C TCP Chat Application

Server Capabilities

1. Multi-Client Support

   • Handles up to 100 concurrent clients using select() multiplexing
   • Thread-safe client management with Client struct array
   • Efficient single-threaded architecture (no race conditions)

2. User Management

   • Automatic username assignment (User<socket_fd>)
   • Username change capability via /name command
   • Duplicate username prevention
   • User list display via /list command

3. Message Broadcasting (Publish-Subscribe)

   • Messages sent to all connected clients except sender
   • Join/leave notifications broadcast to all users
   • Username change notifications

4. Unicast (Private Messaging)

   • Direct messages via /msg <username> <message>
   • Confirmation sent to both sender and recipient
   • User validation before message delivery

5. Robust Error Handling

   • Graceful client disconnection handling
   • Network error detection and recovery
   • Server full notification
   • Invalid command feedback

Client Capabilities

1. Connection Management

   • Connect to server using IP address
   • Graceful disconnection with quit command
   • Connection status feedback

2. Dual Input Monitoring

   • Simultaneous monitoring of server messages and user input
   • Uses select() for non-blocking I/O
   • Real-time message display

3. Command Support

   • /name <username> - Change username
   • /msg <user> <message> - Send private message
   • /list - View online users
   • /help - Display command help
   • quit - Exit application

4. User Experience

   • Clear message formatting with sender identification
   • Private message indicators [PM from/to]
   • System notifications with *** markers

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Usage Instructions

Starting the Server

./chatserver

Output:

Chat server listening on port 6969
Max clients: 100

Connecting Clients

# Connect to localhost
./chatclient 127.0.0.1

# Connect to remote server
./chatclient 192.168.1.100

Example Session

Client 1:

Connected to chat server at 127.0.0.1:6969
Type /help for commands, or just type to chat!
Type 'quit' to exit

Welcome to the chat! Your username is: User4
Type /help for commands
*** User4 has joined the chat ***

/name Alice
*** User4 is now known as Alice ***
Username changed successfully

Hello everyone!
[Alice]: Hello everyone!

/msg Bob Hi there!
[PM to Bob]: Hi there!

Client 2:

Connected to chat server at 127.0.0.1:6969
Welcome to the chat! Your username is: User5

*** User4 has joined the chat ***
*** User4 is now known as Alice ***

/name Bob
Username changed successfully

[Alice]: Hello everyone!

[PM from Alice]: Hi there!

/list

=== Online Users ===
[Alice]
[Bob]  (you)
====================

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Technical Architecture

Server Architecture

┌─────────────────────────────────────┐
│       Main Server Loop              │
│  (select() multiplexing)            │
├─────────────────────────────────────┤
│                                     │
│  ┌──────────────┐  ┌─────────────┐ │
│  │  Listen FD   │  │ Client FDs  │ │
│  │  (new conn)  │  │  (messages) │ │
│  └──────────────┘  └─────────────┘ │
│         │                  │        │
│         ▼                  ▼        │
│  ┌──────────────┐  ┌─────────────┐ │
│  │ accept() new │  │   recv()    │ │
│  │   clients    │  │   messages  │ │
│  └──────────────┘  └─────────────┘ │
│         │                  │        │
│         ▼                  ▼        │
│  ┌──────────────────────────────┐  │
│  │   Client Management System   │  │
│  │  - add_client()              │  │
│  │  - remove_client()           │  │
│  │  - broadcast_message()       │  │
│  │  - handle_command()          │  │
│  └──────────────────────────────┘  │
└─────────────────────────────────────┘

Client Architecture

┌─────────────────────────────────────┐
│       Main Client Loop              │
│  (select() multiplexing)            │
├─────────────────────────────────────┤
│                                     │
│  ┌──────────────┐  ┌─────────────┐ │
│  │  Server FD   │  │  STDIN FD   │ │
│  │ (messages)   │  │ (user input)│ │
│  └──────────────┘  └─────────────┘ │
│         │                  │        │
│         ▼                  ▼        │
│  ┌──────────────┐  ┌─────────────┐ │
│  │   recv()     │  │  fgets()    │ │
│  │   from       │  │  from user  │ │
│  │   server     │  │             │ │
│  └──────────────┘  └─────────────┘ │
│         │                  │        │
│         ▼                  ▼        │
│  ┌──────────────┐  ┌─────────────┐ │
│  │   Display    │  │   send()    │ │
│  │   to user    │  │  to server  │ │
│  └──────────────┘  └─────────────┘ │
└─────────────────────────────────────┘

Key Design Decisions

1. select() vs threads:

   • Single-threaded with select() for I/O multiplexing
   • More efficient than thread-per-client model
   • No need for mutex locks or thread synchronization
   • Lower memory overhead

2. Client Management:

   • Fixed-size array for O(1) access
   • Active flag for efficient iteration
   • Username stored per client for fast lookups

3. Message Protocol:

   • Newline-terminated messages
   • Command prefix: /
   • Private message format: /msg <username> <message>
   • System notifications: *** message ***

4. Error Handling:

   • Graceful degradation on client disconnect
   • Network error logging without server crash
   • Input validation for all commands

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Command Reference

Command	Description	Example
/name <username>	Change your username	/name Alice
/msg <user> <msg>	Send private message	/msg Bob Hello!
/list	List all online users	/list
/help	Show command help	/help
quit	Exit the chat	quit
<message>	Broadcast to all users	Hello everyone!

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Performance Characteristics

• Concurrency Model: Single-threaded with I/O multiplexing
• Maximum Clients: 100 (configurable via MAX_CLIENTS)
• Buffer Size: 4096 bytes per message
• Latency: Near-instant message delivery (LAN)
• Memory Usage: ~50KB per client (minimal overhead)
• CPU Usage: Idle when no activity (event-driven)

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Limitations and Future Enhancements

Current Limitations

• No persistent message history
• No encryption (plaintext communication)
• No file transfer support
• Single server (no distributed architecture)

Potential Enhancements

1. Add SSL/TLS encryption
2. Implement message history storage
3. Add file transfer capability
4. Support for chat rooms/channels
5. Implement rate limiting
6. Add user authentication
7. Create GUI client interface

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