Homework 10 - Socket Chat

This homework is optional. It may replace your lowest homework grade, but it is intended to be extra; that is, you should complete Homework 8-9 first. While due on the last day of class, you may submit this optional homework through 11pm on Friday, December 9, 2022.

In this assignment, you’ll take the server code we supplied and the client code you wrote in lab 9 and combine them to make a command-line 2-party chat program.

Requirements

Write a single file called schat.c. Use only C library functions and code you wrote in it: no third-party libraries or C++.

schat.c should have a main function that accepts command-line arguments. If given no arguments, it should run in server mode; if given two arguments (the first an IP address, the second a port number) it should run in client mode, connecting to that server.

The server should begin by

1. Pick a random ephemeral-range port, like the lab server code did
2. Display that port number to the terminal, like the lab server code did
3. Listen for IPv4 connections on any IP address, like the lab server code did
4. accept only once, unlike the lab server’s accept loop
5. close the listener socket before working with the accepted socket

The client should begin by

1. connecting to the given IP address and port, like your lab client did

Both should then proceed as follows:

1. Repeatedly (i.e., in an infinite loop) use poll¹ to pick either the connected socket (from the server’s accept or the client’s connect) or the standard input stream² to read³ from. Use a 1-minute⁴ timeout for poll.
2. If poll returns a positive number (i.e., it succeeded),
   1. If the standard input has revents including POLLIN, read from standard input and write what you read to the socket.
   2. If the socket has revents including POLLIN, read from the socket and write what you read to standard output⁵. You may assume no single message is more than 4096 bytes, to avoid needing to loop your read/write calls.

Undefined behaviors

This assignment does not require you to handle the following in any particular way:

1. Giving notice of server IP address
2. Failure of any of the socket, read, or write instructions
3. Handling the closing of the remote connection
4. Stopping if the poll call times out
5. Having any way to end the program other than Ctrl+C
6. Avoiding memory leaks
7. Clean display when the remote message arrives while the user it typing
8. Displaying who sent which message

You do need to avoid buffer overflows, use-after-free, and other memory bugs. You are also invited to add sane behaviors for all of the above cases, but are not required to do so.

Example

In a simple implementation, once the client and server connect everything that is typed in one will appear in the other after you press enter.

$ 

$ 

As a reminder, you don’t need to be exactly like the above; the initial display, format of output, and behavior of the server after the client ends are all undefined by this assignment.

Tips

This assignment requires looking things up

You will notice all we said about poll was “check the manual page”. This is by design. We hope you now know enough to learn a bit on your own and use it.

It is also possible to look things up online. Except for online man-pages⁶, this is a bad idea. In general, online example code is explaining something nuanced within a specific context, and it takes some experience before you are able to tease out the part you can use from the context it is discussed within. I expect if you go to a search engine to find example code, you’ll end up with a lot of code that does not work or meet the required specification in some nuanced way.

Testing on a non-server

The CS department servers (notably including servers portal redirects you to) all have static IP Addresses meaning tools like host can learn who they are. Your laptop probably does not.

If your laptop has a C compiler and poll, socket, etc., implemented, you can still test locally but you’ll need to use the IP address 127.0.0.1 which always means “talk to myself”, running the server and client in two different terminal windows at the same time.

Use lldb

When you get a segfault, you should

1. recompile with clang -g schat.c
2. load in a debugger with lldb a.out
3. run and see the line of code that segfaulted

This will be far easier than trying to locate the segfault without a debugger.

You may also find a debugger useful for other kinds of errors.

Feedback

We do not currently have plans to offer automated feedback on your code when you submit. We’re also not planning on testing strange cases: if you can carry on a chat conversation with someone else, where either one of you can type several things in a row and it works, you should be good to go.

Submission

Submit your code to Gradescope.