📇 Visit Card - TCP Client-Server Application

A high-performance, concurrent TCP client-server system built in C for managing digital business cards.

📖 Overview

Visit Card is a robust networking application that allows multiple clients to simultaneously connect, manage, and query a centralized database of digital business cards. Built entirely in C using the POSIX/Linux API, it demonstrates core systems programming concepts including socket communication, multi-processing, and database integration.

🏗️ Architecture

The application follows a classic Client-Server architecture over TCP/IP:

• Concurrent Server: Utilizes the fork() system call to spawn a new child process for every incoming client connection. This ensures that the server can handle multiple clients concurrently without blocking.
• Database Layer: Integrates SQLite3 directly into the server to ensure fast and persistent data storage. It automatically initializes the database and seeds default data on the first run.
• Custom Protocol: Uses a text-based, application-layer protocol for communication, including a challenge-response authentication mechanism for sensitive actions.

✨ Key Features

• Multi-processing: Non-blocking concurrent client handling via process cloning (fork()).
• Persistent Storage: Full CRUD operations backed by an SQLite database.
• Advanced Search: Filter cards dynamically using keyword matching (similar to a browser's F3 search).
• Security & Authentication: Password-protected commands (new_card, edit_card, delete_card) to prevent unauthorized modifications.
• Automated Setup: The server automatically creates the necessary tables and populates initial records if the database is empty.

🛠️ Tech Stack

• Language: C / C++
• Networking: TCP/IP Sockets (POSIX)
• OS API: Linux Systems Programming (fork(), read(), write(), file descriptors)
• Database: SQLite3 C/C++ Interface

💻 Commands / Protocol

Clients interact with the server using the following commands:

• help - Displays the list of available commands.
• get_cards - Retrieves and displays all business cards.
• get_card <id> - Retrieves details for a specific card by its ID.
• search_cards <text> - Searches for cards containing the specified text (e.g., matching a name or email domain).
• new_card,<Name>,<Address>,<Email> - [Requires Password] Adds a new business card.
• edit_card,<id>,<Name>,<Address>,<Email> - [Requires Password] Updates an existing card.
• delete_card <id> - [Requires Password] Removes a card from the database.
• exit - Safely closes the client connection.

🚀 Getting Started

Prerequisites

• GCC Compiler
• SQLite3 Library (libsqlite3-dev on Linux)
• Linux/Unix Environment (or WSL on Windows)

Compilation

Compile the server and client components:

# Compile the Server (link SQLite3)
gcc server.c -o server -lsqlite3

# Compile the Client
gcc client.c -o client