Make One Echo Server Work with Both TCP and Unix Sockets

In the Write a TCP Echo Server From Scratch challenge, you built a server that accepts a connection, reads whatever bytes the client sends, and writes those exact same bytes back. That server listened on a TCP socket - an AF_INET address like 127.0.0.1:4000.

But SOCK_STREAM is not exclusive to TCP. The very same listen()/accept()/recv()/send() workflow also works with a Unix domain socket (AF_UNIX) in the stream mode (SOCK_STREAM) - a local socket addressed by a filesystem path and reachable only from the same host.

Berkeley Sockets API Cheat Sheet.

In this challenge, you'll make one echo server speak over both kinds of connection-oriented stream sockets, chosen at startup by a single command-line argument.

Your task is to write an echo server that takes the listen address as its only command-line argument and:

If the argument looks like unix:///path/to/file, the server creates a Unix domain stream socket and listens on that path.
If the argument looks like <host>:<port>, the server creates an IPv4 TCP socket and listens on that host and port.
Echoes back every byte it receives from a client, exactly as in the original challenge.

To pass the verification, launch your server twice - two instances of the very same program:

./server 127.0.0.1:4000
./server unix:///tmp/echo.sock

The checker connects to both endpoints and verifies that each one echoes correctly.

You're free to use any language to implement the server. The playground already has gcc (for C/C++), Python, and Go installed. Prefer a different stack? You will need to install it yourself.

Hint 1: One program, two address families

The accept loop, the read-until-EOF logic, and the echo-back logic are identical for both stream sockets. The main difference is how you create and bind the listening socket:

For <host>:<port>, create an (AF_INET, SOCK_STREAM) socket and bind it to that host and port - exactly what you did in the original echo server challenge.
For unix:///path, create an (AF_UNIX, SOCK_STREAM) socket and bind it to the filesystem path (the part after unix://).

Parse the single argument first, decide which kind of listening socket to make, and then fall through into the same connection-handling loop.

Hint 2: Unbinding a stale Unix socket

A pathname-based Unix domain socket leaves a socket file behind at its bind path. If that file already exists when you bind(), the call fails with EADDRINUSE ("Address already in use"). Remove a leftover socket file (e.g. with unlink()/os.unlink/os.Remove) before binding. Many languages' higher-level helpers won't do this for you, so it's a common first issue.

If you're reusing the address-creation steps from the How Servers Work: A Hands-On Introduction to TCP Sockets tutorial, note that everything after bind() can stay the same.