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Event-Driven Master-Client Relay (CLI, C)

RUS Version

A small educational networked CLI project written in pure C, demonstrating an event-driven model of client interaction through a relay server with Master-Client Authority.

The project was created as a practical exercise in:

  • network programming in C,
  • working with ENet (UDP + reliable delivery),
  • message serialization and deserialization using MsgPack,
  • multithreading and synchronization,
  • designing client ↔ server architecture without a real-time loop.

🎯 Project Idea

  • There is a relay server that contains no game logic
  • The first connected client becomes the Master
  • The Master:
    • stores the world state
    • processes input from other clients
    • broadcasts the updated state to all clients
  • Other clients (Slaves):
    • send only their input commands
    • apply the state received from the Master

The model is not real-time, but event-driven:

  • the world is updated via the tick command
  • ideal for learning, debugging, and understanding networking patterns

Key principles:

  • the server does not interpret data
  • the server only relays messages
  • all logic is concentrated in the Master client
  • a single shared message protocol for both client and server

📂 Project Structure


.
├── Makefile
├── README.md
└── src
    ├── client.c              # CLI client (Master / Slave)
    ├── server.c              # Relay server
    ├── protocol_msg.h        # Shared network protocol
    ├── help_func/            # Helper functions
    └── thread_safe/          # Thread-safe structures

🔌 Technologies Used

  • C (C17)
  • ENet — UDP networking library with reliable delivery
  • MsgPack-C — binary message serialization
  • POSIX Threads (pthreads) — multithreading
  • Linux / WSL2

The project targets Unix-like systems.

📡 Message Protocol

All messages are defined in protocol_msg.h.

Examples of message types:

  • client registration
  • Master assignment
  • client input (intention to change state)
  • world state snapshot

Serialization is performed using MsgPack, which:

  • removes the need for manual framing
  • makes the protocol extensible
  • allows using identical structures on both client and server

🧵 Threads and Synchronization

  • The ENet networking loop runs in a dedicated thread
  • CLI input is handled independently
  • Inter-thread communication is implemented via thread-safe queues
  • Race conditions when working with ENet are eliminated

▶️ Build

Dependencies

Install required libraries:

sudo apt install libenet-dev libmsgpack-dev

Build

make

This will produce:

  • server
  • client

🚀 Run

Server

./server

Clients

./client <server_ip>

You can run:

  • multiple clients on the same machine
  • clients on different machines within the same LAN / Wi-Fi network

🕹️ Controls (CLI)

Example client commands:

tick
x+
x-
y+
y-
  • tick — triggers a world update (Master only)
  • other commands send input to the Master client

📚 Purpose of This Project

This project was created not as a game, but as:

  • an educational sandbox for network programming

  • a foundation for transitioning to:

    • real-time synchronization
    • prediction / reconciliation
    • an authoritative server model

  • source material for articles and educational content