As discussed in lecture, socket programming is the standard way to write programs that communicate over a network. While originally developed for Unix computers programmed in C, the socket abstraction is general and not tied to any specific operating system or programming language. This allows programmers to use socket to write correct network programs in many contexts.

This part of the assignment will give you experience with basic socket programming. You will write a pair of TCP client and server programs for sending and receiving text messages over the Internet.

The client and server programs in both languages should meet the following specifications. Be sure to read these meticulously before and after programming to make sure your implementation fulfills them:

• Each server program should listen on a socket, wait for a client to connect, receive a message from the client, print the message to stdout, and then wait for the next client indefinitely.
• Each server should take one command-line argument: the port number to listen on for client connections.
• Each server should accept and process client communications in an infinite loop, allowing multiple clients to send messages to the same server. The server should only exit in response to an external signal (e.g. SIGINT from pressing ctrl-c).
• Each server should maintain a short (10) client queue and handle multiple client connection attempts currently via multithreaded programming.

• Each client program should contact a server, read a message from stdin, send the message, and exit.
• Each client should read and send the message exactly as it appears in stdin until reaching an EOF (end-of-file).
• Each client should take two command-line arguments: the IP address of the server and the port number of the server.
• Each client must be able to handle arbitrarily large messages by iteratively reading and sending chunks of the message, rather than reading the whole message into memory first.
• Each client should handle partial sends (when a socket only transmits part of the data given in the last send call) by attempting to re-send the rest of the data until it has all been sent.

The documentation for Python socket programming is located here: https://docs.python.org/3/library/socket.html. The first few paragraphs at the top, the section on socket objects and the first example is particularly relevant. The documentation for Python multithreaded programming is located here: what is a thread, second example on how to use it.

The files client-python.py and server-python.py contain the scaffolding code. You will need to add socket and multithreaded programming code. The Python socket functions will automatically raise Exceptions with helpful error messages. No additional error handling is required.

The server should be run as python3 -u server-python.py [port] > output.txt (DON'T forget the -u argument to force the stdout and stderr streams to be unbuffered). The client should be run as python3 client-python.py [server IP] [server port] < input.txt.

You should test your implementations by attempting to send messages from your clients to your servers. The server can be run in the background (append a & to the command) or in a separate SSH window. You should use 127.0.0.1 as the server IP and a high server port number between 10000 and 60000. If you get the error OSError: [Errno 98] Address already in use, you can run command pkill -9 python3 to kill the process that occupies the port. And, if you get a permissions error, run chmod a+wrx server-python.py client-python.py to give the script execute privileges.

You should compare the two files using diff in.txt out.txt. If nothing returns that means your output file matches your input file. In addition, you can use the provided Robots_of_the_World!_Arise!.txt to test your program.

Submit the assignment by uploading your modified client and server files to Canvas.

We will grade your assignments by running it on the CS lab machine. Double check the specifications above and perform your own tests before submitting.

Code that does not compile is graded harshly; if you want partial credits, make sure your file compiles!