Chat server

From Rosetta Code
Task
Chat server
You are encouraged to solve this task according to the task description, using any language you may know.
Task

Write a server for a minimal text based chat.

People should be able to connect via ‘telnet’, sign on with a nickname, and type messages which will then be seen by all other connected users. Arrivals and departures of chat members should generate appropriate notification messages.

Ada

Library: AdaSockets
with Ada.Containers.Vectors;
with Ada.Command_Line; use Ada.Command_Line;
with Ada.Exceptions;   use Ada.Exceptions;
with Ada.Text_IO;      use Ada.Text_IO;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
with Sockets;          use Sockets;

procedure Chat_Server is

   package Client_Vectors is new Ada.Containers.Vectors
     (Element_Type => Socket_FD, Index_Type => Positive);
   All_Clients : Client_Vectors.Vector;

   procedure Write (S : String) is
      procedure Output (Position : Client_Vectors.Cursor) is
         Sock : Socket_FD := Client_Vectors.Element (Position);
      begin
         Put_Line (Sock, S);
      end Output;
   begin
      All_Clients.Iterate (Output'Access);
   end Write;

   task type Client_Task is
      entry Start (FD : Socket_FD);
   end Client_Task;

   task body Client_Task is
      Sock    : Socket_FD;
      Sock_ID : Positive;
      Name    : Unbounded_String;
   begin
      select
         accept Start (FD : Socket_FD) do
            Sock := FD;
         end Start;
      or
         terminate;
      end select;

      while Name = Null_Unbounded_String loop
         Put (Sock, "Enter Name:");
         Name := To_Unbounded_String (Get_Line (Sock));
      end loop;
      Write (To_String (Name) & " joined.");
      All_Clients.Append (Sock);
      Sock_ID := All_Clients.Find_Index (Sock);
      loop
         declare
            Input : String := Get_Line (Sock);
         begin
            Write (To_String (Name) & ": " & Input);
         end;
      end loop;
   exception
      when Connection_Closed =>
         Put_Line ("Connection closed");
         Shutdown (Sock, Both);
         All_Clients.Delete (Sock_ID);
         Write (To_String (Name) & " left.");
   end Client_Task;

   Accepting_Socket : Socket_FD;
   Incoming_Socket  : Socket_FD;

   type Client_Access is access Client_Task;
   Dummy : Client_Access;
begin
   if Argument_Count /= 1 then
      Raise_Exception (Constraint_Error'Identity,
                       "Usage: " & Command_Name & " port");
   end if;
   Socket (Accepting_Socket, PF_INET, SOCK_STREAM);
   Setsockopt (Accepting_Socket, SOL_SOCKET, SO_REUSEADDR, 1);
   Bind (Accepting_Socket, Positive'Value (Argument (1)));
   Listen (Accepting_Socket);
   loop
      Put_Line ("Waiting for new connection");
      Accept_Socket (Accepting_Socket, Incoming_Socket);
      Put_Line ("New connection acknowledged");

      Dummy := new Client_Task;
      Dummy.Start (Incoming_Socket);
   end loop;
end Chat_Server;

BASIC

BaCon

Requires BaCon 4.2 or higher. Clients have to login with an alias and can use the commands 'say' or 'quit'. Notifications are submitted when users enter the chat or leave the chat.

DECLARE user$ ASSOC STRING
DECLARE connect ASSOC long
OPEN "localhost:51000" FOR SERVER AS mynet
WHILE TRUE
    IF WAIT(mynet, 30) THEN
        fd = ACCEPT(mynet)
        connect(GETPEER$(fd)) = fd
        SEND "Enter your name: " TO fd
    ELSE
        FOR con$ IN OBTAIN$(connect)
            IF WAIT(connect(con$), 10) THEN
                RECEIVE in$ FROM connect(con$)
                IF user$(GETPEER$(connect(con$))) = "" THEN
                    user$(GETPEER$(connect(con$))) = CHOP$(in$)
                    chat$ = chat$ & user$(GETPEER$(connect(con$))) & " joined the chat." & NL$
                    SEND "Welcome, " & CHOP$(in$) & "!" & NL$ TO connect(con$)
                ELIF LEFT$(in$, 4) = "quit" THEN
                    SEND "You're disconnected!" & NL$ TO connect(con$)
                    chat$ = chat$ & user$(GETPEER$(connect(con$))) & " left the chat." & NL$
                    FREE user$(GETPEER$(connect(con$)))
                    FREE connect(con$)
                    CLOSE SERVER connect(con$)
                ELIF LEFT$(in$, 4) = "say " THEN
                    chat$ = chat$ & user$(GETPEER$(connect(con$))) & " said: " & MID$(in$, 5)
                ENDIF
            ENDIF
        NEXT
        IF LEN(chat$) > 0 THEN
            FOR con$ IN OBTAIN$(connect)
                IF user$(GETPEER$(connect(con$))) <> "" THEN SEND chat$ TO connect(con$)
            NEXT
            chat$ = ""
        ENDIF
    ENDIF
WEND

Visual Basic .NET

Translation of: C#
Imports System.Net.Sockets
Imports System.Text
Imports System.Threading

Module Module1

    Class State
        Private ReadOnly client As TcpClient
        Private ReadOnly sb As New StringBuilder

        Public Sub New(name As String, client As TcpClient)
            Me.Name = name
            Me.client = client
        End Sub

        Public ReadOnly Property Name As String

        Public Sub Send(text As String)
            Dim bytes = Encoding.ASCII.GetBytes(String.Format("{0}" & vbCrLf, text))
            client.GetStream().Write(bytes, 0, bytes.Length)
        End Sub
    End Class

    ReadOnly connections As New Dictionary(Of Integer, State)
    Dim listen As TcpListener
    Dim serverThread As Thread

    Sub Main()
        listen = New TcpListener(Net.IPAddress.Parse("127.0.0.1"), 4004)
        serverThread = New Thread(New ThreadStart(AddressOf DoListen))
        serverThread.Start()
    End Sub

    Private Sub DoListen()
        listen.Start()
        Console.WriteLine("Server: Started server")

        Do
            Console.Write("Server: Waiting...")
            Dim client = listen.AcceptTcpClient()
            Console.WriteLine(" Connected")

            ' New thread with client
            Dim clientThread As New Thread(New ParameterizedThreadStart(AddressOf DoClient))

            clientThread.Start(client)
        Loop
    End Sub

    Private Sub DoClient(client As TcpClient)
        Console.WriteLine("Client (Thread: {0}): Connected!", Thread.CurrentThread.ManagedThreadId)
        Dim bytes = Encoding.ASCII.GetBytes("Enter name: ")
        client.GetStream().Write(bytes, 0, bytes.Length)

        Dim done As Boolean
        Dim name As String
        Do
            If Not client.Connected Then
                Console.WriteLine("Client (Thread: {0}): Terminated!", Thread.CurrentThread.ManagedThreadId)
                client.Close()
                Thread.CurrentThread.Abort() ' Kill thread
            End If

            name = Receive(client)
            done = True

            For Each cl In connections
                Dim state = cl.Value
                If state.Name = name Then
                    bytes = Encoding.ASCII.GetBytes("Name already registered. Please enter your name: ")
                    client.GetStream().Write(bytes, 0, bytes.Length)
                    done = False
                End If
            Next
        Loop While Not done

        connections.Add(Thread.CurrentThread.ManagedThreadId, New State(name, client))
        Console.WriteLine(vbTab & "Total connections: {0}", connections.Count)
        Broadcast(String.Format("+++ {0} arrived +++", name))

        Do
            Dim text = Receive(client)
            If text = "/quit" Then
                Broadcast(String.Format("Connection from {0} closed.", name))
                connections.Remove(Thread.CurrentThread.ManagedThreadId)
                Console.WriteLine(vbTab & "Total connections: {0}", connections.Count)
                Exit Do
            End If

            If Not client.Connected Then
                Exit Do
            End If

            Broadcast(String.Format("{0}> {1}", name, text))
        Loop

        Console.WriteLine("Client (Thread: {0}): Terminated!", Thread.CurrentThread.ManagedThreadId)
        client.Close()
        Thread.CurrentThread.Abort()
    End Sub

    Private Function Receive(client As TcpClient) As String
        Dim sb As New StringBuilder
        Do
            If client.Available > 0 Then
                While client.Available > 0
                    Dim ch = Chr(client.GetStream.ReadByte())
                    If ch = vbCr Then
                        ' ignore
                        Continue While
                    End If
                    If ch = vbLf Then
                        Return sb.ToString()
                    End If
                    sb.Append(ch)
                End While

                ' pause
                Thread.Sleep(100)
            End If
        Loop
    End Function

    Private Sub Broadcast(text As String)
        Console.WriteLine(text)
        For Each client In connections
            If client.Key <> Thread.CurrentThread.ManagedThreadId Then
                Dim state = client.Value
                state.Send(text)
            End If
        Next
    End Sub

End Module

C

C has no built-in networking functions, but the POSIX library does provide some low-level networking functions. The functions of interest relating to sockets include bind, listen, select, accept, read, write and close.

The example below was compiled on Cygwin, and accepts PuTTY connections under the RAW protocol.

A glitch occurs if a connection is made using the Telnet protocol - user names are preceded by garbled text.

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>
#include <netinet/ip.h>

int tsocket;
struct sockaddr_in tsockinfo;
fd_set status, current;
void ClientText(int handle, char *buf, int buf_len);

struct client
{
    char buffer[4096];
    int pos;
    char name[32];
} *connections[FD_SETSIZE];

void AddConnection(int handle)
{
    connections[handle] = malloc(sizeof(struct client));
    connections[handle]->buffer[0] = '\0';
    connections[handle]->pos = 0;
    connections[handle]->name[0] = '\0';
}

void CloseConnection(int handle)
{
    char buf[512];
    int j;

    FD_CLR(handle, &status);
    
    if (connections[handle]->name[0])
    {
        sprintf(buf, "* Disconnected: %s\r\n", connections[handle]->name);

        for (j = 0; j < FD_SETSIZE; j++)
        {
            if (handle != j && j != tsocket && FD_ISSET(j, &status))
            {
                if (write(j, buf, strlen(buf)) < 0)
                {
                    CloseConnection(j);
                }
            }
        }
    } else
    {
        printf ("-- Connection %d disconnected\n", handle);
    }
    if (connections[handle])
    {
        free(connections[handle]);
    }
    close(handle);
}

void strip(char *buf)
{
    char *x;
    
    x = strchr(buf, '\n');
    if (x) { *x='\0'; }
    x = strchr(buf, '\r');
    if (x) { *x='\0'; }
}

int RelayText(int handle)
{
    char *begin, *end;
    int ret = 0;
    begin = connections[handle]->buffer;
    if (connections[handle]->pos == 4000)
    {
        if (begin[3999] != '\n')
            begin[4000] = '\0';
        else {
            begin[4000] = '\n';
            begin[4001] = '\0';
        }
    } else {
        begin[connections[handle]->pos] = '\0';
    }
    
    end = strchr(begin, '\n');
    while (end != NULL)
    {
        char output[8000];
        output[0] = '\0';
        if (!connections[handle]->name[0])
        {
            strncpy(connections[handle]->name, begin, 31);
            connections[handle]->name[31] = '\0';
            
            strip(connections[handle]->name);
            sprintf(output, "* Connected: %s\r\n", connections[handle]->name);
            ret = 1;
        } else 
        {
            sprintf(output, "%s: %.*s\r\n", connections[handle]->name, 
                    end-begin, begin);
            ret = 1;
        }
        
        if (output[0])
        {
            int j;
            for (j = 0; j < FD_SETSIZE; j++)
            {
                if (handle != j && j != tsocket && FD_ISSET(j, &status))
                {
                    if (write(j, output, strlen(output)) < 0)
                    {
                        CloseConnection(j);
                    }
                }
            }
        }
        begin = end+1;
        end = strchr(begin, '\n');
    }
    
    strcpy(connections[handle]->buffer, begin);
    connections[handle]->pos -= begin - connections[handle]->buffer;
    return ret;
}

void ClientText(int handle, char *buf, int buf_len)
{
    int i, j;
    if (!connections[handle])
        return;
    j = connections[handle]->pos; 
    
    for (i = 0; i < buf_len; ++i, ++j)
    {
        connections[handle]->buffer[j] = buf[i];
        
        if (j == 4000)
        {
            while (RelayText(handle));
            j = connections[handle]->pos;
        }
    }
    connections[handle]->pos = j;
    
    while (RelayText(handle));
}


int ChatLoop()
{
    int i, j;
    FD_ZERO(&status);

    FD_SET(tsocket, &status);
    FD_SET(0, &status);

    while(1)
    {
        current = status;
        if (select(FD_SETSIZE, &current, NULL, NULL, NULL)==-1)
        {
            perror("Select");
            return 0;
        }
        for (i = 0; i < FD_SETSIZE; ++i)
        {
            if (FD_ISSET(i, &current))
            {
                if (i == tsocket)
                {
                    struct sockaddr_in cliinfo;
                    socklen_t addrlen = sizeof(cliinfo);
                    int handle;
                    handle = accept(tsocket, &cliinfo, &addrlen);
                    if (handle == -1)
                    {
                        perror ("Couldn't accept connection");
                    } else if (handle > FD_SETSIZE)
                    {
                        printf ("Unable to accept new connection.\n");
                        close(handle);
                    }
                    else
                    {
                        if (write(handle, "Enter name: ", 12) >= 0)
                        {
                            printf("-- New connection %d from %s:%hu\n",
                                handle, 
                                inet_ntoa (cliinfo.sin_addr),
                                ntohs(cliinfo.sin_port));
                            FD_SET(handle, &status);

                            AddConnection(handle);
                        }
                    }
                } /* Read data, relay to others. */
                else
                {
                    char buf[512];
                    int b;

                    b = read(i, buf, 500);
                    if (b <= 0)
                    {
                        CloseConnection(i);
                    }
                    else
                    {
                        ClientText(i, buf, b);
                    }
                }
            }
        }
    } /* While 1 */
}

int main (int argc, char*argv[])
{
    tsocket = socket(PF_INET, SOCK_STREAM, 0);

    tsockinfo.sin_family = AF_INET;
    tsockinfo.sin_port = htons(7070);
    if (argc > 1)
    {
        tsockinfo.sin_port = htons(atoi(argv[1]));
    }
    tsockinfo.sin_addr.s_addr = htonl(INADDR_ANY);
    printf ("Socket %d on port %hu\n", tsocket, ntohs(tsockinfo.sin_port));

    if (bind(tsocket, &tsockinfo, sizeof(tsockinfo)) == -1)
    {
        perror("Couldn't bind socket");
        return -1;
    }

    if (listen(tsocket, 10) == -1)
    {
        perror("Couldn't listen to port");
    }

    ChatLoop();

    return 0;
}

C#

using System;
using System.Collections.Generic;
using System.Net.Sockets;
using System.Text;
using System.Threading;

namespace ChatServer {
    class State {
        private TcpClient client;
        private StringBuilder sb = new StringBuilder();

        public string Name { get; }

        public State(string name, TcpClient client) {
            Name = name;
            this.client = client;
        }

        public void Add(byte b) {
            sb.Append((char)b);
        }

        public void Send(string text) {
            var bytes = Encoding.ASCII.GetBytes(string.Format("{0}\r\n", text));
            client.GetStream().Write(bytes, 0, bytes.Length);
        }
    }

    class Program {
        static TcpListener listen;
        static Thread serverthread;
        static Dictionary<int, State> connections = new Dictionary<int, State>();

        static void Main(string[] args) {
            listen = new TcpListener(System.Net.IPAddress.Parse("127.0.0.1"), 4004);
            serverthread = new Thread(new ThreadStart(DoListen));
            serverthread.Start();
        }

        private static void DoListen() {
            // Listen
            listen.Start();
            Console.WriteLine("Server: Started server");

            while (true) {
                Console.WriteLine("Server: Waiting...");
                TcpClient client = listen.AcceptTcpClient();
                Console.WriteLine("Server: Waited");

                // New thread with client
                Thread clientThread = new Thread(new ParameterizedThreadStart(DoClient));
                clientThread.Start(client);
            }
        }

        private static void DoClient(object client) {
            // Read data
            TcpClient tClient = (TcpClient)client;

            Console.WriteLine("Client (Thread: {0}): Connected!", Thread.CurrentThread.ManagedThreadId);
            byte[] bytes = Encoding.ASCII.GetBytes("Enter name: ");
            tClient.GetStream().Write(bytes, 0, bytes.Length);

            string name = string.Empty;
            bool done = false;
            do {
                if (!tClient.Connected) {
                    Console.WriteLine("Client (Thread: {0}): Terminated!", Thread.CurrentThread.ManagedThreadId);
                    tClient.Close();
                    Thread.CurrentThread.Abort();       // Kill thread.
                }

                name = Receive(tClient);
                done = true;

                if (done) {
                    foreach (var cl in connections) {
                        var state = cl.Value;
                        if (state.Name == name) {
                            bytes = Encoding.ASCII.GetBytes("Name already registered. Please enter your name: ");
                            tClient.GetStream().Write(bytes, 0, bytes.Length);
                            done = false;
                        }
                    }
                }
            } while (!done);

            connections.Add(Thread.CurrentThread.ManagedThreadId, new State(name, tClient));
            Console.WriteLine("\tTotal connections: {0}", connections.Count);
            Broadcast(string.Format("+++ {0} arrived +++", name));

            do {
                string text = Receive(tClient);
                if (text == "/quit") {
                    Broadcast(string.Format("Connection from {0} closed.", name));
                    connections.Remove(Thread.CurrentThread.ManagedThreadId);
                    Console.WriteLine("\tTotal connections: {0}", connections.Count);
                    break;
                }

                if (!tClient.Connected) {
                    break;
                }
                Broadcast(string.Format("{0}> {1}", name, text));
            } while (true);

            Console.WriteLine("Client (Thread: {0}): Terminated!", Thread.CurrentThread.ManagedThreadId);
            tClient.Close();
            Thread.CurrentThread.Abort();
        }

        private static string Receive(TcpClient client) {
            StringBuilder sb = new StringBuilder();
            do {
                if (client.Available > 0) {
                    while (client.Available > 0) {
                        char ch = (char)client.GetStream().ReadByte();
                        if (ch == '\r') {
                            //ignore
                            continue;
                        }
                        if (ch == '\n') {
                            return sb.ToString();
                        }
                        sb.Append(ch);
                    }
                }

                // Pause
                Thread.Sleep(100);
            } while (true);
        }

        private static void Broadcast(string text) {
            Console.WriteLine(text);
            foreach (var oClient in connections) {
                if (oClient.Key != Thread.CurrentThread.ManagedThreadId) {
                    State state = oClient.Value;
                    state.Send(text);
                }
            }
        }
    }
}

CoffeeScript

This is ported from the JavaScript version. The tool js2coffee got me a mostly working version, and then I manually converted JS-style classes to CS "classic-style class" syntax.

net = require("net")
sys = require("sys")
EventEmitter = require("events").EventEmitter

isNicknameLegal = (nickname) ->
  return false unless nickname.replace(/[A-Za-z0-9]*/, "") is ""
  for used_nick of @chatters
    return false if used_nick is nickname
  true

class ChatServer
  constructor: ->
    @chatters = {}
    @server = net.createServer @handleConnection
    @server.listen 1212, "localhost"

  handleConnection: (connection) =>
    console.log "Incoming connection from " + connection.remoteAddress
    connection.setEncoding "utf8"
    chatter = new Chatter(connection, this)
    chatter.on "chat", @handleChat
    chatter.on "join", @handleJoin
    chatter.on "leave", @handleLeave

  handleChat: (chatter, message) =>
    @sendToEveryChatterExcept chatter, chatter.nickname + ": " + message

  handleJoin: (chatter) =>
    console.log chatter.nickname + " has joined the chat."
    @sendToEveryChatter chatter.nickname + " has joined the chat."
    @addChatter chatter

  handleLeave: (chatter) =>
    console.log chatter.nickname + " has left the chat."
    @removeChatter chatter
    @sendToEveryChatter chatter.nickname + " has left the chat."

  addChatter: (chatter) =>
    @chatters[chatter.nickname] = chatter

  removeChatter: (chatter) =>
    delete @chatters[chatter.nickname]

  sendToEveryChatter: (data) =>
    for nickname of @chatters
      @chatters[nickname].send data

  sendToEveryChatterExcept: (chatter, data) =>
    for nickname of @chatters
      @chatters[nickname].send data  unless nickname is chatter.nickname


class Chatter extends EventEmitter
  constructor: (socket, server) ->
    EventEmitter.call this
    @socket = socket
    @server = server
    @nickname = ""
    @lineBuffer = new SocketLineBuffer(socket)
    @lineBuffer.on "line", @handleNickname
    @socket.on "close", @handleDisconnect
    @send "Welcome! What is your nickname?"

  handleNickname: (nickname) =>
    if isNicknameLegal(nickname)
      @nickname = nickname
      @lineBuffer.removeAllListeners "line"
      @lineBuffer.on "line", @handleChat
      @send "Welcome to the chat, " + nickname + "!"
      @emit "join", this
    else
      @send "Sorry, but that nickname is not legal or is already in use!"
      @send "What is your nickname?"

  handleChat: (line) =>
    @emit "chat", this, line

  handleDisconnect: =>
    @emit "leave", this

  send: (data) =>
    @socket.write data + "\r\n"


class SocketLineBuffer extends EventEmitter
  constructor: (socket) ->
    EventEmitter.call this
    @socket = socket
    @buffer = ""
    @socket.on "data", @handleData

  handleData: (data) =>
    console.log "Handling data", data
    i = 0

    while i < data.length
      char = data.charAt(i)
      @buffer += char
      if char is "\n"
        @buffer = @buffer.replace("\r\n", "")
        @buffer = @buffer.replace("\n", "")
        @emit "line", @buffer
        console.log "incoming line: #{@buffer}"
        @buffer = ""
      i++

server = new ChatServer()

Common Lisp

Library: usocket
Library: simple-actors

This chat server uses the SIMPLE-ACTORS library to avoid having any explicit locks. Each resource is owned by an actor that receives messages pertaining to that resource. The *USER-MANAGER* actor maintains a list of active users and the actor objects that control their connections. It is responsible for sending users' messages to everyone (by sending messages to the users' actors), announcing when users join and leave (as a regular message from a user named "Server"), and making sure that no two users have the same name (or the special name "Server").

The *USER-MANAGER* also accepts a :WHO message, which causes it to list the names of all the connected users to the user who sent the message.

Each user connection is managed by a user actor, and is responsible for sending and receiving messages over a single user's TCP connection, and for interpreting /COMMANDS (supported: /WHO, /HELP, /QUIT, and also /NICK in the special case that the initially-entered username was rejected by the *USER-MANAGER*). It is also responsible for closing the user's network connection, either on receiving a :CLOSE message from the

  • USER-MANAGER*, or upon an error occurring.
(ql:quickload '(:usocket :simple-actors :bordeaux-threads))

(defpackage :chat-server
  (:use :common-lisp :usocket :simple-actors :bordeaux-threads)
  (:export :accept-connections))

(in-package :chat-server)

(defvar *whitespace* '(#\Space #\Tab #\Page #\Vt #\Newline #\Return))

(defun send-message (users from-user message)
  (loop for (nil . actor) in users
     do (send actor :message from-user message)))

(defun socket-format (socket format-control &rest format-arguments)
  (apply #'format (socket-stream socket) format-control format-arguments)
  (finish-output (socket-stream socket)))

(defvar *log* *standard-output*)

(defmacro log-errors (&body body)
  `(handler-case
       (progn ,@body)
     (t (err)
       (format *log* "Error: ~a" err))))

(defparameter *user-manager*
  (let ((users nil))
    (actor (action &rest args)
       (format *log* "Handling message ~s~%" (cons action args))
       (ecase action
	 (:newuser
	  (destructuring-bind (username session-actor)
	      args
	    (cond ((assoc username users :test #'equalp)
		   (send session-actor :rejected
			 (format nil "Username ~a is already taken. Send /NICK new-nick with a valid name to enter the chat~%" username)))
		  ((equalp username "Server")
		   (send session-actor :rejected
			 (format nil "Server is not a valid username. Send /NICK new-nick with a valid name to enter the chat~%")))
		  (t (send-message users "Server" (format nil "~a has joined the chat." username))
		     (send session-actor :accepted
			   (format nil "Welcome to the Rosetta Code chat server in Common Lisp. ~a users connected.~%"
				   (length users)))
		     (pushnew (cons username session-actor)
			      users
			      :key #'car
			      :test #'equalp)))))
	 (:who
	  (destructuring-bind (username) args
	    (let ((actor (cdr (assoc username users :test #'equalp))))
	      (send actor :message "Server"
		    "Users connected right now:")
	      (loop for (user . nil) in users
		   do (send actor :message "Server" user)))))
	 (:message
	  (apply #'send-message users args))
	 (:dropuser
	  (destructuring-bind (username) args
	    (let ((user-actor (cdr (assoc username users :test #'equalp))))
	      (send user-actor :close)
	      (send user-actor 'stop))
	    (setf users (remove username users
				:key #'car
				:test #'equalp))
	    (send-message users "Server" (format nil "~a has left." username))))))))

(defmacro drop-connection-on-error (&body body)
  `(handler-case (progn ,@body)
     (t (err)
       (format *log* "Error: ~a; Closing connection" err)
       (send self :close)
       (send self 'stop)
       (send *user-manager* :dropuser username))))

(defun parse-command (message)
  (let* ((space-at (position #\Space message))
	 (after-space (and space-at
			   (position-if (lambda (ch)
				     (not (char= ch #\Space)))
					message :start (1+ space-at)))))
    (values (subseq message 0 space-at)
	    (and after-space
		 (string-trim *whitespace*
			      (subseq message after-space))))))

(defun help (socket)
  (socket-format socket "/QUIT to quit, /WHO to list users.~%"))

(defun make-user (username socket)
  (let* ((state :unregistered)
	 (actor 
	  (actor (message &rest args)
	    (drop-connection-on-error
	      (ecase message
		(:register
		 (send *user-manager* :newuser username self))
		(:accepted
		 (destructuring-bind (message) args
		   (write-string message (socket-stream socket))
		   (finish-output (socket-stream socket))
		   (setf state :registered)))
		(:rejected
		 (destructuring-bind (message) args
		   (write-string message (socket-stream socket))
		   (finish-output (socket-stream socket))
		   (setf state :unregistered)))
		(:user-typed
		 (destructuring-bind (message) args
		   (when (> (length message) 0)
		     (if (char= (aref message 0) #\/)
			 (multiple-value-bind (cmd arg)
			     (parse-command message)
			   (cond ((equalp cmd "/nick")
				  (ecase state
				    (:unregistered
				     (setf username arg)
				     (send *user-manager* :newuser username self))
				    (:registered
				     (socket-format socket
						    "Can't change your name after successfully registering~%"))))
				 ((equalp cmd "/help")
				  (help socket))
				 ((equalp cmd "/who")
				  (send *user-manager* :who username))
				 ((equalp cmd "/quit")
				  (socket-format socket
					  "Goodbye.~%")
				  (send *user-manager* :dropuser username))
				 (t
				  (socket-format socket
					  "Unknown command~%"))))
			 (send *user-manager* :message username message)))))
		(:message
		 (destructuring-bind (from-user message) args
		   (socket-format socket "<~a> ~a~%" from-user message)))
		(:close
		 (log-errors 
		   (close (socket-stream socket)))))))))
    (bt:make-thread (lambda ()
		      (handler-case
			  (loop for line = (read-line (socket-stream socket) nil :eof)
			     do (if (eq line :eof)
				    (send *user-manager* :dropuser username)
				    (send actor :user-typed (remove #\Return line))))
			(t () (send *user-manager* :dropuser username))))
			       
		    :name "Reader thread")
    actor))
			       

(defun initialize-user (socket)
  (bt:make-thread
   (lambda ()
     (format *log* "Handling new connection ~s" socket)
     (log-errors
       (loop do
	    (socket-format socket "Your name: ")
	    (let ((name (string-trim *whitespace* (read-line (socket-stream socket)))))
	      (format *log* "Registering user ~a" name)
	      (cond ((equalp name "Server")
		     (socket-format socket
				    "Server is not a valid username.~%"))
		    (t (send *user-manager*
			     :newuser name (make-user name socket))
		       (return)))))))
   :name "INITIALIZE-USER"))
	       

(defun accept-connections ()
  (let ((accepting-socket (socket-listen "0.0.0.0" 7070)))
    (loop for new-connection = (socket-accept accepting-socket)
	 do (initialize-user new-connection))))

(make-thread #'accept-connections)

D

import std.getopt;
import std.socket;
import std.stdio;
import std.string;

struct client {
    int pos;
    char[] name;
    char[] buffer;
    Socket socket;
}

void broadcast(client[] connections, size_t self, const char[] message) {
    writeln(message);
    for (size_t i = 0; i < connections.length; i++) {
        if (i == self) continue;

        connections[i].socket.send(message);
        connections[i].socket.send("\r\n");
    }
}

bool registerClient(client[] connections, size_t self) {
    for (size_t i = 0; i < connections.length; i++) {
        if (i == self) continue;

        if (icmp(connections[i].name, connections[self].name) == 0) {
            return false;
        }
    }

    return true;
}

void main(string[] args) {
    ushort port = 4004;

    auto helpInformation = getopt
    (
        args,
        "port|p", "The port to listen to chat clients on [default is 4004]", &port
    );

    if (helpInformation.helpWanted) {
        defaultGetoptPrinter("A simple chat server based on a task in rosettacode.", helpInformation.options);
        return;
    }

    auto listener = new TcpSocket();
    assert(listener.isAlive);
    listener.blocking = false;
    listener.bind(new InternetAddress(port));
    listener.listen(10);
    writeln("Listening on port: ", port);

    enum MAX_CONNECTIONS = 60;
    auto socketSet = new SocketSet(MAX_CONNECTIONS + 1);
    client[] connections;

    while(true) {
        socketSet.add(listener);

        foreach (con; connections) {
            socketSet.add(con.socket);
        }

        Socket.select(socketSet, null, null);

        for (size_t i = 0; i < connections.length; i++) {
            if (socketSet.isSet(connections[i].socket)) {
                char[1024] buf;
                auto datLength = connections[i].socket.receive(buf[]);

                if (datLength == Socket.ERROR) {
                    writeln("Connection error.");
                } else if (datLength != 0) {
                    if (buf[0] == '\n' || buf[0] == '\r') {
                        if (connections[i].buffer == "/quit") {
                            connections[i].socket.close();
                            if (connections[i].name.length > 0) {
                                writeln("Connection from ", connections[i].name, " closed.");
                            } else {
                                writeln("Connection from ", connections[i].socket.remoteAddress(), " closed.");
                            }

                            connections[i] = connections[$-1];
                            connections.length--;
                            i--;

                            writeln("\tTotal connections: ", connections.length);
                            continue;
                        } else if (connections[i].name.length == 0) {
                            connections[i].buffer = strip(connections[i].buffer);
                            if (connections[i].buffer.length > 0) {
                                connections[i].name = connections[i].buffer;
                                if (registerClient(connections, i)) {
                                    connections.broadcast(i, "+++ " ~ connections[i].name ~ " arrived +++");
                                } else {
                                    connections[i].socket.send("Name already registered. Please enter your name: ");
                                    connections[i].name.length = 0;
                                }
                            } else {
                                connections[i].socket.send("A name is required. Please enter your name: ");
                            }
                        } else {
                            connections.broadcast(i, connections[i].name ~ "> " ~ connections[i].buffer);
                        }
                        connections[i].buffer.length = 0;
                    } else {
                        connections[i].buffer ~= buf[0..datLength];
                    }
                } else {
                    try {
                        if (connections[i].name.length > 0) {
                            writeln("Connection from ", connections[i].name, " closed.");
                        } else {
                            writeln("Connection from ", connections[i].socket.remoteAddress(), " closed.");
                        }
                    } catch (SocketException) {
                        writeln("Connection closed.");
                    }
                }
            }
        }

        if (socketSet.isSet(listener)) {
            Socket sn = null;
            scope(failure) {
                writeln("Error accepting");

                if (sn) {
                    sn.close();
                }
            }
            sn = listener.accept();
            assert(sn.isAlive);
            assert(listener.isAlive);

            if (connections.length < MAX_CONNECTIONS) {
                client newclient;

                writeln("Connection from ", sn.remoteAddress(), " established.");
                sn.send("Enter name: ");

                newclient.socket = sn;
                connections ~= newclient;

                writeln("\tTotal connections: ", connections.length);
            } else {
                writeln("Rejected connection from ", sn.remoteAddress(), "; too many connections.");
                sn.close();
                assert(!sn.isAlive);
                assert(listener.isAlive);
            }
        }

        socketSet.reset();
    }
}

Erlang

-module(chat).

-export([start/0, start/1]).

-record(client, {name=none, socket=none}).

start() -> start(8080).
start(Port) ->
    register(server, spawn(fun() -> server() end)),
    {ok, LSocket} = gen_tcp:listen(Port, [binary, {packet, 0}, {active, false}, {reuseaddr, true}]),
    accept(LSocket).

% main loop for message dispatcher
server() -> server([]).
server(Clients) ->
    receive
        {join, Client=#client{name = Name, socket = Socket}} ->
            self() ! {say, Socket, "has joined." ++ [10, 13]},
            server(Clients ++ [Client]);
        {leave, Socket} ->
            {value, #client{name = Name}, List} = lists:keytake(Socket, 3, Clients),
            self() ! {say, none, Message = "has left."},
            server(List);
        {say, Socket, Data} ->
            {value, #client{name = From}, List} = lists:keytake(Socket, 3, Clients),
            Message = From ++ " : " ++ Data,
            lists:map(fun(#client{socket = S}) ->
                    gen_tcp:send(S, Message)
                end, List)
    end,
    server(Clients).

% accepts connections then spawns the client handler
accept(LSocket) ->
    {ok, Socket} = gen_tcp:accept(LSocket),
    spawn(fun() -> connecting(Socket) end),
    accept(LSocket).

% when client is first connect send prompt for user name
connecting(Socket) ->
    gen_tcp:send(Socket, "What is your name? "),
    case listen(Socket) of
        {ok, N} ->
            Name = binary_to_list(N),
            server ! {join, #client{name =  lists:sublist(Name, 1, length(Name) - 2), socket = Socket} },
            client(Socket);
        _ -> ok
    end.

% main client loop that listens for data
client(Socket) ->
    case listen(Socket) of
        {ok, Data} ->
            server ! {say, Socket, binary_to_list(Data)},
            client(Socket);
        _ -> server ! {leave, Socket}
    end.

% utility function that listens for data on a socket
listen(Socket) ->
    case gen_tcp:recv(Socket, 0) of
        Response -> Response
    end.

Go

This example uses the Go idiom of Do not communicate by sharing memory; instead, share memory by communicating; there are no explicit locks used, instead Go channels are used to safely synchronize where required.

A similar exercise of a chat roulette (different in that messages only have to be written to a single partner rather than broadcast, this simplifies the code greatly) was the topic of a 2012 Go Talk.

This example handles the case of one specific client "falling behind" by relying on the underlying TCP stack to do a reasonable job of buffering. Once that buffer fills, a write to the that client's connection will time out and the connection will dropped. Other minor improvements would include enabling TCP keep alives, handling temporary errors from accept, and better logging. Not ideal, but it should be good enough for this example.

package main

import (
	"bufio"
	"flag"
	"fmt"
	"log"
	"net"
	"strings"
	"time"
)

func main() {
	log.SetPrefix("chat: ")
	addr := flag.String("addr", "localhost:4000", "listen address")
	flag.Parse()
	log.Fatal(ListenAndServe(*addr))
}

// A Server represents a chat server that accepts incoming connections.
type Server struct {
	add  chan *conn  // To add a connection
	rem  chan string // To remove a connection by name
	msg  chan string // To send a message to all connections
	stop chan bool   // To stop early
}

// ListenAndServe listens on the TCP network address addr for
// new chat client connections.
func ListenAndServe(addr string) error {
	ln, err := net.Listen("tcp", addr)
	if err != nil {
		return err
	}
	log.Println("Listening for connections on", addr)
	defer ln.Close()
	s := &Server{
		add:  make(chan *conn),
		rem:  make(chan string),
		msg:  make(chan string),
		stop: make(chan bool),
	}
	go s.handleConns()
	for {
		// TODO use AcceptTCP() so that we can get a TCPConn on which
		// we can call SetKeepAlive() and SetKeepAlivePeriod()
		rwc, err := ln.Accept()
		if err != nil {
			// TODO Could handle err.(net.Error).Temporary()
			// here by adding a backoff delay.
			close(s.stop)
			return err
		}
		log.Println("New connection from", rwc.RemoteAddr())
		go newConn(s, rwc).welcome()
	}
}

// handleConns is run as a go routine to handle adding and removal of
// chat client connections as well as broadcasting messages to them.
func (s *Server) handleConns() {
	// We define the `conns` map here rather than within Server,
	// and we use local function literals rather than methods to be
	// extra sure that the only place that touches this map is this
	// method. In this way we forgo any explicit locking needed as
	// we're the only go routine that can see or modify this.
	conns := make(map[string]*conn)

	var dropConn func(string)
	writeAll := func(str string) {
		log.Printf("Broadcast: %q", str)
		// TODO handle blocked connections
		for name, c := range conns {
			c.SetWriteDeadline(time.Now().Add(500 * time.Millisecond))
			_, err := c.Write([]byte(str))
			if err != nil {
				log.Printf("Error writing to %q: %v", name, err)
				c.Close()
				delete(conns, name)
				// Defer all the disconnect messages until after
				// we've closed all currently problematic conns.
				defer dropConn(name)
			}
		}
	}

	dropConn = func(name string) {
		if c, ok := conns[name]; ok {
			log.Printf("Closing connection with %q from %v",
				name, c.RemoteAddr())
			c.Close()
			delete(conns, name)
		} else {
			log.Printf("Dropped connection with %q", name)
		}
		str := fmt.Sprintf("--- %q disconnected ---\n", name)
		writeAll(str)
	}

	defer func() {
		writeAll("Server stopping!\n")
		for _, c := range conns {
			c.Close()
		}
	}()

	for {
		select {
		case c := <-s.add:
			if _, exists := conns[c.name]; exists {
				fmt.Fprintf(c, "Name %q is not available\n", c.name)
				go c.welcome()
				continue
			}
			str := fmt.Sprintf("+++ %q connected +++\n", c.name)
			writeAll(str)
			conns[c.name] = c
			go c.readloop()
		case str := <-s.msg:
			writeAll(str)
		case name := <-s.rem:
			dropConn(name)
		case <-s.stop:
			return
		}
	}
}

// A conn represents the server side of a single chat connection.
// Note we embed the bufio.Reader and net.Conn (and specifically in
// that order) so that a conn gets the appropriate methods from each
// to be a full io.ReadWriteCloser.
type conn struct {
	*bufio.Reader         // buffered input
	net.Conn              // raw connection
	server        *Server // the Server on which the connection arrived
	name          string
}

func newConn(s *Server, rwc net.Conn) *conn {
	return &conn{
		Reader: bufio.NewReader(rwc),
		Conn:   rwc,
		server: s,
	}
}

// welcome requests a name from the client before attempting to add the
// named connect to the set handled by the server.
func (c *conn) welcome() {
	var err error
	for c.name = ""; c.name == ""; {
		fmt.Fprint(c, "Enter your name: ")
		c.name, err = c.ReadString('\n')
		if err != nil {
			log.Printf("Reading name from %v: %v", c.RemoteAddr(), err)
			c.Close()
			return
		}
		c.name = strings.TrimSpace(c.name)
	}
	// The server will take this *conn and do a final check
	// on the name, possibly starting c.welcome() again.
	c.server.add <- c
}

// readloop is started as a go routine by the server once the initial
// welcome phase has completed successfully. It reads single lines from
// the client and passes them to the server for broadcast to all chat
// clients (including us).
// Once done, we ask the server to remove (and close) our connection.
func (c *conn) readloop() {
	for {
		msg, err := c.ReadString('\n')
		if err != nil {
			break
		}
		//msg = strings.TrimSpace(msg)
		c.server.msg <- c.name + "> " + msg
	}
	c.server.rem <- c.name
}

Groovy

Translation of: Java
class ChatServer implements Runnable {
    private int port = 0
    private List<Client> clientList = new ArrayList<>()

    ChatServer(int port) {
        this.port = port
    }

    @SuppressWarnings("GroovyInfiniteLoopStatement")
    @Override
    void run() {
        try {
            ServerSocket serverSocket = new ServerSocket(port)
            while (true) {
                Socket socket = serverSocket.accept()
                new Thread(new Client(socket)).start()
            }
        } catch (Exception e) {
            e.printStackTrace()
        }
    }

    private synchronized boolean registerClient(Client client) {
        for (Client other : clientList) {
            if (other.clientName.equalsIgnoreCase(client.clientName)) {
                return false
            }
        }
        clientList.add(client)
        return true
    }

    private void deRegisterClient(Client client) {
        boolean wasRegistered
        synchronized (this) {
            wasRegistered = clientList.remove(client)
        }
        if (wasRegistered) {
            broadcast(client, "--- " + client.clientName + " left ---")
        }
    }

    private synchronized String getOnlineListCSV() {
        StringBuilder sb = new StringBuilder()
        sb.append(clientList.size()).append(" user(s) online: ")
        def it = clientList.iterator()
        if (it.hasNext()) {
            sb.append(it.next().clientName)
        }
        while (it.hasNext()) {
            sb.append(", ")
            sb.append(it.next().clientName)
        }
        return sb.toString()
    }

    private void broadcast(Client fromClient, String msg) {
        // Copy client list (don't want to hold lock while doing IO)
        List<Client> clients
        synchronized (this) {
            clients = new ArrayList<>(this.clientList)
        }
        for (Client client : clients) {
            if (client == fromClient) {
                continue
            }
            try {
                client.write(msg + "\r\n")
            } catch (Exception ignored) {
                // empty
            }
        }
    }

    class Client implements Runnable {
        private Socket socket = null
        private Writer output = null
        private String clientName = null

        Client(Socket socket) {
            this.socket = socket
        }

        @Override
        void run() {
            try {
                socket.setSendBufferSize(16384)
                socket.setTcpNoDelay(true)
                BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()))
                output = new OutputStreamWriter(socket.getOutputStream())
                write("Please enter your name: ")
                String line
                while (null != (line = input.readLine())) {
                    if (null == clientName) {
                        line = line.trim()
                        if (line.isEmpty()) {
                            write("A name is required. Please enter your name: ")
                            continue
                        }
                        clientName = line
                        if (!registerClient(this)) {
                            clientName = null
                            write("Name already registered. Please enter your name: ")
                            continue
                        }
                        write(getOnlineListCSV() + "\r\n")
                        broadcast(this, "+++ " + clientName + " arrived +++")
                        continue
                    }
                    if (line.equalsIgnoreCase("/quit")) {
                        return
                    }
                    broadcast(this, clientName + "> " + line)
                }
            } catch (Exception ignored) {
                // empty
            } finally {
                deRegisterClient(this)
                output = null
                try {
                    socket.close()
                } catch (Exception ignored) {
                    // empty
                }
                socket = null
            }
        }

        void write(String msg) {
            output.write(msg)
            output.flush()
        }

        @Override
        boolean equals(client) {
            return (null != client) && (client instanceof Client) && (null != clientName) && clientName == client.clientName
        }

        @Override
        int hashCode() {
            int result
            result = (socket != null ? socket.hashCode() : 0)
            result = 31 * result + (output != null ? output.hashCode() : 0)
            result = 31 * result + (clientName != null ? clientName.hashCode() : 0)
            return result
        }
    }

    static void main(String[] args) {
        int port = 4004
        if (args.length > 0) {
            port = Integer.parseInt(args[0])
        }
        new ChatServer(port).run()
    }
}

Haskell

{-# LANGUAGE OverloadedStrings #-}
import Network
import System.IO
import Control.Concurrent
import qualified Data.Text as T
import Data.Text (Text)
import qualified Data.Text.IO as T
import qualified Data.Map as M
import Data.Map (Map)
import Control.Monad.Reader
import Control.Monad.Error
import Control.Exception
import Data.Monoid 
import Control.Applicative

type ServerApp = ReaderT ThreadData IO
data Speaker = Server | Client Text
data ThreadData = ThreadData { threadHandle :: Handle
                             , userTableMV :: MVar (Map Text Handle)}

echoLocal = liftIO . T.putStrLn
echoRemote = echoMessage . (">> "<>)
echoMessage msg = viewHandle >>= \h -> liftIO $ T.hPutStrLn h msg
getRemoteLine = viewHandle >>= liftIO . T.hGetLine
putMVarT = (liftIO.) . putMVar
takeMVarT = liftIO . takeMVar
readMVarT = liftIO . readMVar
modifyUserTable fn = viewUsers >>= \mv ->
                     liftIO $ modifyMVar_ mv (return . fn)
viewHandle = threadHandle <$> ask
viewUsers = userTableMV <$> ask

userChat :: ServerApp ()
userChat = do
    name <- addUser 
    echoLocal name
    h <- viewHandle
    (flip catchError) (\_ -> removeUser name) $
      do echoLocal $ "Accepted " <> name
         forever $ getRemoteLine >>= broadcast (Client name)

removeUser :: Text -> ServerApp ()
removeUser name = do
    echoLocal $ "Exception with " <> name <> ", removing from userTable"
    broadcast Server $ name <> " has left the server"
    modifyUserTable (M.delete name)

addUser :: ServerApp Text
addUser = do
    h <- viewHandle
    usersMV <- viewUsers
    echoRemote "Enter username" 
    name <- T.filter (/='\r') <$> getRemoteLine
    userTable <- takeMVarT usersMV
    if name `M.member` userTable
      then do echoRemote "Username already exists!" 
              putMVarT usersMV userTable
              addUser
      else do putMVarT usersMV (M.insert name h userTable)
              broadcast Server $ name <> " has joined the server"
              echoRemote "Welcome to the server!\n>> Other users:"
              readMVarT usersMV >>=
                  mapM_ (echoRemote . ("*" <>) . fst) 
                . filter ((/=name). fst) . M.toList
              return name

broadcast :: Speaker -> Text -> ServerApp ()
broadcast user msg =
    viewUsers >>= readMVarT >>= mapM_ (f . snd) . fn . M.toList
  where f h = liftIO $ T.hPutStrLn h $ nm <> msg
        (fn, nm) = case user of
                    Server -> (id, ">> ")
                    Client t -> (filter ((/=t) . fst), t <> "> ")

clientLoop socket users = do
    (h, _, _) <- accept socket
    hSetBuffering h LineBuffering
    forkIO $ runReaderT userChat (ThreadData h users)
    clientLoop socket users

main = do
    server <- listenOn $ PortNumber 5002
    T.putStrLn "Server started"
    newMVar (M.empty) >>= clientLoop server

Icon and Unicon

This is Unicon-specific:

global mlck, nCons, cons

procedure main()
    mlck := mutex()
    nCons := 0
    cons := mutex(set())
    while f := open(":12321","na") do {
        handle_client(f)
        critical mlck: if nCons <= 0 then close(f)
        }
end

procedure handle_client(f)
    critical mlck: nCons +:= 1
    thread {
        select(f,1000) & {
            writes(f, "Name? ")
            nick := (read(f) ? tab(upto('\n\r')))
            every write(!cons, nick," has joined.")
            insert(cons, f)
            while s := read(f) do every write(!cons, nick,": ",s)
            }
        delete(cons, f)
        every write(!cons, nick," has left.")
        critical mlck: nCons -:= 1
        }
end

Java

Broadcasting of messages is done by the thread that received the message, so a bad client could potentially disrupt the server. The output buffer is set to 16K in an attempt to alleviate possible symptoms, but I'm not sure if it's effective. Server does not allow duplicate client names, and lists all users online after a successful connection. Client can type "/quit" to quit.

I think ideally, NIO would be used to select() sockets available/ready for I/O, to eliminate the possibility of a bad connection disrupting the server, but this increases the complexity.

import java.io.*;
import java.net.*;
import java.util.*;

public class ChatServer implements Runnable
{
  private int port = 0;
  private List<Client> clients = new ArrayList<Client>();
  
  public ChatServer(int port)
  {  this.port = port;  }
  
  public void run()
  {
    try
    {
      ServerSocket ss = new ServerSocket(port);
      while (true)
      {
        Socket s = ss.accept();
        new Thread(new Client(s)).start();
      }
    }
    catch (Exception e)
    {  e.printStackTrace();  }
  }

  private synchronized boolean registerClient(Client client)
  {
    for (Client otherClient : clients)
      if (otherClient.clientName.equalsIgnoreCase(client.clientName))
        return false;
    clients.add(client);
    return true;
  }

  private void deregisterClient(Client client)
  {
    boolean wasRegistered = false;
    synchronized (this)
    {  wasRegistered = clients.remove(client);  }
    if (wasRegistered)
      broadcast(client, "--- " + client.clientName + " left ---");
  }
  
  private synchronized String getOnlineListCSV()
  {
    StringBuilder sb = new StringBuilder();
    sb.append(clients.size()).append(" user(s) online: ");
    for (int i = 0; i < clients.size(); i++)
      sb.append((i > 0) ? ", " : "").append(clients.get(i).clientName);
    return sb.toString();
  }
  
  private void broadcast(Client fromClient, String msg)
  {
    // Copy client list (don't want to hold lock while doing IO)
    List<Client> clients = null;
    synchronized (this)
    {  clients = new ArrayList<Client>(this.clients);  }
    for (Client client : clients)
    {
      if (client.equals(fromClient))
        continue;
      try
      {  client.write(msg + "\r\n");  }
      catch (Exception e)
      {  }
    }
  }

  public class Client implements Runnable
  {
    private Socket socket = null;
    private Writer output = null;
    private String clientName = null;
    
    public Client(Socket socket)
    {
      this.socket = socket;
    }
    
    public void run()
    {
      try
      {
        socket.setSendBufferSize(16384);
        socket.setTcpNoDelay(true);
        BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        output = new OutputStreamWriter(socket.getOutputStream());
        write("Please enter your name: ");
        String line = null;
        while ((line = input.readLine()) != null)
        {
          if (clientName == null)
          {
            line = line.trim();
            if (line.isEmpty())
            {
              write("A name is required. Please enter your name: ");
              continue;
            }
            clientName = line;
            if (!registerClient(this))
            {
              clientName = null;
              write("Name already registered. Please enter your name: ");
              continue;
            }
            write(getOnlineListCSV() + "\r\n");
            broadcast(this, "+++ " + clientName + " arrived +++");
            continue;
          }
          if (line.equalsIgnoreCase("/quit"))
            return;
          broadcast(this, clientName + "> " + line);
        }
      }
      catch (Exception e)
      {  }
      finally
      {
        deregisterClient(this);
        output = null;
        try
        {  socket.close();  }
        catch (Exception e)
        {  }
        socket = null;
      }
    }
    
    public void write(String msg) throws IOException
    {
      output.write(msg);
      output.flush();
    }
    
    public boolean equals(Client client)
    {
      return (client != null) && (client instanceof Client) && (clientName != null) && (client.clientName != null) && clientName.equals(client.clientName);
    }
  }
  
  public static void main(String[] args)
  {
    int port = 4004;
    if (args.length > 0)
      port = Integer.parseInt(args[0]);
    new ChatServer(port).run();
  }
}

JavaScript

Works with: Node.js
const net = require("net");
const EventEmitter = require("events").EventEmitter;
 
/*******************************************************************************
 * ChatServer
 *
 * Manages connections, users, and chat messages.
 ******************************************************************************/
 
class ChatServer {
    constructor() {
        this.chatters = {};
        this.server = net.createServer(this.handleConnection.bind(this));
        this.server.listen(1212, "localhost");
    }
    isNicknameLegal(nickname) {
        // A nickname may contain letters or numbers only,
        // and may only be used once.
        if (nickname.replace(/[A-Za-z0-9]*/, '') !== "") {
            return false;
        }
        for (const used_nick in this.chatters) {
            if (used_nick === nickname) {
                return false;
            }
        }
        return true;
    }
    handleConnection(connection) {
        console.log(`Incoming connection from ${connection.remoteAddress}`);
        connection.setEncoding("utf8");

        let chatter = new Chatter(connection, this);
        chatter.on("chat", this.handleChat.bind(this));
        chatter.on("join", this.handleJoin.bind(this));
        chatter.on("leave", this.handleLeave.bind(this));
    }
    handleChat(chatter, message) {
        this.sendToEveryChatterExcept(chatter, chatter.nickname + ": " + message);
    }
    handleJoin(chatter) {
        console.log(`${chatter.nickname} has joined the chat.`);
        this.sendToEveryChatter(`${chatter.nickname} has joined the chat.`);
        this.addChatter(chatter);
    }
    handleLeave(chatter) {
        console.log(`${chatter.nickname} has left the chat.`);
        this.removeChatter(chatter);
        this.sendToEveryChatter(`${chatter.nickname} has left the chat.`);
    }
    addChatter(chatter) {
        this.chatters[chatter.nickname] = chatter;
    }
    removeChatter(chatter) {
        delete this.chatters[chatter.nickname];
    }
    sendToEveryChatter(data) {
        for (const nickname in this.chatters) {
            this.chatters[nickname].send(data);
        }
    }
    sendToEveryChatterExcept(chatter, data) {
        for (const nickname in this.chatters) {
            if (nickname !== chatter.nickname) {
                this.chatters[nickname].send(data);
            }
        }
    }
}
 
 
/*******************************************************************************
 * Chatter
 *
 * Represents a single user/connection in the chat server.
 ******************************************************************************/
 
class Chatter extends EventEmitter {
    constructor(socket, server) {
        super();

        this.socket = socket;
        this.server = server;
        this.nickname = "";
        this.lineBuffer = new SocketLineBuffer(socket);

        this.lineBuffer.on("line", this.handleNickname.bind(this));
        this.socket.on("close", this.handleDisconnect.bind(this));

        this.send("Welcome! What is your nickname?");
    }
    handleNickname(nickname) {
        if (server.isNicknameLegal(nickname)) {
            this.nickname = nickname;
            this.lineBuffer.removeAllListeners("line");
            this.lineBuffer.on("line", this.handleChat.bind(this));
            this.send(`Welcome to the chat, ${nickname}!`);
            this.emit("join", this);
        } else {
            this.send("Sorry, but that nickname is not legal or is already in use!");
            this.send("What is your nickname?");
        }
    }
    handleChat(line) {
        this.emit("chat", this, line);
    }
    handleDisconnect() {
        this.emit("leave", this);
    }
    send(data) {
        this.socket.write(data + "\r\n");
    }
};
 
 
/*******************************************************************************
 * SocketLineBuffer
 *
 * Listens for and buffers incoming data on a socket and emits a 'line' event
 * whenever a complete line is detected.
 ******************************************************************************/
 
class SocketLineBuffer extends EventEmitter {
    constructor(socket) {
        super();

        this.socket = socket;
        this.buffer = "";

        this.socket.on("data", this.handleData.bind(this));
    }
    handleData(data) {
        for (let i = 0; i < data.length; i++) {
            const char = data.charAt(i);
            this.buffer += char;
            if (char == "\n") {
                this.buffer = this.buffer.replace("\r\n", "");
                this.buffer = this.buffer.replace("\n", "");
                this.emit("line", this.buffer);
                this.buffer = "";
            }
        }
    }
};
 
 
// Start the server!
server = new ChatServer();

Julia

Modified to fit the Rosetta Code task from example code for the WebSockets module written by Leah Hanson. To test, start the code and use a browser to connect to localhost:8000.

using HttpServer
using WebSockets

const connections = Dict{Int,WebSocket}()
const usernames   = Dict{Int,String}()

function decodeMessage( msg )
    String(copy(msg))
end


wsh = WebSocketHandler() do req, client
    global connections
    @show connections[client.id] = client
    println("req is $req")
    notifyonline = "Connection from user number $(client.id) is now online."
    for (k,v) in connections
        if k != client.id
            try
                write(v, notifyonline)
            catch
                continue
            end
        end
    end
    while true
        try
            msg = read(client)
        catch
            telloffline = "User $(usernames[client.id]) disconnected."
            println(telloffline, "(The client id was $(client.id).)")
            delete!(connections, client.id)
            if haskey(usernames, client.id)
                delete!(usernames, client.id)
            end
            for (k,v) in connections
                try
                    write(v, telloffline)
                catch
                    continue
                end
            end
            return
        end
        msg = decodeMessage(msg)
        if startswith(msg, "setusername:")
            println("SETTING USERNAME: $msg")
            usernames[client.id] = msg[13:end]
            notifyusername = "User number $(client.id) chose $(usernames[client.id]) as name handle."
            for (k,v) in connections
                try
                    write(v, notifyusername)
                catch
                    println("Caught exception writing to user $k")
                    continue
                end
            end
        end
        if startswith(msg, "say:")
            println("EMITTING MESSAGE: $msg")
            for (k,v) in connections
                if k != client.id
                    try
                        write(v, (usernames[client.id] * ": " * msg[5:end]))
                    catch
                        println("Caught exception writing to user $k")
                        continue
                    end
                end
            end
        end
    end
end

onepage = readstring(Pkg.dir("WebSockets","examples","chat-client.html"))
httph = HttpHandler() do req::Request, res::Response
  Response(onepage)
end

server = Server(httph, wsh)
println("Chat server listening on 8000...")
run(server,8000)

Kotlin

Translation of: Java
import java.io.BufferedReader
import java.io.IOException
import java.io.InputStreamReader
import java.io.OutputStreamWriter
import java.io.Writer
import java.net.ServerSocket
import java.net.Socket
import java.util.ArrayList
import java.util.Collections

class ChatServer private constructor(private val port: Int) : Runnable {
    private val clients = ArrayList<Client>()

    private val onlineListCSV: String
        @Synchronized get() {
            val sb = StringBuilder()
            sb.append(clients.size).append(" user(s) online: ")
            for (i in clients.indices) {
                sb.append(if (i > 0) ", " else "").append(clients[i].clientName)
            }
            return sb.toString()
        }

    override fun run() {
        try {
            val ss = ServerSocket(port)
            while (true) {
                val s = ss.accept()
                Thread(Client(s)).start()
            }
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }

    @Synchronized
    private fun registerClient(client: Client): Boolean {
        for (otherClient in clients) {
            if (otherClient.clientName!!.equals(client.clientName!!, ignoreCase = true)) {
                return false
            }
        }
        clients.add(client)
        return true
    }

    private fun deRegisterClient(client: Client) {
        var wasRegistered = false
        synchronized(this) {
            wasRegistered = clients.remove(client)
        }
        if (wasRegistered) {
            broadcast(client, "--- " + client.clientName + " left ---")
        }
    }

    private fun broadcast(fromClient: Client, msg: String) {
        // Copy client list (don't want to hold lock while doing IO)
        var clients: List<Client> = Collections.emptyList()
        synchronized(this) {
            clients = ArrayList(this.clients)
        }
        for (client in clients) {
            if (client.equals(fromClient)) {
                continue
            }
            try {
                client.write(msg + "\r\n")
            } catch (e: Exception) {
                e.printStackTrace()
            }

        }
    }

    inner class Client internal constructor(private var socket: Socket?) : Runnable {
        private var output: Writer? = null
        var clientName: String? = null

        override fun run() {
            try {
                socket!!.sendBufferSize = 16384
                socket!!.tcpNoDelay = true
                val input = BufferedReader(InputStreamReader(socket!!.getInputStream()))
                output = OutputStreamWriter(socket!!.getOutputStream())
                write("Please enter your name: ")
                var line: String
                while (true) {
                    line = input.readLine()
                    if (null == line) {
                        break
                    }
                    if (clientName == null) {
                        line = line.trim { it <= ' ' }
                        if (line.isEmpty()) {
                            write("A name is required. Please enter your name: ")
                            continue
                        }
                        clientName = line
                        if (!registerClient(this)) {
                            clientName = null
                            write("Name already registered. Please enter your name: ")
                            continue
                        }
                        write(onlineListCSV + "\r\n")
                        broadcast(this, "+++ $clientName arrived +++")
                        continue
                    }
                    if (line.equals("/quit", ignoreCase = true)) {
                        return
                    }
                    broadcast(this, "$clientName> $line")
                }
            } catch (e: Exception) {
                e.printStackTrace()
            } finally {
                deRegisterClient(this)
                output = null
                try {
                    socket!!.close()
                } catch (e: Exception) {
                    e.printStackTrace()
                }

                socket = null
            }
        }

        @Throws(IOException::class)
        internal fun write(msg: String) {
            output!!.write(msg)
            output!!.flush()
        }

        internal fun equals(client: Client?): Boolean {
            return (client != null
                && clientName != null
                && client.clientName != null
                && clientName == client.clientName)
        }
    }

    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            var port = 4004
            if (args.isNotEmpty()) {
                port = Integer.parseInt(args[0])
            }
            ChatServer(port).run()
        }
    }
}

Nim

import asyncnet, asyncdispatch

type
  Client = tuple
    socket: AsyncSocket
    name: string
    connected: bool

var clients {.threadvar.}: seq[Client]

proc sendOthers(client: Client, line: string) {.async.} =
  for c in clients:
    if c != client and c.connected:
      await c.socket.send(line & "\c\L")

proc processClient(socket: AsyncSocket) {.async.} =
  await socket.send("Please enter your name: ")
  var client: Client = (socket, await socket.recvLine(), true)

  clients.add(client)
  asyncCheck client.sendOthers("+++ " & client.name & " arrived +++")

  while true:
    let line = await client.socket.recvLine()
    if line == "":
      asyncCheck client.sendOthers("--- " & client.name & " leaves ---")
      client.connected = false
      return
    asyncCheck client.sendOthers(client.name & "> " & line)

proc serve() {.async.} =
  clients = @[]
  var server = newAsyncSocket()
  server.bindAddr(Port(4004))
  server.listen()

  while true:
    let socket = await server.accept()
    asyncCheck processClient(socket)

asyncCheck serve()
runForever()

Objeck

use System.IO.Net;
use System.Concurrency;
use Collection;

bundle Default {
  class ChatServer {
    @clients : StringMap;
    @clients_mutex : ThreadMutex;
    
    New() {
      @clients := StringMap->New();
      @clients_mutex := ThreadMutex->New("clients_mutex");
    }

    method : ValidLogin(login_name : String, clients : StringMap) ~ Bool {
      if(clients->Has(login_name)) {
        return false;
      };
      
      return true;
    }

    function : Main(args : String[]) ~ Nil {
      chat_server := ChatServer->New();
      chat_server->Run();
    }
    
    method : public : Broadcast(message : String, sender : Client) ~ Nil {
      client_array : Vector;
      critical(@clients_mutex) {
        client_array := @clients->GetValues();
      };
      each(i : client_array) {
        client := client_array->Get(i)->As(Client);
        if(client <> sender) {
          client->Send(message);
        };
      };      
    }

    method : public : Disconnect(sender : Client) ~ Nil {
      send_name := sender->GetName();
      Broadcast("+++ {$send_name} has left +++", sender);
      critical(@clients_mutex) {
        @clients->Remove(sender->GetName());
      };
      sender->Close();
    }

    method : public : Run() ~ Nil {
      server := TCPSocketServer->New(4661);
      if(server->Listen(5)) {
        while(true) {
          client_sock := server->Accept();
          critical(@clients_mutex) {
            client_sock->WriteString("login: ");
            login_name := client_sock->ReadString();
            if(ValidLogin(login_name, @clients)) {
              client := Client->New(login_name, client_sock, @self);
              @clients->Insert(client->GetName(), client);
              client->Execute(Nil);
            }
            else {
              client_sock->WriteString("+++ login in use +++\r\n");
              client_sock->Close();
            };
          };
        };
      };
      server->Close();
    }
  }

  class Client from Thread {
    @client_sock : TCPSocket;
    @server : ChatServer;
    
    New(login_name : String, client_sock : TCPSocket, server : ChatServer) {
      Parent(login_name);
      @client_sock := client_sock;
      @server := server;
    }
    
    method : public : Close() ~ Nil {
      @client_sock->Close();
    }
    
    method : public : Send(message : String) ~ Nil {
      if(@client_sock->IsOpen() & message->Size() > 0) {
        @client_sock->WriteString("{$message}\r\n");
      }
      else {
        @server->Disconnect(@self);
      };
    }

    method : public : Run(param : Base) ~ Nil {
      client_name := GetName();
      @server->Broadcast("+++ {$client_name} has arrived +++", @self);
      
      message := @client_sock->ReadString();
      while(message->Size() > 0 & message->Equals("/quit") = false) {
        @server->Broadcast("{$client_name}> {$message}", @self);      
        message := @client_sock->ReadString();
      };
      @server->Disconnect(@self);
    }
  }
}

Ol

(define (timestamp) (syscall 201 "%c"))

(fork-server 'chat-room (lambda ()
(let this ((visitors #empty))
(let* ((envelope (wait-mail))
       (sender msg envelope))
   (case msg
      (['join who name]
         (let ((visitors (put visitors who name)))
            (for-each (lambda (who)
                  (print-to (car who) name " joined to as"))
               (ff->alist visitors))
            (this visitors)))
      (['talk message]
         (for-each (lambda (who)
               (print-to (car who) (cdr who) ": " message))
            (ff->alist visitors))
         (this visitors))
      (['part who]
         (for-each (lambda (who)
               (print-to (car who) (visitors (car who) "unknown") " leaved"))
            (ff->alist visitors))
         (let ((visitors (del visitors who)))
            (this visitors))))))))


(define (on-accept name fd)
(lambda ()
   (print "# " (timestamp) "> we got new visitor: " name)
   (mail 'chat-room ['join fd name])

   (let*((ss1 ms1 (clock)))
      (let loop ((str #null) (stream (force (port->bytestream fd))))
         (cond
            ((null? stream)
               #false)
            ((function? stream)
               (mail 'chat-room ['talk (list->string (reverse str))])
               (loop #null (force stream)))
            (else
               (loop (cons (car stream) str) (cdr stream)))))
      (syscall 3 fd)
      (let*((ss2 ms2 (clock)))
         (print "# " (timestamp) "> visitor leave us. It takes "  (+ (* (- ss2 ss1) 1000) (- ms2 ms1)) "ms.")))
   (mail 'chat-room ['part fd])
   ))

(define (run port)
(let ((socket (syscall 41)))
   ; bind
   (let loop ((port port))
      (if (not (syscall 49 socket port)) ; bind
         (loop (+ port 2))
         (print "Server binded to " port)))
   ; listen
   (if (not (syscall 50 socket)) ; listen
      (shutdown (print "Can't listen")))

   ; accept
   (let loop ()
      (if (syscall 23 socket) ; select
         (let ((fd (syscall 43 socket))) ; accept
            ;(print "\n# " (timestamp) ": new request from " (syscall 51 fd))
            (fork (on-accept (syscall 51 fd) fd))))
      (sleep 0)
      (loop))))

(run 8080)
Output:

Usecase:

  1. First client connected.
  2. Second client connected.
  3. First client say "I'm the first. hello!".
  4. Second client say "I'm the second :)".
  5. Second client say "see you..".
  6. Second client disconnected.
  7. First client disconnected.

Server:

$ ol chat_server.scm 
Server binded to 8080
# Fri Jul 24 00:29:49 2020> we got new visitor: (127.0.0.1 . 55316)
# Fri Jul 24 00:29:55 2020> we got new visitor: (127.0.0.1 . 55320)
# Fri Jul 24 00:30:34 2020> visitor leave us. It takes 38977ms.
# Fri Jul 24 00:30:51 2020> visitor leave us. It takes 61962ms.

First client:

$ telnet 127.0.0.1 8080
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
(127.0.0.1 . 55316) joined to as
(127.0.0.1 . 55320) joined to as
I'm the first. hello!
(127.0.0.1 . 55316): I'm the first. hello!

(127.0.0.1 . 55316): I'm the second :)

(127.0.0.1 . 55316): see you..

(127.0.0.1 . 55316) leaved

Second client:

$ telnet 127.0.0.1 8080
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
(127.0.0.1 . 55320) joined to as
(127.0.0.1 . 55320): I'm the first. hello!

I'm the second :)
(127.0.0.1 . 55320): I'm the second :)

see you..
(127.0.0.1 . 55320): see you..

Perl

Translation of: Python
use 5.010;
use strict;
use warnings;

use threads;
use threads::shared;

use IO::Socket::INET;
use Time::HiRes qw(sleep ualarm);

my $HOST = "localhost";
my $PORT = 4004;

my @open;
my %users : shared;

sub broadcast {
    my ($id, $message) = @_;
    print "$message\n";
    foreach my $i (keys %users) {
        if ($i != $id) {
            $open[$i]->send("$message\n");
        }
    }
}

sub sign_in {
    my ($conn) = @_;

    state $id = 0;

    threads->new(
        sub {
            while (1) {
                $conn->send("Please enter your name: ");
                $conn->recv(my $name, 1024, 0);

                if (defined $name) {
                    $name = unpack('A*', $name);

                    if (exists $users{$name}) {
                        $conn->send("Name entered is already in use.\n");
                    }
                    elsif ($name ne '') {
                        $users{$id} = $name;
                        broadcast($id, "+++ $name arrived +++");
                        last;
                    }
                }
            }
        }
    );

    ++$id;
    push @open, $conn;
}

my $server = IO::Socket::INET->new(
                                   Timeout   => 0,
                                   LocalPort => $PORT,
                                   Proto     => "tcp",
                                   LocalAddr => $HOST,
                                   Blocking  => 0,
                                   Listen    => 1,
                                   Reuse     => 1,
                                  );

local $| = 1;
print "Listening on $HOST:$PORT\n";

while (1) {
    my ($conn) = $server->accept;

    if (defined($conn)) {
        sign_in($conn);
    }

    foreach my $i (keys %users) {

        my $conn = $open[$i];
        my $message;

        eval {
            local $SIG{ALRM} = sub { die "alarm\n" };
            ualarm(500);
            $conn->recv($message, 1024, 0);
            ualarm(0);
        };

        if ($@ eq "alarm\n") {
            next;
        }

        if (defined($message)) {
            if ($message ne '') {
                $message = unpack('A*', $message);
                broadcast($i, "$users{$i}> $message");
            }
            else {
                broadcast($i, "--- $users{$i} leaves ---");
                delete $users{$i};
                undef $open[$i];
            }
        }
    }

    sleep(0.1);
}

Alternate with both read and write queuing

#!/usr/bin/perl

use strict; # http://www.rosettacode.org/wiki/Chat_server
use warnings;
use IO::Socket;
use IO::Select; # with write queueing

my $port = shift // 6666;
my (%nicks, @users, %data);

my $listen = IO::Socket::INET->new(LocalPort => $port, Listen => 9,
  Reuse => 1) or die "$@ opening socket on port $port";
my $rsel = IO::Select->new($listen);
my $wsel = IO::Select->new();
print "ready on $port...\n";

sub to
  {
  my $text = pop;
  for ( @_ )
    {
    length $data{$_}{out} or $wsel->add( $_ );
    length( $data{$_}{out} .= $text ) > 1e4 and left( $_ );
    }
  return $text;
  }

sub left
  {
  my $h = shift;
  @users = grep $h != $_, @users;
  if( defined( my $nick = delete $nicks{$h} ) )
    {
    print to @users, "$nick has left\n";
    }
  delete $data{$h};
  $rsel->remove($h);
  }

while( 1 )
  {
  my ($reads, $writes) = IO::Select->select($rsel, $wsel, undef, 5);
  for my $h ( @{ $writes // [] } )
    {
    my $len = syswrite $h, $data{$h}{out};
    $len and substr $data{$h}{out}, 0, $len, '';
    length $data{$h}{out} or $wsel->remove( $h );
    }
  for my $h ( @{ $reads // [] } )
    {
    if( $h == $listen )                                    # new connection
      {
      $rsel->add( my $client = $h->accept );
      $data{$client} = { h => $client, out => "enter nick: ", in => '' };
      $wsel->add( $client );
      }
    elsif( not sysread $h, $data{$h}{in}, 4096, length $data{$h}{in} ) # closed
      {
      left $h;
      }
    elsif( exists $nicks{$h} )                             # user is signed in
      {
      my @others = grep $h != $_, @users;
      to @others, "$nicks{$h}> $&" while $data{$h}{in} =~ s/.*\n//;
      }
    elsif( $data{$h}{in} =~ s/^(\w+)\r?\n.*//s and
      not grep lc $1 eq lc, values %nicks )
      {                                                    # user has joined
      my $all = join ' ', sort values %nicks;
      $nicks{$h} = $1;
      push @users, $h;
      print to @users, "$nicks{$h} has joined $all\n";
      }
    else                                                   # bad nick
      {
      to $h, "nick invalid or in use, enter nick: ";
      $data{$h}{in} = '';
      }
    }
  }

Phix

server

--
-- demo\rosetta\ChatServer.exw
-- ========================
--
-- translation of (qchat) chatServ.exw
--
-- Run this first, then ChatClient.exw, see also IRC_Gateway.exw
--
-- Note that I've only got a 32-bit windows eulibnet.dll, but it should not
-- be dificult to use a "real" libnet.dll/so, or something a little newer.
--
without js
include pGUI.e
constant dl = `Download rosetta\eulibnet\ from http://phix.x10.mx/pmwiki/pmwiki.php?n=Main.Eulibnet`
assert(get_file_type("eulibnet")=FILETYPE_DIRECTORY,dl)
include eulibnet/eulibnet.ew
Ihandle log_list, log_window, statusbar, timer

atom listconn
constant IP = "127.0.0.1",
         port = "29029",
         IPaddress = IP & ":" & port
constant MAX_MSG = 550
sequence connections = {},
         nicknames = {}

constant max_log_len = 300

procedure log_to_window(string txt)
    integer count = to_number(IupGetAttribute(log_list,"COUNT"))
    if count > max_log_len then
        for t=1 to count-max_log_len do
            IupSetAttribute(log_list,"REMOVEITEM","1")
        end for
    end if
    IupSetAttribute(log_list,"APPENDITEM",txt)
    IupSetAttribute(log_list,"TOPITEM",IupGetAttribute(log_list,"COUNT"))
    IupUpdate(log_list)
end procedure

procedure message(string msg, sequence args={})
    if length(args) then msg = sprintf(msg,args) end if
    IupSetStrAttribute(statusbar, "TITLE", msg)
    log_to_window(msg)
end procedure

procedure shutDown()
    message("Shutting down euLibnet...")
    for i = 1 to length(connections) do
        if net_closeconn(connections[i]) then crash("Error closing connection!") end if
    end for
    if net_closeconn(listconn) then crash("Error closing listconn!") end if
    if net_shutdown() then crash("Error shutting down euLibnet!") end if
end procedure

procedure sendToAll(string msg)
    -- Send msg to all clients
    for i=1 to length(connections) do
        atom ci = connections[i]
        message("Sending to connection %d (%s)",{ci,nicknames[i]})
        if net_send_rdm(ci, msg) then
            message("Error sending to connection %d",{ci})
        end if
    end for
end procedure

procedure mainWindow_onOpen()
    message("Initializing euLibnet...")
    if net_init() then crash("Error initializing euLibnet!") end if
    message("done.")
    message("Initializing driver...")
    if net_initdriver(NET_DRIVER_WSOCK_WIN) != 1 then
        crash("Error initializing WinSock driver!")
    end if
    message("done.")
    message("Opening port " & IPaddress & "...")
    listconn = net_openconn(NET_DRIVER_WSOCK_WIN, IPaddress)
    if listconn = NULL then
        crash("Couldn't open connection (server already running?)")
    end if
    message("done.")
    if net_listen(listconn) then
        crash("Error trying to listen to port")
    end if
    message("Listening on port " & IPaddress)
end procedure

function timer_cb(Ihandle /*timer*/)
    integer conn = net_poll_listen(listconn)
    if conn != NULL then
         connections = append(connections, conn)
         nicknames = append(nicknames, "")
         message("New connection open from " & net_getpeer(conn))
    end if

    -- Check for messages from clients
    for i = 1 to length(connections) do
        integer ci = connections[i]
        if net_query_rdm(ci) > 0 then
            string ni = nicknames[i]
            --Get the message
            sequence msg = net_receive_rdm(ci, MAX_MSG)
            message("received msg \"%s\" of length %d from %d aka %s",{msg[2],msg[1],ci,ni})
            if msg[1] < 0 then --Exit on error
                {} = net_ignore_rdm(ci)
                sendToAll("Server error: some data may be lost")
                exit
            end if

            msg = msg[2]
            if length(msg) > 4 and equal(msg[1..3], "/n:") then
                if find(msg[4..length(msg)], nicknames) then
                    if net_send_rdm(ci, "/nt") then
                        message("Error sending to %d",{ci})
                    end if
                else
                    string prevname = ni
                    ni = msg[4..length(msg)]
                    nicknames[i] = ni
                    if length(prevname) = 0 then
                        sendToAll("/j:" & ni & " has joined")
                        -- send fake "has joined"s to the new joiner,
                        -- so that it can build a full members list (see allj)
                        for n=1 to length(nicknames) do
                            if n!=i then
                                msg = "/j:" & nicknames[n] & " has joined"
                                if net_send_rdm(ci, msg) then
                                    message("Error sending to connection %d",{ci})
                                    exit
                                end if
                            end if
                        end for
                    else
                        sendToAll("/c:" & prevname & " has changed name to " & ni)
                    end if
                end if
            elsif equal(msg, "/d") then
                msg = "/l:"& ni & " has left"
                message(msg)
                nicknames[i..i] = {}
                connections[i..i] = {}
                sendToAll(msg)
                exit
            -- Aside: bunch of popup and file transfer stuff was here in qchat.exw,
            --        all ripped out in the name of keeping this short and sweet.
            else
                sendToAll(ni & ": " & msg) -- (Add nickname to message)
            end if
        end if
    end for
    return IUP_IGNORE
end function

function close_cb(Ihandle /*ih*/)
    shutDown()
    return IUP_DEFAULT
end function

IupOpen()
log_list = IupList("EXPAND=YES, CANFOCUS=NO, MULTIPLE=YES")
statusbar = IupLabel("Loading...","EXPAND=HORIZONTAL")
log_window = IupDialog(IupVbox({log_list,statusbar}),
            `TITLE="Chat Server", RASTERSIZE=400x400`)
IupSetCallback(log_window,"CLOSE_CB",Icallback("close_cb"))
IupShow(log_window)
mainWindow_onOpen()
timer = IupTimer(Icallback("timer_cb"), 500)
IupMainLoop()
IupClose()

client

--
-- demo\rosetta\ChatClient.exw
-- ===========================
--
-- translation of (qchat) QChat.exw
--
-- Probably best to run ChatServer.exw before this.
--
without js
constant bool bViaGateway = false
--constant bool bViaGateway = true  -- see IRC_Gateway.exw
include pGUI.e
constant dl = `Download rosetta\eulibnet\ from http://phix.x10.mx/pmwiki/pmwiki.php?n=Main.Eulibnet`
assert(get_file_type("eulibnet")=FILETYPE_DIRECTORY,dl)
include eulibnet/eulibnet.ew
Ihandle about, help, quit, nickle, nickname, login, 
        memble, members, logle, log_list, 
        messle, input, statusbar, chat_window, timer
integer conn = -1,
        conFlag = 0
bool allj = true -- (suppress initial flurry of fake "has joined" messages)
string nick = ""
string conTextBack = ""
constant IP = "127.0.0.1",
         port = iff(bViaGateway?"29030":"29029"), -- See IRC_Gateway.exw
         timeout = 20,
         IPaddress = IP & ":" & port,
         cr = "\r\n",
         MAX_MSG = 80

constant about_text = """
Translated by Pete Lomax from qchat by Andrew/Norman (and win32lib ==> pGUI).
Uses Libnet by George Foot and Chad Catlet as wrapped by Ray Smith."
"""

function about_cb(Ihandle /*ih*/)
    IupMessage("About",about_text)
    return IUP_DEFAULT
end function

constant help_text = """
Make sure ChatServer.exw is running first...

Enter a nickname and press the Connect Button (or Return), then
enter your messages in the message area.
"""

function help_cb(Ihandln /*ih*/)
    IupMessage("Chat client",help_text)
    return IUP_DEFAULT
end function

procedure message(string message)
    IupSetStrAttribute(statusbar, "TITLE", message)
    -- This should probably be cropped once it gets too long...
    conTextBack &= message & cr 
    IupSetAttribute(log_list,"VALUE",conTextBack)
    IupSetInt(log_list,"SCROLLTOPOS",length(conTextBack))
end procedure

procedure sendMsg(sequence msg)
    if conFlag = 0 then
        message("You must be connected to do this")
        return
    end if
    if net_send_rdm(conn, msg) then
        message("Error sending message \'" & msg & "\'")
    end if
end procedure

procedure shutDown()
    if length(nick) > 0 then
        message("Disconnecting from server...")
        if net_send_rdm(conn, "/d") then
            message("Error disconnecting from server!")
        end if
    end if
    message("Shutting down euLibnet...")
    if conn > -1 then
        if net_closeconn(conn) then
            crash("Error closing connection!")
        end if
    end if
    if net_shutdown() then
        crash("Error shutting down euLibnet!")
    end if
    conFlag = 0
end procedure

function quit_cb(Ihandle /*ih*/)
    shutDown()
    return IUP_CLOSE
end function

function connect_cb(Ihandle /*ih*/)
    nick = IupGetAttribute(nickname,"VALUE")
    if length(nick) then
        message("Opening connection...")
        conn = net_openconn(NET_DRIVER_WSOCK_WIN, NULL)
        if conn = NULL then
            message("Couldn't open connection.")
            return IUP_DEFAULT
        end if
        message("done.")
        message("Attempting to connect to chat server...")
        integer ret = net_connect_wait_time(conn, IPaddress, timeout)
        if ret < 0 then
            message("Error trying to establish connection.")
            return IUP_DEFAULT
        elsif ret > 0 then
            message("Timeout trying to establish connection.")
            return IUP_DEFAULT
        end if
        message("done.")

        conFlag = 1

        IupSetStrAttribute(nickname,"VALUE","Chat Client - " & nick)
        IupSetInt({nickname,login},"ACTIVE",false)
        sendMsg("/n:" & nick)
        IupSetInt(timer,"RUN",true)
        IupSetAttribute(members,"VALUE",nick)
        message("")
        IupSetInt(input,"ACTIVE",true)
        IupSetFocus(input)
    end if
    return IUP_DEFAULT
end function

procedure mainWindow_onOpen()
    message("Initializing euLibnet...")
    if net_init() then crash("Error initializing euLibnet!") end if
    message("done.")
    message("Initializing driver...")
    if net_initdriver(NET_DRIVER_WSOCK_WIN) != 1 then
        crash("Error initializing WinSock driver!")
    end if
    message("done.")
    IupSetFocus(nickname)
end procedure

procedure member_has_joined(string joiner)
    string mt = IupGetAttribute(members,"VALUE")
    sequence ml = split(mt,'\n')
    mt = join(unique(append(ml,joiner)),'\n')
    IupSetStrAttribute(members,"VALUE",mt)
end procedure

procedure member_has_left(string leaver)
    string mt = IupGetAttribute(members,"VALUE")
    sequence ml = split(mt,'\n')
    integer k = find(leaver,ml)
    ml[k..k] = {}   
    IupSetStrAttribute(members,"VALUE",join(ml,'\n'))
end procedure

function timer_cb(Ihandle /*timer*/)
    if net_query_rdm(conn) > 0 then --Check for message
        sequence msg = net_receive_rdm(conn, MAX_MSG)
        if msg[1] < 0 then
           message("Error receiving message!")
           {} = net_ignore_rdm(conn)
        end if
        msg = msg[2]
        if equal(msg, "/nt") then
           message("Nickname " & nick & " already taken")
           nick &= "b"
           message("Trying " & nick & " instead")
           message("Type /n:nickname to try a different one")
           sendMsg("/n:" & nick)
        -- As per ChatServer, bunch of popup and file transfer stuff was 
        -- all ripped out in the name of keeping this short and sweet.
        else
            if length(msg)>3 and equal(msg[1..3], "/j:") then
                member_has_joined(msg[4..match(" has joined",msg)-1])
                if allj then return IUP_DEFAULT end if
                msg = msg[4..$]
            elsif length(msg)>3 and equal(msg[1..3], "/l:") then
                member_has_left(msg[4..match(" has left",msg)-1])
                msg = msg[4..$]
            elsif length(msg)>3 and equal(msg[1..3], "/c:") then
                string shcnts = " has changed name to "
                integer k = match(shcnts,msg)
                member_has_left(msg[4..k-1])
                member_has_joined(msg[k+length(shcnts)-1..$])
                msg = msg[4..$]
            end if
            message(msg)
        end if
        allj = false
    end if
    return IUP_DEFAULT
end function

function key_cb(Ihandle ih, atom c)
    if c=K_ESC then shutDown() return IUP_CLOSE end if
    if c=K_F1 then return help_cb(NULL) end if
    if c='\r' then
        if ih=input then
            sendMsg(IupGetAttribute(input,"VALUE"))
            IupSetAttribute(input,"VALUE","")
        elsif ih=nickname then
            return connect_cb(ih)
        end if
    end if
    return IUP_DEFAULT
end function

IupOpen()
about = IupButton("About",Icallback("about_cb"))
help = IupButton("Help",Icallback("help_cb"))
quit = IupButton("Exit",Icallback("quit_cb"))
nickle = IupLabel("Nickname")
nickname = IupText("EXPAND=HORIZONTAL")
login = IupButton("Connect",Icallback("connect_cb"))
memble = IupLabel("Members online")
members = IupText("EXPAND=VERTICAL, CANFOCUS=NO, MULTILINE=YES, SIZE=70x")
logle = IupLabel("Incoming text")
log_list = IupText("EXPAND=YES, CANFOCUS=NO, MULTILINE=YES")
messle = IupLabel("Message")
input = IupText("EXPAND=HORIZONTAL")
IupSetCallback({nickname,input},"KEY_CB",Icallback("key_cb"))
IupSetAttribute(input,"CUEBANNER","This Is Where You Type")
IupSetInt(input,"NC",72) -- Max 72 characters allowed
IupSetInt(input,"ACTIVE",false)
statusbar = IupLabel("Loading...","EXPAND=HORIZONTAL")
chat_window = IupDialog(IupVbox({IupHbox({about,help,quit,nickle,nickname,login},
                                      "NORMALIZESIZE=VERTICAL,GAP=10,MARGIN=5x5"),
                                IupHbox({IupVbox({memble,members}),
                                         IupVbox({logle,log_list})},
                                                "NGAP=10,NMARGIN=5x5"),
                                IupHbox({messle,input},"GAP=10,MARGIN=5x5"),
                                statusbar}),
            `TITLE="Chat Client", RASTERSIZE=400x400`)
IupSetCallback(chat_window,"CLOSE_CB",Icallback("quit_cb"))
IupSetAttributeHandle(NULL,"PARENTDIALOG",chat_window)
IupShow(chat_window)
mainWindow_onOpen()
timer = IupTimer(Icallback("timer_cb"), 500, false)
IupMainLoop()
IupClose()

PicoLisp

#!/usr/bin/picolisp /usr/lib/picolisp/lib.l

(de chat Lst
   (out *Sock
      (mapc prin Lst)
      (prinl) ) )

(setq *Port (port 4004))

(loop
   (setq *Sock (listen *Port))
   (NIL (fork) (close *Port))
   (close *Sock) )

(out *Sock
   (prin "Please enter your name: ")
   (flush) )
(in *Sock (setq *Name (line T)))

(tell 'chat "+++ " *Name " arrived +++")

(task *Sock
   (in @
      (ifn (eof)
         (tell 'chat *Name "> " (line T))
         (tell 'chat "--- " *Name " left ---")
         (bye) ) ) )
(wait)

After starting the above script, connect to the chat server from two terminals:

           Terminal 1            |           Terminal 2
---------------------------------+---------------------------------
$ telnet localhost 4004          |
Trying ::1...                    |
Trying 127.0.0.1...              |
Connected to localhost.          |
Escape character is '^]'.        |
Please enter your name: Ben      |
                                 | $ telnet localhost 4004
                                 | Trying ::1...
                                 | Trying 127.0.0.1...
                                 | Connected to localhost.
                                 | Escape character is '^]'.
                                 | Please enter your name: Tom
+++ Tom arrived +++              |
Hi Tom                           |
                                 | Ben> Hi Tom
                                 | Hi Ben
Tom> Hi Ben                      |
                                 | How are you?
Tom> How are you?                |
Thanks, fine!                    |
                                 | Ben> Thanks, fine!
                                 | See you!
Tom> See you!                    |
                                 | ^]
                                 | telnet> quit
--- Tom left ---                 |
                                 | Connection closed.
                                 | $

Prolog

Works with Swi-Prolog as of Jan 2019.

This version will load the server automatically on port 5000, adapt to your needs.

:- initialization chat_server(5000).

chat_server(Port) :-
      tcp_socket(Socket),
      tcp_bind(Socket, Port),
      tcp_listen(Socket, 5),
      tcp_open_socket(Socket, AcceptFd, _),
      dispatch(AcceptFd).

dispatch(AcceptFd) :-
        tcp_accept(AcceptFd, Socket, _),
        thread_create(process_client(Socket, _), _, [detached(true)]),
        dispatch(AcceptFd).

process_client(Socket, _) :-
        setup_call_cleanup(
            tcp_open_socket(Socket, Str),
            handle_connection(Str),
            close(Str)).

% a connection was made, get the username and add the streams so the
% client can be broadcast to.
handle_connection(Str) :-
      send_msg(Str, msg_welcome, []),
      repeat,
      send_msg(Str, msg_username, []),
      read_line_to_string(Str, Name),
      connect_user(Name, Str), !.

% connections are stored here
:- dynamic(connected/2).

connect_user(Name, Str) :-
      connected(Name, _),
      send_msg(Str, msg_username_taken, []),
      fail.
connect_user(Name, Str) :-
      \+ connected(Name, _),
      send_msg(Str, msg_welcome_name, Name),

      % make sure that the connection is removed when the client leaves.
      setup_call_cleanup(
          assert(connected(Name, Str)),
          (
              broadcast(Name, msg_joined, Name),
              chat_loop(Name, Str), !,
              broadcast(Name, msg_left, Name)
          ),
          retractall(connected(Name, _))
      ).

% wait for a line to be sent then broadcast to the rest of the clients
% finish this goal when the client disconnects (end of stream)
chat_loop(Name, Str) :-
      read_line_to_string(Str, S),
      dif(S, end_of_file),
      broadcast(Name, msg_by_user, [Name, S]),
      chat_loop(Name, Str).
chat_loop(_, Str) :- at_end_of_stream(Str).

% send a message to all connected clients except Name (the sender)
broadcast(Name, Msg, Params) :-
    forall(
        (connected(N, Str), dif(N, Name)),
        (send_msg(Str, Msg, Params), send_msg(Str, msg_new_line, []))
    ).

send_msg(St, MsgConst, Params) :-
      call(MsgConst, Msg),
      format(St, Msg, Params),
      flush_output(St).

% constants for the various message types that are sent
msg_welcome('Welcome to Chatalot\n\r').
msg_username('Please enter your nickname: ').
msg_welcome_name('Welcome ~p\n\r').
msg_joined(' -- "~w" has joined the chat --').
msg_left(' -- "~w" has left the chat. --').
msg_username_taken('That username is already taken, choose another\n\r').
msg_new_line('\n\r').
msg_by_user('~w> ~w').

Python

#!/usr/bin/env python

import socket
import thread
import time

HOST = ""
PORT = 4004

def accept(conn):
    """
    Call the inner func in a thread so as not to block. Wait for a 
    name to be entered from the given connection. Once a name is 
    entered, set the connection to non-blocking and add the user to 
    the users dict.
    """
    def threaded():
        while True:
            conn.send("Please enter your name: ")
            try:
                name = conn.recv(1024).strip()
            except socket.error:
                continue
            if name in users:
                conn.send("Name entered is already in use.\n")
            elif name:
                conn.setblocking(False)
                users[name] = conn
                broadcast(name, "+++ %s arrived +++" % name)
                break
    thread.start_new_thread(threaded, ())

def broadcast(name, message):
    """
    Send a message to all users from the given name.
    """
    print message
    for to_name, conn in users.items():
        if to_name != name:
            try:
                conn.send(message + "\n")
            except socket.error:
                pass

# Set up the server socket.
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server.setblocking(False)
server.bind((HOST, PORT))
server.listen(1)
print "Listening on %s" % ("%s:%s" % server.getsockname())

# Main event loop.
users = {}
while True:
    try:
        # Accept new connections.
        while True:
            try:
                conn, addr = server.accept()
            except socket.error:
                break
            accept(conn)
        # Read from connections.
        for name, conn in users.items():
            try:
                message = conn.recv(1024)
            except socket.error:
                continue
            if not message:
                # Empty string is given on disconnect.
                del users[name]
                broadcast(name, "--- %s leaves ---" % name)
            else:
                broadcast(name, "%s> %s" % (name, message.strip()))
        time.sleep(.1)
    except (SystemExit, KeyboardInterrupt):
        break

R

This implementation relies on the new server socket connection type introduced in R 4.0.0.

chat_loop <- function(server, sockets, delay = 0.5) {
    repeat {
        Sys.sleep(delay) # Saves CPU resources

        ## Exhausts queue each iteration
        while (in_queue(server))
            sockets <- new_socket_entry(server, sockets)

        ## Update which sockets have sent messages
        sockets <- check_messages(sockets)

        ## No sockets, nothing to do
        if (nrow(sockets) == 0)
            next

        ## No new messages, nothing to do
        if (all(!sockets$message_ready))
            next


        sockets <- read_messages(sockets) # Messages are stored until sent
        sockets <- drop_dead(sockets) # Dead = ready to read, but no data

        ## In case all sockets were dropped
        if (nrow(sockets) == 0)
            next

        sockets <- update_nicknames(sockets)
        sockets <- send_messages(sockets) # Only to users with nicknames
    }
}

check_messages <- function(sockets) {
    if (nrow(sockets) != 0)
        sockets$message_ready <- socketSelect(sockets$conn, timeout = 0)

    sockets
}

drop_dead <- function(sockets) {
    lapply(with(sockets, conn[!alive]), close)
    dropped <- with(sockets, nickname[nickname_exists(sockets) & !alive])
    sockets <- sockets[sockets$alive, ]

    if (length(dropped) != 0) {
        send_named(sockets, paste0(dropped, " has disconnected."))
    }

    sockets
}

in_queue <- function(server) socketSelect(list(server), timeout = 0)
is_valid_name <- function(nicks) gsub("[A-Za-z0-9]*", "", nicks) == ""
message_exists <- function(sockets) !is.na(sockets$message)

new_row <- function(df) {
    df[nrow(df) + 1, ] <- NA
    df
}

new_socket_entry <- function(server, sockets) {
    sockets <- new_row(sockets)
    n <- nrow(sockets)
    within(sockets, {
        conn[[n]] <- new_user(server)
        alive[n] <- TRUE
        message_ready[n] <- FALSE
    })
}

new_user <- function(server) {
    conn <- socketAccept(server)
    writeLines("Hello! Please enter a nickname.", conn)
    conn
}

nickname_exists <- function(sockets) !is.na(sockets$nickname)

read_messages <- function(sockets) {
    if (all(!sockets$message_ready))
        return(sockets)

    msgs <- lapply(with(sockets, conn[message_ready]), readLines, n = 1)
    empty_msgs <- sapply(msgs, identical, character(0))
    sockets <- within(sockets, alive[message_ready & empty_msgs] <- FALSE)
    msgs <- unlist(ifelse(empty_msgs, NA, msgs))
    within(sockets, message[message_ready] <- msgs)
}

send_messages <- function(sockets) {
    named_message <- message_exists(sockets) & nickname_exists(sockets)

    if (all(!named_message))
        return(sockets)

    rows <- which(named_message)
    socksub <- sockets[rows, ]
    time <- format(Sys.time(), "[%H:%M:%S] ")
    with(socksub, send_named(sockets, paste0(time, nickname, ": ", message)))
    within(sockets, message[rows] <- NA)
}

send_named <- function(sockets, msg) {
    has_nickname <- nickname_exists(sockets)
    invisible(lapply(sockets$conn[has_nickname], writeLines, text = msg))
}

start_chat_server <- function(port = 50525) {
    server <- serverSocket(port) # Start listening
    on.exit(closeAllConnections()) # Cleanup connections

    ## All socket data is stored and passed using this object
    sockets <- data.frame(conn = I(list()), nickname = character(),
                          message = character(), alive = logical(),
                          message_ready = logical())

    ## Main event loop
    chat_loop(server, sockets)
}

update_nicknames <- function(sockets) {
    sent_nickname <- message_exists(sockets) & !nickname_exists(sockets)
    nickname_valid <- is_valid_name(sockets$message)

    if (all(!sent_nickname))
        return(sockets)

    is_taken <- with(sockets, (tolower(message) %in% tolower(sockets$nickname)) &
                              !is.na(message))
    sent_ok <- sent_nickname & nickname_valid & !is_taken
    sockets <- within(sockets, {
        nickname[sent_ok] <- message[sent_ok]
        message[sent_nickname] <- NA
        lapply(conn[sent_nickname & !nickname_valid], writeLines,
               text = "Alphanumeric characters only. Try again.")
        lapply(conn[is_taken], writeLines,
               text = "Name already taken. Try again.")
    })

    if (any(sent_ok))
        send_named(sockets, paste0(sockets$nickname[sent_ok], " has connected."))

    sockets
}

start_chat_server()

Racket

This is a very basic chat server, but it does everything that is needed for this task.

#lang racket

(define outs (list (current-output-port)))
(define ((tell-all who o) line)
  (for ([c outs] #:unless (eq? o c)) (displayln (~a who ": " line) c)))

(define ((client i o))
  (define nick (begin (display "Nick: " o) (read-line i)))
  (define tell (tell-all nick o))
  (let loop ([line "(joined)"])
    (if (eof-object? line)
      (begin (tell "(left)") (set! outs (remq o outs)) (close-output-port o))
      (begin (tell line) (loop (read-line i))))))

(define (chat-server listener)
  (define-values [i o] (tcp-accept listener))
  (for ([p (list i o)]) (file-stream-buffer-mode p 'none))
  (thread (client i o)) (set! outs (cons o outs)) (chat-server listener))

(void (thread (λ() (chat-server (tcp-listen 12321)))))
((client (current-input-port) (current-output-port)))

Raku

(formerly Perl 6)

Translation of: Python
(or at least started out that way)
Works with: Rakudo version 2016.07
react {
    my %connections;
    
    whenever IO::Socket::Async.listen('localhost', 4004) -> $conn {
        my $name;
        
        $conn.print: "Please enter your name: ";
        
        whenever $conn.Supply.lines -> $message {
            if !$name {
                if %connections{$message} {
                    $conn.print: "Name already taken, choose another one: ";
                }
                else {
                    $name = $message;
                    %connections{$name} = $conn;
                    broadcast "+++ %s arrived +++", $name;
                }
            }
            else {
                broadcast "%s> %s", $name, $message;
            }
            LAST {
                broadcast "--- %s left ---", $name;
                %connections{$name}:delete;
                $conn.close ;
            }
            QUIT {
                 default {
                     say "oh no, $_";
                }
            }
        }
    }
    
    sub broadcast ($format, $from, *@message) {
        my $text = sprintf $format, $from, |@message;
        say $text;
        for %connections.kv -> $name, $conn {
            $conn.print: "$text\n" if $name ne $from;
        }
    }
}

Notes:

  • It operates asynchronously (using IO::Socket::Async), so a slow connection to one client won't affect other clients.
  • It accepts messages encoded in UTF-8.
  • It tokenizes the message streams at newline boundaries (using the Supply.lines method), which I think makes the most sense for a chat application.

Ruby

require 'gserver'

class ChatServer < GServer
  def initialize *args
    super

    #Keep a list for broadcasting messages
    @chatters = []

    #We'll need this for thread safety
    @mutex = Mutex.new 
  end

  #Send message out to everyone but sender
  def broadcast message, sender = nil
    #Need to use \r\n for our Windows friends
    message = message.strip << "\r\n"

    #Mutex for safety - GServer uses threads
    @mutex.synchronize do
      @chatters.each do |chatter|
        begin
          chatter.print message unless chatter == sender
        rescue
          @chatters.delete chatter
        end
      end
    end
  end

  #Handle each connection
  def serve io
    io.print 'Name: '
    name = io.gets

    #They might disconnect
    return if name.nil?

    name.strip!

    broadcast "--+ #{name} has joined +--"

    #Add to our list of connections
    @mutex.synchronize do
      @chatters << io
    end

    #Get and broadcast input until connection returns nil
    loop do
      message = io.gets

      if message
        broadcast "#{name}> #{message}", io
      else
        break
      end
    end

    broadcast "--+ #{name} has left +--"
  end
end

#Start up the server on port 7000
#Accept connections for any IP address
#Allow up to 100 connections
#Send information to stderr
#Turn on informational messages
ChatServer.new(7000, '0.0.0.0', 100, $stderr, true).start.join

Rust

use std::collections::HashMap;
use std::io;
use std::io::prelude::*;
use std::io::BufReader;
use std::net::{TcpListener, TcpStream};
use std::sync::{Arc, RwLock};
use std::thread;

type Username = String;

/// Sends a message to all clients except the sending client.
fn broadcast_message(
    user: &str,
    clients: &mut HashMap<String, TcpStream>,
    message: &str,
) -> io::Result<()> {
    for (client, stream) in clients.iter_mut() {
        if client != user {
            writeln!(stream, "{}", message)?;
        }
    }

    Ok(())
}

fn chat_loop(listener: &TcpListener) -> io::Result<()> {
    let local_clients: Arc<RwLock<HashMap<Username, TcpStream>>> =
        Arc::new(RwLock::new(HashMap::new()));

    println!("Accepting connections on {}", listener.local_addr()?.port());

    for stream in listener.incoming() {
        match stream {
            Ok(stream) => {
                let client_clients = Arc::clone(&local_clients);
                thread::spawn(move || -> io::Result<()> {
                    let mut reader = BufReader::new(stream.try_clone()?);
                    let mut writer = stream;

                    let mut name = String::new();
                    loop {
                        write!(writer, "Please enter a username: ")?;
                        reader.read_line(&mut name)?;
                        name = name.trim().to_owned();

                        let clients = client_clients.read().unwrap();
                        if !clients.contains_key(&name) {
                            writeln!(writer, "Welcome, {}!", &name)?;
                            break;
                        }

                        writeln!(writer, "That username is taken.")?;
                        name.clear();
                    }

                    {
                        let mut clients = client_clients.write().unwrap();
                        clients.insert(name.clone(), writer);
                        broadcast_message(
                            &name,
                            &mut *clients,
                            &format!("{} has joined the chat room.", &name),
                        )?;
                    }

                    for line in reader.lines() {
                        let mut clients = client_clients.write().unwrap();
                        broadcast_message(&name, &mut *clients, &format!("{}: {}", &name, line?))?;
                    }

                    {
                        let mut clients = client_clients.write().unwrap();
                        clients.remove(&name);
                        broadcast_message(
                            &name,
                            &mut *clients,
                            &format!("{} has left the chat room.", &name),
                        )?;
                    }

                    Ok(())
                });
            }
            Err(e) => {
                println!("Connection failed: {}", e);
            }
        }
    }

    Ok(())
}

fn main() {
    let listener = TcpListener::bind(("localhost", 7000)).unwrap();
    chat_loop(&listener).unwrap();
}

Tcl

Works with: Tcl version 8.6
package require Tcl 8.6

# Write a message to everyone except the sender of the message
proc writeEveryoneElse {sender message} {
    dict for {who ch} $::cmap {
	if {$who ne $sender} {
	    puts $ch $message
	}
    }
}

# How to read a line (up to 256 chars long) in a coroutine
proc cgets {ch var} {
    upvar 1 $var v
    while {[gets $ch v] < 0} {
	if {[eof $ch] || [chan pending input $ch] > 256} {
	    return false
	}
	yield
    }
    return true
}

# The chatting, as seen by one user
proc chat {ch addr port} {
    ### CONNECTION CODE ###
    #Log "connection from ${addr}:${port} on channel $ch"
    fconfigure $ch -buffering none -blocking 0 -encoding utf-8
    fileevent $ch readable [info coroutine]
    global cmap
    try {

	### GET THE NICKNAME OF THE USER ###
	puts -nonewline $ch "Please enter your name: "
	if {![cgets $ch name]} {
	    return
	}
	#Log "Mapping ${addr}:${port} to ${name} on channel $ch"
	dict set cmap $name $ch
	writeEveryoneElse $name "+++ $name arrived +++"

	### MAIN CHAT LOOP ###
	while {[cgets $ch line]} {
	    writeEveryoneElse $name "$name> $line"
	}

    } finally {
	### DISCONNECTION CODE ###
	if {[info exists name]} {
	    writeEveryoneElse $name "--- $name left ---"
	    dict unset cmap $name
	}
	close $ch
	#Log "disconnection from ${addr}:${port} on channel $ch"
    }
}

# Service the socket by making corouines running [chat]
socket -server {coroutine c[incr count] chat} 4004
set ::cmap {};		# Dictionary mapping nicks to channels
vwait forever;		# Run event loop

Wren

An embedded solution using a C host since Wren has no built in support for either networking or multi-threading.

The following is based on the C code here and runs fine on my Ubuntu 20.04 box.

As Wren's VM is single threaded we create separate VMs to service each potential client connection (limited to 10) which run in their own thread. As the only way for the VMs to share mutable state is to use global variables within the host, synchronization is needed when accessing such variables.

/* Chat_server.wren */

class Clients {
    foreign static max
    foreign static count
    foreign static isActive(vmi)
    foreign static connfd(vmi)
    foreign static uid(vmi)
    foreign static name(vmi)
    foreign static setName(vmi, s)
    foreign static printAddr(vmi)
    foreign static delete(vmi)
}

class Mutex {
    foreign static clientsLock()
    foreign static clientsUnlock()

    foreign static topicLock()
    foreign static topicUnlock()
}

class Chat {
    // send message to all clients but the sender
    static sendMessage(s, uid) {
        Mutex.clientsLock()
        for (i in 0...Clients.max) {
            if (Clients.isActive(i) && Clients.uid(i) != uid) {
                if (write(Clients.connfd(i), s, s.bytes.count) < 0) {
                    System.print("Write to descriptor %(Clients.connfd(i)) failed.")
                    break
                }
            }
        }
        Mutex.clientsUnlock()
    }

    // send message to all clients
    static sendMessageAll(s) {
        Mutex.clientsLock()
        for (i in 0...Clients.max) {
            if (Clients.isActive(i)) {
                if (write(Clients.connfd(i), s, s.bytes.count) < 0) {
                    System.print("Write to descriptor %(Clients.connfd(i)) failed.")
                    break
                }
            }
        }
        Mutex.clientsUnlock()
    }

    // send message to sender
    static sendMessageSelf(s, connfd) {
        if (write(connfd, s, s.bytes.count) < 0) {
            Fiber.abort("Write to descriptor %(connfd) failed.")
        }
    }

    // send message to client
    static sendMessageClient(s, uid) {
        Mutex.clientsLock()
        for (i in 0...Clients.max) {
            if (Clients.isActive(i) && Clients.uid(i) == uid) {
                if (write(Clients.connfd(i), s, s.bytes.count) < 0) {
                    System.print("Write to descriptor %(Clients.connfd(i)) failed.")
                    break
                }
            }
        }
        Mutex.clientsUnlock()
    }

    // send list of active clients
    static sendActiveClients(connfd) {
        Mutex.clientsLock()
        for (i in 0...Clients.max) {
            if (Clients.isActive(i)) {
                var s = "<< [%(Clients.uid(i))] %(Clients.name(i))\r\n"
                sendMessageSelf(s, connfd)
            }
        }
        Mutex.clientsUnlock()
    }

    // handle all communication with the client
    static handleClient(vmi) {
        if (!Clients.isActive(vmi)) {
            Fiber.abort("The client handled by VM[%(vmi)] is inactive.")
        }
        var connfd = Clients.connfd(vmi)
        var uid    = Clients.uid(vmi)
        var name   = Clients.name(vmi)
        System.write("<< accept ")
        Clients.printAddr(vmi)
        System.print(" referenced by %(uid)")
        var buffOut = "<< %(name) has joined\r\n"
        sendMessageAll(buffOut)
        Mutex.topicLock()
        if (topic != "") {
            buffOut = "<< topic: %(topic)\r\n"
            sendMessageSelf(buffOut, connfd)
        }
        Mutex.topicUnlock()
        sendMessageSelf("<< see /help for assistance\r\n", connfd)

        /* receive input from client */
        var buffIn = ""
        while ((buffIn = read(connfd, bufferSize/2 - 1)) && buffIn.bytes.count > 0) {
            buffOut = ""
            buffIn = buffIn.trimEnd("\r\n") 
 
            /* ignore empty buffer */
            if (buffIn == "") continue

            /* special options */
            if (buffIn[0] == "/") {
                var split = buffIn.split(" ")
                var command = split[0]
                if (command == "/quit") {
                    break
                } else if (command == "/ping") {
                    sendMessageSelf("<< pong\r\n", connfd)
                } else if (command == "/topic") {
                    if (split.count > 0) {
                        Mutex.topicLock()
                        topic = split[1..-1].join(" ")
                        Mutex.topicUnlock()
                        buffOut = "<< topic changed to: %(topic)\r\n"
                        sendMessageAll(buffOut)
                    } else {
                        sendMessageSelf("<< message cannot be null\r\n", connfd)
                    }
                } else if (command == "/nick") {
                    if (split.count > 0) {
                        var newName = split[1..-1].join(" ")
                        buffOut = "<< %(name) is now known as %(newName)\r\n"
                        Clients.setName(vmi, newName)
                        name = newName
                        sendMessageAll(buffOut)
                    } else {
                        sendMessageSelf("<< name cannot be null\r\n", connfd)
                    }
                } else if (command == "/msg") {
                    if (split.count > 0) {
                        var toUid = Num.fromString(split[1])
                        if (split.count > 1) {
                            buffOut = "[PM][%(name)] "
                            buffOut = buffOut + split[2..-1].join(" ") + "\r\n"
                            sendMessageClient(buffOut, toUid)
                        } else {
                            sendMessageSelf("<< message cannot be null\r\n", connfd)
                        }
                    } else {
                        sendMessageSelf("<< reference cannot be null\r\n", connfd)
                    }
                } else if (command == "/list") {
                    buffOut = "<< clients %(Clients.count)\r\n"
                    sendMessageSelf(buffOut, connfd)
                    sendActiveClients(connfd)
                } else if (command == "/help") {
                    buffOut = ""
                    buffOut = buffOut + "<< /quit      Quit chatroom\r\n"
                    buffOut = buffOut + "<< /ping      Server test\r\n"
                    buffOut = buffOut + "<< /topic     <message> Set chat topic\r\n"
                    buffOut = buffOut + "<< /nick      <name> Change nickname\r\n"
                    buffOut = buffOut + "<< /msg       <reference> <message> Send private message\r\n"
                    buffOut = buffOut + "<< /list      Show active clients\r\n"
                    buffOut = buffOut + "<< /help      Show help\r\n"
                    sendMessageSelf(buffOut, connfd)
                } else {
                    sendMessageSelf("<< unknown command\r\n", connfd)
                }
            } else {
                /* send message */
                buffOut = "[%(name)] %(buffIn)\r\n"
                sendMessage(buffOut, uid)
            }
         }

         /* close connection */
         buffOut = "<< [%(name)] has left\r\n"
         sendMessageAll(buffOut)
         close(connfd)

         /* delete client from queue and yield thread (from C side) */
         System.write("<< quit ")
         Clients.printAddr(vmi)
         System.print(" referenced by %(uid)")          
         Clients.delete(vmi)         
     }

     foreign static topic 
     foreign static topic=(s)

     foreign static bufferSize
     foreign static write(connfd, buf, count)
     foreign static read(connfd, count)
     foreign static close(connfd)
}


We now embed this in the following C program, build and run it to start the server. To end the server, just press control-C. For testing purposes, clients can use telnet from separate terminals to connect to the server on port 5000.

/* gcc Chat_server.c -o Chat_server -lpthread -lwren -lm */

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <pthread.h>
#include <sys/types.h>
#include <signal.h>
#include <wren.h>

#define MAX_CLIENTS 10
#define BUFFER_SZ 2048

static _Atomic unsigned int cli_count = 0;
static int listenfd = 0, uid = 10;
char *script = NULL;

/* Client structure */
typedef struct {
    struct sockaddr_in addr; /* Client remote address */
    int connfd;              /* Connection file descriptor */
    int uid;                 /* Client unique identifier */
    int vmi;                 /* The index of the VM which handles this client */
    char name[32];           /* Client name */
} client_t;

client_t *clients[MAX_CLIENTS];

pthread_mutex_t clients_mutex = PTHREAD_MUTEX_INITIALIZER;

static char topic[BUFFER_SZ/2];

pthread_mutex_t topic_mutex = PTHREAD_MUTEX_INITIALIZER;

WrenVM* vms[MAX_CLIENTS]; // array of VMs

/* add client to queue */
void queue_add(client_t *cl){
    pthread_mutex_lock(&clients_mutex);
    for (int i = 0; i < MAX_CLIENTS; ++i) {
        if (!clients[i]) {
            cl->vmi = i;
            clients[i] = cl; 
            break;
        }
    }
    pthread_mutex_unlock(&clients_mutex);
}

/* Delete client from queue */
void queue_delete(int uid){
    pthread_mutex_lock(&clients_mutex);
    for (int i = 0; i < MAX_CLIENTS; ++i) {
        if (clients[i]) {
            if (clients[i]->uid == uid) {
                clients[i] = NULL;
                break;
            }
        }
    }
    pthread_mutex_unlock(&clients_mutex);
}

/* print ip address */
void print_client_addr(struct sockaddr_in addr){
    printf("%d.%d.%d.%d",
        addr.sin_addr.s_addr & 0xff,
        (addr.sin_addr.s_addr & 0xff00) >> 8,
        (addr.sin_addr.s_addr & 0xff0000) >> 16,
        (addr.sin_addr.s_addr & 0xff000000) >> 24);
}

/* enable Wren to handle all client communication */
void *handle_client(void *arg) {
    client_t *cli = (client_t *)arg;
    cli_count++;
    int vmi = cli->vmi;
    WrenHandle *callHandle = wrenMakeCallHandle(vms[vmi], "handleClient(_)");
    wrenEnsureSlots(vms[vmi], 2);
    wrenGetVariable(vms[vmi], "main", "Chat", 0);
    wrenSetSlotDouble(vms[vmi], 1, (double)vmi);
    wrenCall(vms[vmi], callHandle);
}

/* C <= Wren interface functions */

void C_max(WrenVM* vm) {
    wrenSetSlotDouble(vm, 0, (double)MAX_CLIENTS);
}

void C_count(WrenVM* vm) {
    wrenSetSlotDouble(vm, 0, (double)cli_count);
}

void C_isActive(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    bool res = clients[vmi] != NULL;
    wrenSetSlotBool(vm, 0, res);
}

void C_connfd(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    wrenSetSlotDouble(vm, 0, (double)clients[vmi]->connfd);
}

void C_uid(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    wrenSetSlotDouble(vm, 0, (double)clients[vmi]->uid);
}

void C_name(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    wrenSetSlotString(vm, 0, (const char *)clients[vmi]->name);
}

void C_setName(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    const char *name = wrenGetSlotString(vm, 2);
    size_t size = sizeof(clients[vmi]->name);
    strncpy(clients[vmi]->name, name, size);
    clients[vmi]->name[size-1] = '\0';
}

void C_clientsLock(WrenVM* vm) {
    pthread_mutex_lock(&clients_mutex);
}

void C_clientsUnlock(WrenVM* vm) {
    pthread_mutex_unlock(&clients_mutex);
}

void C_topicLock(WrenVM* vm) {
    pthread_mutex_lock(&topic_mutex);
}

void C_topicUnlock(WrenVM* vm) {
    pthread_mutex_unlock(&topic_mutex);
}

void C_printAddr(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    print_client_addr(clients[vmi]->addr);
}

void C_topic(WrenVM* vm) {
    wrenSetSlotString(vm, 0, (const char *)topic);
}

void C_setTopic(WrenVM* vm) {
    const char *t = wrenGetSlotString(vm, 1);
    strncpy(topic, t, sizeof(topic));
    topic[sizeof(topic)-1] = '\0';
}

void C_bufferSize(WrenVM* vm) {
    wrenSetSlotDouble(vm, 0, (double)BUFFER_SZ);
}

void C_write(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    const void *buf = (const void *)wrenGetSlotString(vm, 2);
    size_t count = (size_t)wrenGetSlotDouble(vm, 3);
    ssize_t res = write(fd, buf, count);
    wrenSetSlotDouble(vm, 0, (double)res);
}

void C_read(WrenVM* vm) {
    char buf[BUFFER_SZ / 2];
    int fd = (int)wrenGetSlotDouble(vm, 1);
    size_t count = (size_t)wrenGetSlotDouble(vm, 2);
    ssize_t rlen = read(fd, buf, count);
    buf[rlen] = '\0';
    wrenSetSlotString(vm, 0, (const char *)buf);
}

void C_close(WrenVM* vm) {
    int connfd = (int)wrenGetSlotDouble(vm, 1);
    close(connfd);
}

void C_delete(WrenVM* vm) {
    int vmi = (int)wrenGetSlotDouble(vm, 1);
    client_t *cli = clients[vmi];
    queue_delete(cli->uid);
    free(cli);
    cli_count--;
    pthread_detach(pthread_self());
}

WrenForeignMethodFn bindForeignMethod(
    WrenVM* vm,
    const char* module,
    const char* className,
    bool isStatic,
    const char* signature) {
    if (strcmp(module, "main") == 0) {
        if (strcmp(className, "Clients") == 0) {
            if (isStatic && strcmp(signature, "max") == 0)             return C_max;
            if (isStatic && strcmp(signature, "count") == 0)           return C_count;
            if (isStatic && strcmp(signature, "isActive(_)") == 0)     return C_isActive;
            if (isStatic && strcmp(signature, "connfd(_)") == 0)       return C_connfd;
            if (isStatic && strcmp(signature, "uid(_)") == 0)          return C_uid;
            if (isStatic && strcmp(signature, "name(_)") == 0)         return C_name;
            if (isStatic && strcmp(signature, "setName(_,_)") == 0)    return C_setName;
            if (isStatic && strcmp(signature, "printAddr(_)") == 0)    return C_printAddr;
            if (isStatic && strcmp(signature, "delete(_)") == 0)       return C_delete;       
        } else if (strcmp(className, "Mutex") == 0) {
            if (isStatic && strcmp(signature, "clientsLock()") == 0)   return C_clientsLock;
            if (isStatic && strcmp(signature, "clientsUnlock()") == 0) return C_clientsUnlock;
            if (isStatic && strcmp(signature, "topicLock()") == 0)     return C_topicLock;
            if (isStatic && strcmp(signature, "topicUnlock()") == 0)   return C_topicUnlock;
        } else if (strcmp(className, "Chat") == 0) {
            if (isStatic && strcmp(signature, "topic") == 0)           return C_topic;
            if (isStatic && strcmp(signature, "topic=(_)") == 0)       return C_setTopic;
            if (isStatic && strcmp(signature, "bufferSize") == 0)      return C_bufferSize;
            if (isStatic && strcmp(signature, "write(_,_,_)") == 0)    return C_write;
            if (isStatic && strcmp(signature, "read(_,_)") == 0)       return C_read;
            if (isStatic && strcmp(signature, "close(_)") == 0)        return C_close;
        }
    }
    return NULL;
}

static void writeFn(WrenVM* vm, const char* text) {
    printf("%s", text);
}

void errorFn(WrenVM* vm, WrenErrorType errorType, const char* module, const int line, const char* msg) {
    switch (errorType) {
        case WREN_ERROR_COMPILE:
            printf("[%s line %d] [Error] %s\n", module, line, msg);
            break;
        case WREN_ERROR_STACK_TRACE:
            printf("[%s line %d] in %s\n", module, line, msg);
            break;
        case WREN_ERROR_RUNTIME:
            printf("[Runtime Error] %s\n", msg);
            break;
    }
}

char *readFile(const char *fileName) {
    FILE *f = fopen(fileName, "r");
    fseek(f, 0, SEEK_END);
    long fsize = ftell(f);
    rewind(f);
    char *script = malloc(fsize + 1);
    fread(script, 1, fsize, f);
    fclose(f);
    script[fsize] = 0;
    return script;
}

void catch_ctrl_c(int sig) {
    /* clean up and exit */
    for (int i = 0; i < MAX_CLIENTS; ++i) {
        wrenFreeVM(vms[i]);
        if (clients[i]) {
            close(clients[i]->connfd);
            free(clients[i]);
        }
    }
    close(listenfd);
    free(script);
    printf("\n<[ SERVER ENDED ]>\n");
    exit(EXIT_SUCCESS);
}

int main(int argc, char **argv) {
    WrenConfiguration config;
    wrenInitConfiguration(&config);
    config.writeFn = &writeFn;
    config.errorFn = &errorFn;
    config.bindForeignMethodFn = &bindForeignMethod;
    const char* module = "main";
    const char* fileName = "Chat_server.wren";
    script = readFile(fileName);

    /* config the VMs and interpret the script */
    for (int i = 0; i < MAX_CLIENTS; ++i) {
        vms[i] = wrenNewVM(&config);
        wrenInterpret(vms[i], module, script);
    }

    /* prepare to start the server */
    int connfd = 0;
    struct sockaddr_in serv_addr;
    struct sockaddr_in cli_addr;
    pthread_t tid;

    /* socket settings */
    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(5000);

    /* ignore pipe signals */
    signal(SIGPIPE, SIG_IGN);

    /* catch ctrl-c being pressed */
    signal(SIGINT, catch_ctrl_c);

    /* bind */
    if (bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("Socket binding failed");
        return EXIT_FAILURE;
    }

    /* listen */
    if (listen(listenfd, 10) < 0) {
        perror("Socket listening failed");
        return EXIT_FAILURE;
    }

    printf("<[ SERVER STARTED ]>\n");

    /* accept clients */
    while (1) {
        socklen_t clilen = sizeof(cli_addr);
        connfd = accept(listenfd, (struct sockaddr*)&cli_addr, &clilen);

        /* check if max clients is reached */
        if ((cli_count + 1) == MAX_CLIENTS) {
            printf("<< max clients reached\n");
            printf("<< reject ");
            print_client_addr(cli_addr);
            printf("\n");
            close(connfd);
            continue;
        }

        /* client settings */
        client_t *cli = (client_t *)malloc(sizeof(client_t));
        cli->addr = cli_addr;
        cli->connfd = connfd;
        cli->uid = uid++;
        sprintf(cli->name, "%d", cli->uid);

        /* add client to the queue and fork thread */
        queue_add(cli);
        pthread_create(&tid, NULL, &handle_client, (void*)cli);

        /* reduce CPU usage */
        sleep(1);
    }

    return 0;
}

zkl

Translation of: Python

On my Linux box, telnet seems to only want to connect to port 23.

const PORT=23;
 
var users=Dictionary(); // ( handle:socket, ...)
pipe:=Thread.Pipe(); // how server tells thread to connect to user

fcn accept(pipe){ // a thread waiting for the server to send a socket
   while(socket:=pipe.read()){
      println("Somebody is connecting ...");
      socket.read();  // telnet stuff
      while(True){    // get credentials
	 reg name;
	 socket.write("Your handle: ");  // bottle neck
	 try{ name = socket.read().text.strip() } catch(IOError){ continue }
	 if(users.holds(name)) socket.write("Handle is already in use.\n");
	 else if(name){
	    users[name] = socket;
	    chat.launch(name,socket);	// thread
	    broadcast(name, "+++ %s arrived +++".fmt(name));
	    break;	// wait for next connection
	 }
      }//while
   }//while
}.launch(pipe); // thread
 
fcn chat(name,socket){		// a thread, one per user
   try{
      socket.write("^D to disconnect\n");
      while(True){
	 message:=socket.read().text.strip();
	 if(message=="\xff\xec") break; // ^D to disconnect.
	 broadcast(name, "%s> %s".fmt(name,message));
      }
   }catch{}  // eg socket pukes
   users.del(name); socket.close(); 
   broadcast(name, "--- %s leaves ---".fmt(name));
}

    // Send a message to all users from the given name.
fcn broadcast(name, message){ // called from user thread
   println(message);  // log message to server console
   users.pump(Void,'wrap([(toName,socket)]){
      if(toName != name) try{ socket.write(message + "\n") } catch(IOError){}
   });
}

   // Set up the server socket.
server:=Network.TCPServerSocket.open(PORT);
println("Listening on %s:%s".fmt(server.hostname,server.port));
server.listen(pipe);  // Main event loop
Output:

Start the server:

$ sudo ../../../Bin/zkl chatServer.zkl
Listening on Octavius:23
Somebody is connecting ...
+++ sam arrived +++
--- sam leaves ---
Somebody is connecting ...
+++ sam arrived +++
Somebody is connecting ...
+++ craig arrived +++
craig> this is a test
sam> sam i am
--- sam leaves ---
^\

Another terminal:

$ telnet localhost
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Your handle: sam
^D to disconnect
+++ craig arrived +++
craig> this is a test
sam i am
^D
Connection closed by foreign host.

And yet another terminal:

$ telnet localhost
...
Your handle: craig
^D to disconnect
this is a test
sam> sam i am
--- sam leaves ---
Connection closed by foreign host.