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NIX-chat

A minimal two-client chat app built with UNIX domain sockets in C. Written as a systems programming exercise covering sockets, select(), and signal handling.

How it works

A server process binds a UNIX domain socket to /tmp/chat.sock and waits for exactly two clients to connect. Once both are connected, the server relays messages between them. Each client uses select() to simultaneously watch stdin and the socket, so it can send and receive without blocking.

client A  <---->  server  <---->  client B

Project structure

.
├── server.c    # Binds, listens, accepts two clients, relays messages
├── client.c    # Connects to server, sends/receives messages
└── Makefile

Building

make

Running

Open three terminals:

# Terminal 1 — start the server first
./server

# Terminal 2 — first client
./client

# Terminal 3 — second client
./client

Once both clients connect, type in either terminal and the message will appear in the other.

Example session

Terminal 1 — server

$ ./server
Server listening on /tmp/chat.sock
Client 1 connected.
Client 2 connected.
Both clients connected, starting relay.
Client 1 disconnected.

Terminal 2 — client A

$ ./client
Connected to server!
hello from A
Them: hey, this is B
how's it going?
Them: pretty good!
^C
Disconnecting...

Terminal 3 — client B

$ ./client
Connected to server!
Them: hello from A
hey, this is B
Them: how's it going?
pretty good!
Server disconnected.

Stopping

Ctrl+C in any terminal shuts down gracefully. The server handles SIGINT, SIGTERM, and SIGPIPE — closing both client connections and unlinking the socket file on exit.

Key concepts

• UNIX domain sockets — local-only sockets identified by a filesystem path instead of an IP/port
• SOCK_STREAM — reliable, ordered byte stream (analogous to TCP)
• select() — multiplexes multiple file descriptors so a single thread can watch stdin and a socket simultaneously
• SIGPIPE — suppressed so the server/client don't crash if the other end disconnects mid-send

Limitations

• Exactly two clients — no more, no less
• Messages are plain text, newline-terminated
• No usernames or timestamps
• Sequential accept() — client A must connect before client B

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Made by Alejandro Alvarado