/nd-cse-20289-sp23/cse-20289-sp23-assignments/master/reading12/Makefile

# Download Starter code
$ curl -LO https://raw.githubusercontent.com/nd-cse-20289-sp23/cse-20289-sp23-assignments/master/reading12/ncat.c

# Download, build, and execute tests
$ make test


Record the answers to the following Reading 12 Quiz questions in your reading12 branch:


Given the provided Makefile and ncat.c, you are to do the following:

  1. Modify Makefile to include a rule for the ncat program. Be sure to use the CC and CFLAGS variables in your rule.

    Once you have a working Makefile, you should be able to use the make command to run your recipes:

    $ make clean                                # Remove targets
    rm -f ncat
    $ make                                      # Build targets
    gcc -g -Wall -Werror -std=gnu99 -o ncat ncat.c
  2. Modify ncat.c so that it uses system calls in C to implement the equivalent Python script:

    #!/usr/bin/env python3
    import socket
    import sys
    # Parse command line options
        HOST = sys.argv[1]
        PORT = int(sys.argv[2])
    except IndexError:
        print("Usage: {} HOST PORT".format(sys.argv[0]), file=sys.stderr)
    # Create socket and connect to specified HOST and PORT
        csocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        csocket.connect((HOST, PORT))
        cstream = csocket.makefile('w')
    except socket.error as e:
        print('Socket Error: {}'.format(e))
    # Read from stdin and write to socket
    for line in sys.stdin:
    # Cleanup

This is basically a partial implementation of the nc command you have used previously: given a HOST and PORT, the program creates a socket, connects to the server, and then sends data from standard input to the remote server via the socket connection.

Fill in the Blanks

The code has a few ____ placeholders. You will need to replace these placeholders with the appropriate functions and variables to complete the program.

Here is a list of some possible functions and variables you may wish to use:

Once you have a working ncat.c, you should be able to build and run it:

$ make                            # Build ncat program
gcc -g -Wall -Werror -std=gnu99 -o ncat ncat.c

$ ./ncat                          # Usage
Usage: ./ncat HOST PORT

$ nc -l 9123 &                    # Start netcat server in background
[1] 12532

$ date | ./ncat localhost 9123    # Send message to server
Connected to localhost:9123
Sun Apr 15 16:50:38 EDT 2018
[1]+  Done                    nc -l -p 9123

$ ./ncat fakehost 9999
Could not look up fakehost:9999: Name or service not known

$ ./ncat localhost 0
Unable to connect to localhost:0: Connection refused

$ make clean                      # Cleanup
rm -f ncat


To submit you work, follow the same process outlined in Reading 01:

$ git checkout master                 # Make sure we are in master branch
$ git pull --rebase                   # Make sure we are up-to-date with GitHub

$ git checkout -b reading12           # Create reading12 branch and check it out

$ mkdir reading12                     # Create reading12 folder

$ cd reading12                        # Go into reading12 folder

$ $EDITOR answers.json                # Edit your answers.json file

$ ../.scripts/check.py                # Check reading12 quiz
Checking reading12 quiz ...
      Q1 0.25
      Q2 0.75
   Score 1.00 / 1.00
  Status Success

$ git add answers.json                # Add answers.json to staging area
$ git commit -m "Reading 12: Quiz"    # Commit work

$ $EDITOR ncat.c                      # Edit source code

$ make test                           # Build and Run tests
Checking reading12 ncat ...
 ncat (syscalls)                                              ... Success
 ncat (usage, output)                                         ... Success
 ncat (usage, valgrind)                                       ... Success
 ncat (fakehost 9999, client)                                 ... Success
 ncat (fakehost 9999, valgrind)                               ... Success
 ncat (localhost 0, client)                                   ... Success
 ncat (localhost 0, valgrind)                                 ... Success
 ncat (localhost 9770, client)                                ... Success
 ncat (localhost 9770, server)                                ... Success
 ncat (localhost 9770, valgrind)                              ... Success
 ncat (weasel.h4x0r.space 9910, client)                       ... Success
 ncat (weasel.h4x0r.space 9910, server)                       ... Success
 ncat (weasel.h4x0r.space 9910, valgrind)                     ... Success

   Score 3.00 / 3.00
  Status Success

$ git add Makefile                    # Add Makefile to staging area
$ git add ncat.c                      # Add source code to staging area
$ git commit -m "Reading 12: Code"    # Commit work

$ git push -u origin reading12        # Push branch to GitHub

Pull Request

Remember to create a Pull Request and assign the appropriate TA from the Reading 12 TA List.

DO NOT MERGE your own Pull Request. The TAs use open Pull Requests to keep track of which assignments to grade. Closing them yourself will cause a delay in grading and confuse the TAs.