File Sharing Feed

A TCP client-server file sharing system built for CSC209 (Software Tools and Systems Programming) at the University of Toronto St. George, Winter 2026.

Overview

Multiple clients connect to a central server to list, upload, and download files. When a client uploads a file, all other connected clients receive a real-time broadcast notification. The server runs a single-threaded select() event loop — no threads, no fork() per client.

Build & Run

make                  # build both server and client (default port 4242)
make PORT=9000        # build with a custom default port
make clean            # remove build artifacts

Start the server

./server <port>

Example:

./server 4242

The server creates a ./storage/ directory if it doesn't already exist, then begins accepting connections.

Connect a client

./client <server_ip> <port>

• Same machine: use localhost or 127.0.0.1
• LAN: use the server machine's IP (hostname -I on Linux, ifconfig on macOS)
• teach.cs: use the hostname directly (e.g. teach.cs.toronto.edu)

./client 127.0.0.1 4242
./client teach.cs.toronto.edu 4242

Client commands

fileshare> list                   # list files on the server
fileshare> upload path/to/file    # upload a local file
fileshare> download filename      # download a file to the current directory
fileshare> help                   # show available commands
fileshare> quit                   # disconnect and exit

Testing with multiple clients

# Terminal 1
./server 4242

# Terminal 2
./client 127.0.0.1 4242

# Terminal 3
./client 127.0.0.1 4242

When one client uploads a file, all others receive:

[notification] New file available: example.txt

Architecture

File	Role
protocol.h	Shared message types, constants, wire-format header
network.c/h	Socket setup, send_all/recv_all, send_message
file_ops.c/h	Storage directory init, file listing, filename validation
client_state.c/h	Per-client state machine, read/write buffers
server.c	Server entry point, select() loop, upload/download/broadcast logic
client.c	Client entry point, select() on stdin + socket, command parsing

Wire Protocol

Messages use a 5-byte length-prefixed header over TCP:

[type : uint8_t (1 byte)][payload_length : uint32_t (4 bytes, network order)][payload]

Message	Direction	Description
MSG_REQ_LIST	Client → Server	Request file list
MSG_RES_LIST	Server → Client	File list response
MSG_REQ_UPLOAD	Client → Server	Begin upload (filename + file size)
MSG_RES_UPLOAD_OK	Server → Client	Upload accepted
MSG_REQ_DOWNLOAD	Client → Server	Request a file download
MSG_RES_DOWNLOAD	Server → Client	Download starting (file size)
MSG_NEW_FILE	Server → Client	Broadcast: new file available
MSG_DATA_CHUNK	Either	Raw file data chunk (≤ 4096 bytes)
MSG_DONE	Either	End of file transfer
MSG_ERR	Server → Client	Error message

Known Limitations

• File list is truncated to 4095 bytes if the server has many files.
• Files are limited to ~4 GB due to uint32_t size fields in the protocol.
• Downloaded files are saved to the client's current working directory using the server-side filename.