Echo server

From Rosetta Code
(Redirected from Echo Server)
Task
Echo server
You are encouraged to solve this task according to the task description, using any language you may know.

Create a network service that sits on TCP port 12321, which accepts connections on that port, and which echoes complete lines (using a carriage-return/line-feed sequence as line separator) back to clients. No error handling is required. For the purposes of testing, it is only necessary to support connections from localhost (127.0.0.1 or perhaps ::1). Logging of connection information to standard output is recommended.

The implementation must be able to handle simultaneous connections from multiple clients. A multi-threaded or multi-process solution may be used. Each connection must be able to echo more than a single line.

The implementation must not stop responding to other clients if one client sends a partial line or stops reading responses.

Ada[edit]

Works with: GNAT

single-threaded, one client served at a time.

with Ada.Text_IO;
with Ada.IO_Exceptions;
with GNAT.Sockets;
procedure Echo_Server is
   Receiver   : GNAT.Sockets.Socket_Type;
   Connection : GNAT.Sockets.Socket_Type;
   Client     : GNAT.Sockets.Sock_Addr_Type;
   Channel    : GNAT.Sockets.Stream_Access;
begin
   GNAT.Sockets.Create_Socket (Socket => Receiver);
   GNAT.Sockets.Set_Socket_Option
     (Socket => Receiver,
      Level  => GNAT.Sockets.Socket_Level,
      Option => (Name    => GNAT.Sockets.Reuse_Address, Enabled => True));
   GNAT.Sockets.Bind_Socket
     (Socket  => Receiver,
      Address => (Family => GNAT.Sockets.Family_Inet,
                  Addr   => GNAT.Sockets.Inet_Addr ("127.0.0.1"),
                  Port   => 12321));
   GNAT.Sockets.Listen_Socket (Socket => Receiver);
   loop
      GNAT.Sockets.Accept_Socket
        (Server  => Receiver,
         Socket  => Connection,
         Address => Client);
      Ada.Text_IO.Put_Line
        ("Client connected from " & GNAT.Sockets.Image (Client));
      Channel := GNAT.Sockets.Stream (Connection);
      begin
         loop
            Character'Output (Channel, Character'Input (Channel));
         end loop;
      exception
         when Ada.IO_Exceptions.End_Error =>
            null;
      end;
      GNAT.Sockets.Close_Socket (Connection);
   end loop;
end Echo_Server;

Multi-threaded, multiple clients served. On OS X 10.10.5 with gcc 4.9.1, serves a maximum of about 2000 threads (communication tasks) per process.

with Ada.Text_IO;
with Ada.IO_Exceptions;
with GNAT.Sockets;
procedure echo_server_multi is
-- Multiple socket connections example based on Rosetta Code echo server.

   Tasks_To_Create : constant := 3; -- simultaneous socket connections.

-------------------------------------------------------------------------------
-- Use stack to pop the next free task index. When a task finishes its
-- asynchronous (no rendezvous) phase, it pushes the index back on the stack.
   type Integer_List is array (1..Tasks_To_Create) of integer;
   subtype Counter is integer range 0 .. Tasks_To_Create;
   subtype Index is integer range 1 .. Tasks_To_Create;
   protected type Info is
      procedure Push_Stack (Return_Task_Index : in Index);
      procedure Initialize_Stack;
      entry Pop_Stack (Get_Task_Index : out Index);
   private
      Task_Stack   : Integer_List; -- Stack of free-to-use tasks.
      Stack_Pointer: Counter := 0;
   end Info;

   protected body Info is
      procedure Push_Stack (Return_Task_Index : in Index) is
      begin -- Performed by tasks that were popped, so won't overflow.
         Stack_Pointer := Stack_Pointer + 1;
         Task_Stack(Stack_Pointer) := Return_Task_Index;
      end;

      entry Pop_Stack (Get_Task_Index : out Index) when Stack_Pointer /= 0 is
      begin -- guarded against underflow.
         Get_Task_Index := Task_Stack(Stack_Pointer);
         Stack_Pointer := Stack_Pointer -  1;
      end;         

      procedure Initialize_Stack is
      begin
         for I in Task_Stack'range loop
            Push_Stack (I);
         end loop;
      end;
   end Info;

   Task_Info : Info;
     
-------------------------------------------------------------------------------
   task type SocketTask is
      -- Rendezvous the setup, which sets the parameters for entry Echo.
      entry Setup (Connection : GNAT.Sockets.Socket_Type;
                   Client     : GNAT.Sockets.Sock_Addr_Type;
                   Channel    : GNAT.Sockets.Stream_Access;
                   Task_Index : Index);
      -- Echo accepts the asynchronous phase, i.e. no rendezvous. When the
      -- communication is over, push the task number back on the stack.
      entry Echo;
   end SocketTask;

   task body SocketTask is
      my_Connection : GNAT.Sockets.Socket_Type;
      my_Client     : GNAT.Sockets.Sock_Addr_Type;
      my_Channel    : GNAT.Sockets.Stream_Access;
      my_Index      : Index;
   begin
      loop -- Infinitely reusable
         accept Setup (Connection : GNAT.Sockets.Socket_Type; 
                       Client  : GNAT.Sockets.Sock_Addr_Type; 
                       Channel : GNAT.Sockets.Stream_Access;
                       Task_Index   : Index) do
            -- Store parameters and mark task busy.
            my_Connection := Connection;
            my_Client     := Client;
            my_Channel    := Channel;
            my_Index      := Task_Index;
         end;
   
         accept Echo; -- Do the echo communications.
         begin
            Ada.Text_IO.Put_Line ("Task " & integer'image(my_Index));
            loop
               Character'Output (my_Channel, Character'Input(my_Channel));
            end loop;
         exception
            when Ada.IO_Exceptions.End_Error =>
              Ada.Text_IO.Put_Line ("Echo " & integer'image(my_Index) & " end");
            when others => 
              Ada.Text_IO.Put_Line ("Echo " & integer'image(my_Index) & " err");
         end;
         GNAT.Sockets.Close_Socket (my_Connection);
         Task_Info.Push_Stack (my_Index); -- Return to stack of unused tasks.
      end loop;
   end SocketTask;

-------------------------------------------------------------------------------
-- Setup the socket receiver, initialize the task stack, and then loop,
-- blocking on Accept_Socket, using Pop_Stack for the next free task from the
-- stack, waiting if necessary.
   task type SocketServer (my_Port : GNAT.Sockets.Port_Type) is
      entry Listen;
   end SocketServer;

   task body SocketServer is
   Receiver   : GNAT.Sockets.Socket_Type;
   Connection : GNAT.Sockets.Socket_Type;
   Client     : GNAT.Sockets.Sock_Addr_Type;
   Channel    : GNAT.Sockets.Stream_Access;
   Worker     : array (1..Tasks_To_Create) of SocketTask;
   Use_Task   : Index;

   begin
      accept Listen;
      GNAT.Sockets.Create_Socket (Socket => Receiver);
      GNAT.Sockets.Set_Socket_Option
        (Socket => Receiver,
         Level  => GNAT.Sockets.Socket_Level,
         Option => (Name    => GNAT.Sockets.Reuse_Address, Enabled => True));
      GNAT.Sockets.Bind_Socket
        (Socket  => Receiver,
         Address => (Family => GNAT.Sockets.Family_Inet,
                     Addr   => GNAT.Sockets.Inet_Addr ("127.0.0.1"),
                     Port   => my_Port));
      GNAT.Sockets.Listen_Socket (Socket => Receiver);
      Task_Info.Initialize_Stack;
Find: loop -- Block for connection and take next free task.
         GNAT.Sockets.Accept_Socket
           (Server  => Receiver,
            Socket  => Connection,
            Address => Client);
         Ada.Text_IO.Put_Line ("Connect " & GNAT.Sockets.Image(Client));
         Channel := GNAT.Sockets.Stream (Connection);
         Task_Info.Pop_Stack(Use_Task); -- Protected guard waits if full house.
         -- Setup the socket in this task in rendezvous.
         Worker(Use_Task).Setup(Connection,Client, Channel,Use_Task);
         -- Run the asynchronous task for the socket communications.
         Worker(Use_Task).Echo; -- Start echo loop.
      end loop Find;
   end SocketServer;

   Echo_Server : SocketServer(my_Port => 12321);

-------------------------------------------------------------------------------
begin
   Echo_Server.Listen;
end echo_server_multi;

Aime[edit]

void
readc(dispatch w, file i, file o, data b)
{
    integer e;
    data t;

    while (1) {
        e = f_b_read(i, t, 1 << 10);
        if (e < 1) {
            if (e == -1) {
                w_resign(w, i);
            }

            break;
        } else {
            e = b_frame(t, '\n');
            if (e != -1) {
                e += 1;
                b_rule(b, -1, t, 0, e);
                f_data(o, b);
                w_register(w, o);
                b_ecopy(b, t, e, ~t - e);
            } else {
                b_add(b, t);
            }
        }
    }
}

void
serve(dispatch w, file s)
{
    file i, o;
    data b;

    accept(i, o, s, NONBLOCKING_INPUT | NONBLOCKING_OUTPUT);
    w.watch(i, readc, w, i, o, b);
}

integer
main(void)
{
    dispatch w;
    file s;

    tcpip_listen(s, 12321, 0);
    w.watch(s, serve, w, s);
    w.press;

    0;
}

AutoHotkey[edit]

echoserver.ahk, modified from script by zed gecko.

#SingleInstance Force
Network_Port = 12321
Network_Address = 127.0.0.1

NewData := false
DataReceived =
Gosub Connection_Init
return

Connection_Init:
OnExit, ExitSub
socket := PrepareForIncomingConnection(Network_Address, Network_Port)
if socket = -1
    ExitApp

Process, Exist
DetectHiddenWindows On
ScriptMainWindowId := WinExist("ahk_class AutoHotkey ahk_pid " . ErrorLevel)
DetectHiddenWindows Off

NotificationMsg = 0x5555
OnMessage(NotificationMsg, "ReceiveData")

ExitMsg = 0x6666
OnMessage(ExitMsg, "ExitData")

FD_READ = 1
FD_CLOSE = 32
FD_CONNECT = 20

if DllCall("Ws2_32\WSAAsyncSelect", "UInt", socket, 
      "UInt", ScriptMainWindowId, "UInt", ExitMsg, "Int", FD_CLOSE)
{
    msgbox, closed
}

if DllCall("Ws2_32\WSAAsyncSelect", "UInt", socket, 
        "UInt", ScriptMainWindowId, "UInt", NotificationMsg, "Int",
        FD_READ|FD_CONNECT) 
{
    MsgBox % "WSAAsyncSelect() indicated Winsock error "
          . DllCall("Ws2_32\WSAGetLastError")
    DllCall("Ws2_32\WSAAsyncSelect", "UInt", socket, 
          "UInt", ScriptMainWindowId, "UInt", ExitMsg, "Int", FD_CLOSE)
    ExitApp
}

SetTimer, NewConnectionCheck, 500
return

PrepareForIncomingConnection(IPAddress, Port)
{
    VarSetCapacity(wsaData, 32)
    result := DllCall("Ws2_32\WSAStartup", "UShort", 0x0002, "UInt", &wsaData)
    if ErrorLevel
    {
        MsgBox % "WSAStartup() could not be called due to error %ErrorLevel%. "
                . "Winsock 2.0 or higher is required."
        return -1
    }
    if result
    {
        MsgBox % "WSAStartup() indicated Winsock error "
                . DllCall("Ws2_32\WSAGetLastError")
        return -1
    }
    AF_INET = 2
    SOCK_STREAM = 1
    IPPROTO_TCP = 6
    socket := DllCall("Ws2_32\socket", "Int", AF_INET, 
          "Int", SOCK_STREAM, "Int", IPPROTO_TCP)
    if socket = -1
    {
        MsgBox % "socket() indicated Winsock error "
                . DllCall("Ws2_32\WSAGetLastError")
        return -1
    }
    SizeOfSocketAddress = 16
    VarSetCapacity(SocketAddress, SizeOfSocketAddress)
    InsertInteger(2, SocketAddress, 0, AF_INET)
    InsertInteger(DllCall("Ws2_32\htons", "UShort", Port), SocketAddress, 2, 2)
    InsertInteger(DllCall("Ws2_32\inet_addr", "Str", IPAddress),
            SocketAddress, 4, 4)
    if DllCall("Ws2_32\bind", "UInt", socket, 
            "UInt", &SocketAddress, "Int", SizeOfSocketAddress)
    {
        MsgBox % "bind() indicated Winsock error "
                . DllCall("Ws2_32\WSAGetLastError") . "?"
        return -1
    }
    if DllCall("Ws2_32\listen", "UInt", socket, "UInt", "SOMAXCONN")
    {
        MsgBox % "LISTEN() indicated Winsock error "
                . DllCall("Ws2_32\WSAGetLastError") . "?"
        return -1
    }
    return socket
}

ReceiveData(wParam, lParam)
{
    global DataReceived
    global NewData
    global mydata
    global ConnectionList
    socket := wParam
    ReceivedDataSize = 4096
    Loop
    {
        VarSetCapacity(ReceivedData, ReceivedDataSize, 0)
        ReceivedDataLength := DllCall("Ws2_32\recv", "UInt", 
              socket, "Str", ReceivedData, "Int", ReceivedDataSize, "Int", 0)
	if ReceivedDataLength = 0
        {   
            StringReplace, ConnectionList, ConnectionList, %socket%`n
            DllCall("Ws2_32\closesocket", "UInt", socket)
        } 
        if ReceivedDataLength = -1
        {
            WinsockError := DllCall("Ws2_32\WSAGetLastError")
            if WinsockError = 10035
            {
                DataReceived = %TempDataReceived%
                NewData := true
                return 1
            }
            if WinsockError <> 10054
            {
                MsgBox % "recv() indicated Winsock error " . WinsockError
                StringReplace, ConnectionList, ConnectionList, %socket%`n
                DllCall("Ws2_32\closesocket", "UInt", socket)
            }
        }
        mydata := ReceivedData
        gosub myreceive
	if (A_Index = 1)
            TempDataReceived =
                TempDataReceived = %TempDataReceived%%ReceivedData%
    }
    return 1
}

ExitData(wParam, lParam)
{
    global ConnectionList
    socket := wParam
    ReceivedDataSize = 16
    VarSetCapacity(ReceivedData, ReceivedDataSize, 0)
    ReceivedDataLength := DllCall("Ws2_32\recv", "UInt", socket, 
          "Str", ReceivedData, "Int", ReceivedDataSize, "Int", 0)
    StringReplace, ConnectionList, ConnectionList, %socket%`n
    DllCall("Ws2_32\closesocket", "UInt", socket)
    return 1
}

SendData(wParam,SendData)
{
    SendDataSize := VarSetCapacity(SendData)
    SendDataSize += 1
    Loop, parse, wParam, `n
    {
        If A_LoopField =
           Continue
        socket := A_LoopField
        sendret := DllCall("Ws2_32\send", "UInt", socket, 
              "Str", SendData, "Int", SendDatasize, "Int", 0)
    }
}


InsertInteger(pInteger, ByRef pDest, pOffset = 0, pSize = 4)
{
    Loop %pSize%
        DllCall("RtlFillMemory", "UInt", &pDest + pOffset + A_Index-1, 
                "UInt", 1, "UChar", pInteger >> 8*(A_Index-1) & 0xFF)
}

NewConnectionCheck:
ConnectionCheck := DllCall("Ws2_32\accept", "UInt", socket, 
      "UInt", &SocketAddress, "Int", SizeOfSocketAddress)
if ConnectionCheck > 1
    ConnectionList = %ConnectionList%%ConnectionCheck%`n
Return

SendProcedure:
If ConnectionList <>
{
    SendText = %A_Hour%:%A_Min%:%A_Sec%
    SendData(ConnectionList,SendText)
}
Return

myreceive:
 TrayTip, server, %mydata%, ,16
  return

GuiClose:
ExitSub:
DllCall("Ws2_32\WSACleanup")
ExitApp

A client is also available for testing this code.

BaCon[edit]

OPEN "localhost:12321" FOR SERVER AS echo
WHILE TRUE
    fd = ACCEPT(echo)
    PRINT "Incoming connection from: ", GETPEER$(fd)
    RECEIVE data$ FROM fd
    SEND data$ & CR$ & NL$ TO fd
    CLOSE SERVER fd
WEND

Input from other terminal:

# echo "Hello world" | netcat 127.0.0.1 12321
Hello world

# echo "Hello world" | netcat 127.0.0.1 12321
Hello world

# echo "Hello world" | netcat 127.0.0.1 12321
Hello world

# echo "Hello world" | netcat 127.0.0.1 12321
Hello world

Output:

Incoming connection from: 127.0.0.1:36778
Incoming connection from: 127.0.0.1:36780
Incoming connection from: 127.0.0.1:36782
Incoming connection from: 127.0.0.1:36784

BBC BASIC[edit]

      INSTALL @lib$+"SOCKLIB"
      PROC_initsockets
      
      maxSess% = 8
      DIM sock%(maxSess%-1), rcvd$(maxSess%-1), Buffer% 255
      
      ON ERROR PRINT REPORT$ : PROC_exitsockets : END
      ON CLOSE PROC_exitsockets : QUIT
      
      crlf$ = CHR$13 + CHR$10
      port$ = "12321"
      host$ = FN_gethostname
      PRINT "Host name is " host$
      
      listen% = FN_tcplisten(host$, port$)
      PRINT "Listening on port ";port$
      
      REPEAT
        socket% = FN_check_connection(listen%)
        IF socket% THEN
          FOR i% = 0 TO maxSess%-1
            IF sock%(i%) = 0 THEN
              sock%(i%) = socket%
              rcvd$(i%) = ""
              PRINT "Connection on socket "; sock%(i%) " opened"
              EXIT FOR
            ENDIF
          NEXT i%
          listen% = FN_tcplisten(host$, port$)
        ENDIF
        
        FOR i% = 0 TO maxSess%-1
          IF sock%(i%) THEN
            res% = FN_readsocket(sock%(i%), Buffer%, 256)
            IF res% >= 0 THEN
              Buffer%?res% = 0
              rcvd$(i%) += $$Buffer%
              crlf% = INSTR(rcvd$(i%), crlf$)
              IF crlf% THEN
                echo$ = LEFT$(rcvd$(i%), crlf%-1)
                res% = FN_writelinesocket(sock%(i%), echo$)
                rcvd$(i%) = MID$(rcvd$(i%), crlf%+2)
              ENDIF
            ELSE
              PROC_closesocket(sock%(i%))
              PRINT "Connection on socket " ; sock%(i%) " closed"
              sock%(i%) = 0
            ENDIF
          ENDIF
        NEXT i%
        
        WAIT 0
      UNTIL FALSE
      END

Sample output:

Host name is PC236
Listening on port 12321
Connection on socket 1016 opened
Connection on socket 1012 opened
Connection on socket 1016 closed
Connection on socket 1016 opened
Connection on socket 1016 closed
Connection on socket 1012 closed

C[edit]

Works with: POSIX

This is a rather standard code (details apart); the reference guide for such a code is the Beej's Guide to Network programming. The dependency from POSIX is mainly in the use of the read and write functions, (using the socket as a file descriptor sometimes make things simpler).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <sys/wait.h>
#include <signal.h>

#define MAX_ENQUEUED 20
#define BUF_LEN 256
#define PORT_STR "12321"

/* ------------------------------------------------------------ */
/* How to clean up after dead child processes                   */
/* ------------------------------------------------------------ */

void wait_for_zombie(int s)
{
    while(waitpid(-1, NULL, WNOHANG) > 0) ;
}

/* ------------------------------------------------------------ */
/* Core of implementation of a child process                    */
/* ------------------------------------------------------------ */

void echo_lines(int csock)
{
    char buf[BUF_LEN];
    int r;

    while( (r = read(csock, buf, BUF_LEN)) > 0 ) {
        (void)write(csock, buf, r);
    }
    exit(EXIT_SUCCESS);
}

/* ------------------------------------------------------------ */
/* Core of implementation of the parent process                 */
/* ------------------------------------------------------------ */

void take_connections_forever(int ssock)
{
    for(;;) {
        struct sockaddr addr;
        socklen_t addr_size = sizeof(addr);
        int csock;

        /* Block until we take one connection to the server socket */
        csock = accept(ssock, &addr, &addr_size);

        /* If it was a successful connection, spawn a worker process to service it */
        if ( csock == -1 ) {
            perror("accept");
        } else if ( fork() == 0 ) {
            close(ssock);
            echo_lines(csock);
        } else {
            close(csock);
        }
    }
}

/* ------------------------------------------------------------ */
/* The server process's one-off setup code                      */
/* ------------------------------------------------------------ */

int main()
{
    struct addrinfo hints, *res;
    struct sigaction sa;
    int sock;

    /* Look up the address to bind to */
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    if ( getaddrinfo(NULL, PORT_STR, &hints, &res) != 0 ) {
        perror("getaddrinfo");
        exit(EXIT_FAILURE);
    }

    /* Make a socket */
    if ( (sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol)) == -1 ) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    /* Arrange to clean up child processes (the workers) */
    sa.sa_handler = wait_for_zombie;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if ( sigaction(SIGCHLD, &sa, NULL) == -1 ) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }

    /* Associate the socket with its address */
    if ( bind(sock, res->ai_addr, res->ai_addrlen) != 0 ) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    freeaddrinfo(res);

    /* State that we've opened a server socket and are listening for connections */
    if ( listen(sock, MAX_ENQUEUED) != 0 ) {
        perror("listen");
        exit(EXIT_FAILURE);
    }

    /* Serve the listening socket until killed */
    take_connections_forever(sock);
    return EXIT_SUCCESS;
}

C#[edit]

using System.Net.Sockets;
using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        static TcpListener listen;
        static Thread serverthread;

        static void Main(string[] args)
        {
            listen = new TcpListener(System.Net.IPAddress.Parse("127.0.0.1"), 12321);
            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);
            do
            {
                if (!tClient.Connected)
                { 
                    tClient.Close();
                    Thread.CurrentThread.Abort();       // Kill thread.
                }

                if (tClient.Available > 0)
                {
                    // Resend
                    byte pByte = (byte)tClient.GetStream().ReadByte();
                    Console.WriteLine("Client (Thread: {0}): Data {1}", Thread.CurrentThread.ManagedThreadId, pByte);
                    tClient.GetStream().WriteByte(pByte);
                }

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

Clojure[edit]

(use '[clojure.contrib.server-socket :only (create-server)])
(use '[clojure.contrib.duck-streams :only (read-lines write-lines)])

(defn echo [input output]
  (write-lines (java.io.PrintWriter. output true) (read-lines input)))

(create-server 12321 echo)

Note here that an auto-flushing PrintWriter needs to be created, otherwise 'output' could simply be passed to write-lines.

CoffeeScript[edit]

Translation of: JavaScript
Works with: node.js
net = require("net")
server = net.createServer (conn) ->
  console.log "Connection from #{conn.remoteAddress} on port  #{conn.remotePort}"
  conn.setEncoding "utf8"
  buffer = ""
  conn.on "data", (data) ->
    i = 0

    while i <= data.length
      char = data.charAt(i)
      buffer += char
      if char is "\n"
        conn.write buffer
        buffer = ""
      i++

server.listen 12321, "localhost"

Common Lisp[edit]

Here is a basic :usocket example (it should work with any Common Lisp):

(ql:quickload (list :usocket))
(defpackage :echo (:use :cl :usocket))
(in-package :echo)

(defun read-all (stream)
  (loop for char = (read-char-no-hang stream nil :eof)
     until (or (null char) (eql char :eof)) collect char into msg
     finally (return (values msg char))))

(defun echo-server (port &optional (log-stream *standard-output*))
  (let ((connections (list (socket-listen "127.0.0.1" port :reuse-address t))))
    (unwind-protect
	 (loop (loop for ready in (wait-for-input connections :ready-only t)
		  do (if (typep ready 'stream-server-usocket)
			 (push (socket-accept ready) connections)
			 (let* ((stream (socket-stream ready))
				(msg (concatenate 'string "You said: " (read-all stream))))
			   (format log-stream "Got message...~%")
			   (write-string msg stream)
			   (socket-close ready)
			   (setf connections (remove ready connections))))))
      (loop for c in connections do (loop while (socket-close c))))))

(echo-server 12321)

It's single threaded, so you can't REPL around with a running server. You'll need to start a second Lisp prompt, load the above and

(defun echo-send (message port)
  (with-client-socket (sock str "127.0.0.1" port)
    (write-string message str)
    (force-output str)
    (when (wait-for-input sock :timeout 5)
      (coerce (read-all str) 'string))))

(echo-send "Hello echo!" 12321)

The return value of that call should be "You said: Hello echo!".


The usocket library notwithstanding, sockets are not a standard part of Common Lisp, but many implementations provide them. Here is a CLISP-specific example:
Works with: CLISP
(defvar *clients* '()
    "This is a list of (socket :input status) which is used with
`socket:socket-status' to test for data ready on a socket.")

(defun echo-server (port)
    "Listen on `port' for new client connections and for data arriving on
any existing client connections"
    (let ((server (socket:socket-server port)))
        (format t "Echo service listening on port ~a:~d~%"
            (socket:socket-server-host server)
            (socket:socket-server-port server))
        (unwind-protect
            (loop 
                (when (socket:socket-status server 0 1)
                    (echo-accept-client (socket:socket-accept server 
                                            :external-format :dos
                                            :buffered t)))
                (when *clients*
                    (socket:socket-status *clients* 0 1)
                    (mapcar #'(lambda (client)
                                  (when (eq :input (cddr client))
                                      (echo-service-client (car client)))
                                  (when (eq :eof (cddr client))
                                      (echo-close-client (car client)))) *clients*)))
            (socket-server-close server))))

(defun echo-accept-client (socket)
    "Accept a new client connection and add it to the watch list."
    (multiple-value-bind 
        (host port) (socket:socket-stream-peer socket)
        (format t "Connect from ~a:~d~%" host port))
    (push (list socket :input nil) *clients*))
    
(defun echo-service-client (socket)
    (let ((line (read-line socket nil nil)))
        (princ line socket)
        (finish-output socket)))

(defun echo-close-client (socket)
    "Close a client connection and remove it from the watch list."
    (multiple-value-bind 
        (host port) (socket:socket-stream-peer socket)
        (format t "Closing connection from ~a:~d~%" host port))
    (close socket)
    (setq *clients* (remove socket *clients* :key #'car)))

(echo-server 12321)

D[edit]

This is a very basic server that processes the buffers one character at a time. In a real-world application, the buffers would be larger. More seriously, it processes one listener at a time. If the currSock.receive() blocks, the loop will not process other clients. This opens the door for a trivial denial-of-service attack. A realistic echo service must multiplex clients.

import std.array, std.socket;

void main() {
    auto listener = new TcpSocket;
    assert(listener.isAlive);
    listener.bind(new InternetAddress(12321));
    listener.listen(10);

    Socket currSock;
    uint bytesRead;
    ubyte[1] buff;

    while (true) {
        currSock = listener.accept();
        while ((bytesRead = currSock.receive(buff)) > 0)
            currSock.send(buff);
        currSock.close();
        buff.clear();
    }
}

This example will handle many connections.

import std.stdio, std.socket, std.array;

void main() {
    enum ushort port = 7;
    enum int backlog = 10;
    enum int max_connections = 60;
    enum BUFFER_SIZE = 16;

    auto listener = new TcpSocket;
    assert(listener.isAlive);
    listener.bind(new InternetAddress(port));
    listener.listen(backlog);
    debug writeln("Listening on port ", port);

    // Room for listener.
    auto sset = new SocketSet(max_connections + 1);

    Socket[] sockets;
    char[BUFFER_SIZE] buf;

    for (;; sset.reset()) {
        sset.add(listener);
        foreach (each; sockets)
            sset.add(each);

        // Update socket set with only those sockets that have data
        // avaliable for reading. Options are for read, write,
        // and error.
        Socket.select(sset, null, null);

        // Read the data from each socket remaining, and handle
        // the request.
        for (int i = 0; ; i++) {
NEXT:
            if (i == sockets.length)
                break;
            if (sset.isSet(sockets[i])) {
                int read = sockets[i].receive(buf);
                if (Socket.ERROR == read) {
                    debug writeln("Connection error.");
                    goto SOCK_DOWN;
                } else if (read == 0) {
                    debug {
                        try {
                            // If the connection closed due to an
                            // error, remoteAddress() could fail.
                            writefln("Connection from %s closed.",
                                     sockets[i].remoteAddress()
                                     .toString());
                        } catch (SocketException) {
                            writeln("Connection closed.");
                        }
                    }
SOCK_DOWN:
                    sockets[i].close(); //Release socket resources now.

                    // Remove from socket from sockets, and id from
                    // threads.
                    if (i != sockets.length - 1)
                        sockets[i] = sockets.back;
                    sockets.length--;
                    debug writeln("\tTotal connections: ",
                                  sockets.length);
                    goto NEXT; // -i- is still the NEXT index.
                } else {
                    debug
                        writefln("Received %d bytes from %s:"
                                 ~ "\n-----\n%s\n-----",
                                 read,
                                 sockets[i].remoteAddress().toString(),
                                 buf[0 .. read]);

                    // Echo what was sent.
                    sockets[i].send(buf[0 .. read]);
                }
            }
        }

        // Connection request.
        if (sset.isSet(listener)) {
            Socket sn;
            try {
                if (sockets.length < max_connections) {
                    sn = listener.accept();
                    debug writefln("Connection from %s established.",
                                   sn.remoteAddress().toString());
                    assert(sn.isAlive);
                    assert(listener.isAlive);
                    sockets ~= sn;
                    debug writefln("\tTotal connections: %d",
                                   sockets.length);
                } else {
                    sn = listener.accept();
                    debug writefln("Rejected connection from %s;"
                                   ~ " too many connections.",
                                   sn.remoteAddress().toString());
                    assert(sn.isAlive);
                    sn.close();
                    assert(!sn.isAlive);
                    assert(listener.isAlive);
                }
            } catch (Exception e) {
                debug writefln("Error accepting: %s", e.toString());
                if (sn)
                    sn.close();
            }
        }
    }
}

Delphi[edit]

program EchoServer;

{$APPTYPE CONSOLE}

uses SysUtils, IdContext, IdTCPServer;

type
  TEchoServer = class
  private
    FTCPServer: TIdTCPServer;
  public
    constructor Create;
    destructor Destroy; override;
    procedure TCPServerExecute(AContext: TIdContext);
  end;

constructor TEchoServer.Create;
begin
  FTCPServer := TIdTCPServer.Create(nil);
  FTCPServer.DefaultPort := 12321;
  FTCPServer.OnExecute := TCPServerExecute;
  FTCPServer.Active := True;
end;

destructor TEchoServer.Destroy;
begin
  FTCPServer.Active := False;
  FTCPServer.Free;
  inherited Destroy;
end;

procedure TEchoServer.TCPServerExecute(AContext: TIdContext);
var
  lCmdLine: string;
begin
  lCmdLine := AContext.Connection.IOHandler.ReadLn;
  Writeln('>' + lCmdLine);
  AContext.Connection.IOHandler.Writeln('>' + lCmdLine);

  if SameText(lCmdLine, 'QUIT') then
  begin
    AContext.Connection.IOHandler.Writeln('Disconnecting');
    AContext.Connection.Disconnect;
  end;
end;

var
  lEchoServer: TEchoServer;
begin
  lEchoServer := TEchoServer.Create;
  try
    Writeln('Delphi Echo Server');
    Writeln('Press Enter to quit');
    Readln;
  finally
    lEchoServer.Free;
  end;
end.

Erlang[edit]

-module(echo).
-export([start/0]).

start() ->
    spawn(fun () -> {ok, Sock} = gen_tcp:listen(12321, [{packet, line}]),
                    echo_loop(Sock)
          end).

echo_loop(Sock) ->
    {ok, Conn} = gen_tcp:accept(Sock),
    io:format("Got connection: ~p~n", [Conn]),
    Handler = spawn(fun () -> handle(Conn) end),
    gen_tcp:controlling_process(Conn, Handler),
    echo_loop(Sock).

handle(Conn) ->
    receive
        {tcp, Conn, Data} ->
            gen_tcp:send(Conn, Data),
            handle(Conn);
        {tcp_closed, Conn} ->
            io:format("Connection closed: ~p~n", [Conn])
    end.

Elixir[edit]

defmodule Echo.Server do
  def start(port) do
    tcp_options = [:binary, {:packet, 0}, {:active, false}]
    {:ok, socket} = :gen_tcp.listen(port, tcp_options)
    listen(socket)
  end

  defp listen(socket) do
    {:ok, conn} = :gen_tcp.accept(socket)
    spawn(fn -> recv(conn) end)
    listen(socket)
  end

  defp recv(conn) do
    case :gen_tcp.recv(conn, 0) do
      {:ok, data} ->
        :gen_tcp.send(conn, data)
        recv(conn)
      {:error, :closed} ->
        :ok
    end
  end
end

F#[edit]

open System.IO
open System.Net
open System.Net.Sockets

let service (client:TcpClient) =
    use stream = client.GetStream()
    use out = new StreamWriter(stream, AutoFlush = true)
    use inp = new StreamReader(stream)
    while not inp.EndOfStream do
        match inp.ReadLine() with
        | line -> printfn "< %s" line
                  out.WriteLine(line)
    printfn "closed %A" client.Client.RemoteEndPoint
    client.Close |> ignore

let EchoService = 
    let socket = new TcpListener(IPAddress.Loopback, 12321)
    do socket.Start()
    printfn "echo service listening on %A" socket.Server.LocalEndPoint
    while true do
        let client = socket.AcceptTcpClient()
        printfn "connect from %A" client.Client.RemoteEndPoint
        let job = async {
            use c = client in try service client with _ -> () }
        Async.Start job

[<EntryPoint>]
let main _ =
    EchoService
    0

Factor[edit]

Connections get logged to /place-where-factor-is/logs/echo-server.

USING: accessors io io.encodings.utf8 io.servers io.sockets threads ;
IN: rosetta.echo

CONSTANT: echo-port 12321

: handle-client ( -- )
   [ print flush ] each-line ;

: <echo-server> ( -- threaded-server )
    utf8 <threaded-server>
        "echo server" >>name
        echo-port >>insecure
        [ handle-client ] >>handler ;

: start-echo-server ( -- )
    <echo-server> [ start-server ] in-thread start-server drop ;

Forth[edit]

Works with: GNU Forth version 0.7.0
include unix/socket.fs

128 constant size

: (echo) ( sock buf -- sock buf )
  begin
    cr ." waiting..."
    2dup 2dup size read-socket nip
    dup 0>
  while
    ."  got: " 2dup type
    rot write-socket
  repeat
  drop drop drop ;

create buf size allot

: echo-server ( port -- )
  cr ." Listening on " dup .
  create-server
  dup 4 listen
  begin
    dup accept-socket
    cr ." Connection!"
    buf ['] (echo) catch
    cr ." Disconnected (" . ." )"
    drop close-socket
  again ;

12321 echo-server

TODO: use tasker.fs and non-blocking semantics to handle mutliple connections

Go[edit]

package main

import (
	"fmt"
	"net"
	"bufio"
)

func echo(s net.Conn, i int) {
	defer s.Close();

	fmt.Printf("%d: %v <-> %v\n", i, s.LocalAddr(), s.RemoteAddr())
	b := bufio.NewReader(s)
	for {
		line, e := b.ReadBytes('\n')
		if e != nil {
			break
		}
		s.Write(line)
	}
	fmt.Printf("%d: closed\n", i)
}

func main() {
	l, e := net.Listen("tcp", ":12321")
	for i := 0; e == nil; i++ {
		var s net.Conn
		s, e = l.Accept()
		go echo(s, i)
	}
}

Haskell[edit]

module Main where
import Network (withSocketsDo, accept, listenOn, sClose, PortID(PortNumber))
import Control.Monad (forever)
import System.IO (hGetLine, hPutStrLn, hFlush, hClose)
import System.IO.Error (isEOFError)
import Control.Concurrent (forkIO)
import Control.Exception (bracket)

-- For convenience in testing, ensure that the listen socket is closed if the main loop is aborted
withListenOn port body = bracket (listenOn port) sClose body

echo (handle, host, port) = catch (forever doOneLine) stop where
  doOneLine = do line <- hGetLine handle
                 print (host, port, init line)
                 hPutStrLn handle line
                 hFlush handle
  stop error = do putStrLn $ "Closed connection from " ++ show (host, port) ++ " due to " ++ show error
                  hClose handle

main = withSocketsDo $
  withListenOn (PortNumber 12321) $ \listener ->
    forever $ do
      acc@(_, host, port) <- accept listener
      putStrLn $ "Accepted connection from " ++ show (host, port)
      forkIO (echo acc)

Icon and Unicon[edit]

The following is Unicon-specific:

global mlck, nCons

procedure main()
    mlck := mutex()
    nCons := 0
    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) & repeat writes(f,reads(f))
        critical mlck: nCons -:= 1
        }
end

Java[edit]

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;

public class EchoServer {

    public static void main(String[] args) throws IOException {
        try (ServerSocket listener = new ServerSocket(12321)) {
            while (true) {
                Socket conn = listener.accept();
                Thread clientThread = new Thread(() -> handleClient(conn));
                clientThread.start();
            }
        }
    }

    private static void handleClient(Socket connArg) {
        Charset utf8 = StandardCharsets.UTF_8;

        try (Socket conn = connArg) {
            BufferedReader in = new BufferedReader(
                    new InputStreamReader(conn.getInputStream(), utf8));

            PrintWriter out = new PrintWriter(
                    new OutputStreamWriter(conn.getOutputStream(), utf8),
                    true);

            String line;
            while ((line = in.readLine()) != null) {
                out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

JavaScript[edit]

Works with: Node.js
const net = require('net');

function handleClient(conn) {
    console.log('Connection from ' + conn.remoteAddress + ' on port ' + conn.remotePort);

    conn.setEncoding('utf-8');

    let buffer = '';

    function handleData(data) {
        for (let i = 0; i < data.length; i++) {
            const char = data.charAt(i);
            buffer += char;
            if (char === '\n') {
                conn.write(buffer);
                buffer = '';
            }
        }
    }

    conn.on('data', handleData);
}

net.createServer(handleClient).listen(12321, 'localhost');

Julia[edit]

using Sockets # for version 1.0
println("Echo server on port 12321")
try
    server = listen(12321)
    instance = 0
    while true
        sock = accept(server)
        instance += 1
        socklabel = "$(getsockname(sock)) number $instance"
        @async begin 
            println("Server connected to socket $socklabel")
            write(sock, "Connected to echo server.\r\n")
            while isopen(sock)
                str = readline(sock)
                write(sock,"$str\r\n")
                println("Echoed $str to socket $socklabel")
            end
            println("Closed socket $socklabel")
        end
    end
catch y
    println("Caught exception: $y")
end

Kotlin[edit]

Translation of: Java
import java.net.ServerSocket
import java.net.Socket

fun main() {

    fun handleClient(conn: Socket) {
        conn.use {
            val input = conn.inputStream.bufferedReader()
            val output = conn.outputStream.bufferedWriter()

            input.forEachLine { line ->
                output.write(line)
                output.newLine()
                output.flush()
            }
        }
    }

    ServerSocket(12321).use { listener ->
        while (true) {
            val conn = listener.accept()
            Thread { handleClient(conn) }.start()
        }
    }
}
Output:

Quick test using netcat:

nc localhost 12321
Hello
Hello
Goodbye
Goodbye

LFE[edit]

Translation of: Erlang

Paste into the LFE REPL:

(defun start ()
  (spawn (lambda ()
           (let ((`#(ok ,socket) (gen_tcp:listen 12321 `(#(packet line)))))
             (echo-loop socket)))))

(defun echo-loop (socket)
  (let* ((`#(ok ,conn) (gen_tcp:accept socket))
         (handler (spawn (lambda () (handle conn)))))
    (lfe_io:format "Got connection: ~p~n" (list conn))
    (gen_tcp:controlling_process conn handler)
    (echo-loop socket)))

(defun handle (conn)
  (receive
    (`#(tcp ,conn ,data)
     (gen_tcp:send conn data))
    (`#(tcp_closed ,conn)
     (lfe_io:format "Connection closed: ~p~n" (list conn)))))

Usage:

> (set server (start))
<0.38.0>
> (! server "hey!")
"hey!"
> (! server "wassup?")
"wassup?"

Lua[edit]

Works with: Lua version 5.3
Library: LuaSocket
local socket = require("socket")

local function has_value(tab, value)
    for i, v in ipairs(tab) do
        if v == value then return i end
    end
    return false
end

local function checkOn(client)
   local line, err = client:receive()
	if line then
		client:send(line .. "\n")
	end
	if err and err ~= "timeout" then
		print(tostring(client) .. " " .. err)
		client:close()
		return true  -- end this connection
   end
	return false  -- do not end this connection
end

local server = assert(socket.bind("*",12321))
server:settimeout(0)  -- make non-blocking
local connections = { }  -- a list of the client connections
while true do
	local newClient = server:accept()
	if newClient then
		newClient:settimeout(0)  -- make non-blocking
		table.insert(connections, newClient)
	end
	local readList = socket.select({server, table.unpack(connections)})
	for _, conn in ipairs(readList) do
		if conn ~= server and checkOn(conn) then
			table.remove(connections, has_value(connections, conn))
		end
	end
end

The following implementation uses tiny delays rather than socket.select. It uses a table of not-quite-non-blocking socket client objects (they block for 4 milliseconds), which is iterated over to check on whether each one has either a line to echo or an error to warrant deletion. Without the millisecond delays, the whole thing would become one 'hot' loop and eat all the CPU time for one core. With them, it uses close to zero percent.

local socket=require("socket")

function checkOn (client)
    local line, err = client:receive()
    if line then
        print(tostring(client) .. " said " .. line)
        client:send(line .. "\n")
    end
    if err and err ~= "timeout" then
        print(tostring(client) .. " " .. err)
        client:close()
        return true  -- end this connection
    end
    return false    -- do not end this connection
end

local delay = 0.004  -- anything less than this uses up my CPU
local connections = {}  -- an array of connections
local newClient
local server = assert(socket.bind("*", 12321))
server:settimeout(delay)
while true do
    repeat
        newClient = server:accept()
        for idx, client in ipairs(connections) do
            if checkOn(client) then table.remove(connections, idx) end
        end
    until newClient
    newClient:settimeout(delay)
    print(tostring(newClient) .. " connected")
    table.insert(connections, newClient)
end

Works with: Luvit
local http = require("http")

http.createServer(function(req, res)
	print(("Connection from %s"):format(req.socket:address().ip))

	local chunks = {}
	local function dumpChunks()
		for i=1,#chunks do
			res:write(table.remove(chunks, 1))
		end
	end

	req:on("data", function(data)
		for line, nl in data:gmatch("([^\n]+)(\n?)") do
			if nl == "\n" then
				dumpChunks()
				res:write(line)
				res:write("\n")
			else
				table.insert(chunks, line)
			end
		end
	end)

	req:on("end", function()
		dumpChunks()
		res:finish()
	end)
end):listen(12321, "127.0.0.1")

print("Server running at http://127.0.0.1:12321/")

Mathematica/Wolfram Language[edit]

This will be able to handle multiple connections and multiple echoes:

server = SocketOpen[12321];
SocketListen[server, Function[{assoc},
  With[{client = assoc["SourceSocket"], input = assoc["Data"]},
   WriteString[client, ByteArrayToString[input]];
   ]
  ]]

Nim[edit]

import asyncnet, asyncdispatch

proc processClient(client: AsyncSocket) {.async.} =
  while true:
    let line = await client.recvLine()
    await client.send(line & "\c\L")

proc serve() {.async.} =
  var server = newAsyncSocket()
  server.bindAddr(Port(12321))
  server.listen()

  while true:
    let client = await server.accept()
    echo "Accepting connection from client", client.getLocalAddr[0]
    discard processClient(client)

discard serve()
runForever()

Objeck[edit]

use Net;
use Concurrency;

bundle Default {
  class SocketServer {
    id : static : Int;

    function : Main(args : String[]) ~ Nil {
      server := TCPSocketServer->New(12321);
      if(server->Listen(5)) {
        while(true) {
          client := server->Accept();
          service := Service->New(id->ToString());
          service->Execute(client);
          id += 1;
        };
      };
      server->Close();
    }
  }

  class Service from Thread {
    New(name : String) {
      Parent(name);
    }

    method : public : Run(param : Base) ~ Nil {
      client := param->As(TCPSocket);
      line := client->ReadString();
      while(line->Size() > 0) {
        line->PrintLine();
        line := client->ReadString();
      };
    }
  }
}

Ol[edit]

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

(define (on-accept name fd)
(lambda ()
   (print "# " (timestamp) "> we got new visitor: " name)

   (let*((ss1 ms1 (clock)))
      (let loop ((str #null) (stream (force (port->bytestream fd))))
         (cond
            ((null? stream)
               #false)
            ((function? stream)
               (let ((message (list->string (reverse str))))
                  (print "# " (timestamp) "> client " name " wrote " message)
                  (print-to fd message))
               (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.")))))

(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
            (fork (on-accept (syscall 51 fd) fd))))
      (sleep 0)
      (loop))))

(run 12321)

Oz[edit]

declare
  ServerSocket = {New Open.socket init}

  proc {Echo Socket}
     case {Socket getS($)} of false then skip
     [] Line then
        {System.showInfo "Received line: "#Line}
        {Socket write(vs:Line#"\n")}
        {Echo Socket}
     end
  end

  class TextSocket from Open.socket Open.text end
in
  {ServerSocket bind(takePort:12321)}
  {System.showInfo "Socket bound."}

  {ServerSocket listen}
  {System.showInfo "Started listening."}

  for do
     ClientHost ClientPort
     ClientSocket = {ServerSocket accept(accepted:$
					 acceptClass:TextSocket
					 host:?ClientHost
					 port:?ClientPort
					)}
  in
     {System.showInfo "Connection accepted from "#ClientHost#":"#ClientPort#"."}
     thread
        {Echo ClientSocket}

	{System.showInfo "Connection lost: "#ClientHost#":"#ClientPort#"."}
        {ClientSocket close}
     end
  end

Client test code:

declare
  Socket = {New class $ from Open.socket Open.text end init}
in
  {Socket connect(port:12321)}
  {Socket write(vs:"Hello\n")}
  {System.showInfo "Client received: "#{Socket getS($)}}
  {Socket close}

Example session:

Socket bound.
Started listening.
Connection accepted from localhost:2048.
Received line: Hello
Client received: Hello
Connection lost: localhost:2048.

Perl[edit]

This server will run indefinitely listening in the port 12321 and forking every time a client connects, the childs listen to the client and write back.

This is an example using the IO::Socket module:

use IO::Socket;
my $use_fork = 1;

my $sock = new IO::Socket::INET (
                                 LocalHost => '127.0.0.1',
                                 LocalPort => '12321',
                                 Proto => 'tcp',
                                 Listen => 1,   # maximum queued connections
                                 Reuse => 1,
                                )
		or die "socket: $!";	# no newline, so perl appends stuff

$SIG{CHLD} = 'IGNORE' if $use_fork;	# let perl deal with zombies

print "listening...\n";
while (1) {
	# declare $con 'my' so it's closed by parent every loop
        my $con = $sock->accept()
		or die "accept: $!";
	fork and next if $use_fork;	# following are for child only

	print "incoming..\n";
	print $con $_ while(<$con>);	# read each line and write back
	print "done\n";

	last	if $use_fork;	# if not forking, loop
}

# child will reach here and close its copy of $sock before exit

This is an equivalent program using the Net::Server module:

package Echo;
use base 'Net::Server::Fork';
sub process_request {
    print while <STDIN>;
}
Echo->run(port => 12321, log_level => 3);

It also prints the IP address and port number of every connection.

This is a more complicated example using preforking:

package Echo;
use base 'Net::Server::PreFork';
sub process_request {
    print while <STDIN>;
}
Echo->run(port => 12321, log_level => 3);

By default it spawns 5 child processes at startup, makes sure there are always at least 2 and at most 10 spare children available for new requests, each of which will be killed after processing 1000 requests and new ones will take their place.

Phix[edit]

-- demo\rosetta\EchoServer.exw
without js
include builtins\sockets.e
constant ESCAPE = #1B
procedure echo(atom sockd)
    ?{"socket opened",sockd}
    string buffer = ""
    integer bytes_sent
    bool first = true
    while true do
        {integer len, string s} = recv(sockd)
        if len<=0 then exit end if
        if first then
            bytes_sent = send(sockd, s) -- partial echo, see note
            first = false
        end if
        buffer &= s
        if s[$]='\n' then
            bytes_sent = send(sockd, buffer)
            buffer = ""
        end if
    end while
    ?{"socket disconnected",sockd}
end procedure

atom list_s = socket(AF_INET,SOCK_STREAM,NULL),
     pSockAddr = sockaddr_in(AF_INET, "", 12321)
if list_s<0 then ?9/0 end if
if bind(list_s, pSockAddr)=SOCKET_ERROR then crash("bind (%v)",{get_socket_error()}) end if
if listen(list_s,100)=SOCKET_ERROR then crash("listen (%v)",{get_socket_error()}) end if
puts(1,"echo server started, press escape or q to exit\n")
while not find(get_key(),{ESCAPE,'q','Q'}) do
    {integer code} = select({list_s},{},{},250000)  -- (0.25s)
    if code=SOCKET_ERROR then crash("select (%v)",{get_socket_error()}) end if
    if code>0 then  -- (not timeout)
        atom conn_s = accept(list_s)
        if conn_s=SOCKET_ERROR then ?9/0 end if
        atom hThread = create_thread(echo,{conn_s})
    end if
end while
list_s = closesocket(list_s)
WSACleanup()

Tested using telnet [-e q] localhost 12321
Note: on windows, keying "abc" did not echo anything until the first return, so I added a partial echo: remove if not needed/wanted.

PHP[edit]

$socket = socket_create(AF_INET,SOCK_STREAM,SOL_TCP);
socket_bind($socket, '127.0.0.1', 12321);
socket_listen($socket);

$client_count = 0;
while (true){
  if (($client = socket_accept($socket)) === false) continue;
  $client_count++;

  $client_name = 'Unknown';
  socket_getpeername($client, $client_name);
  echo "Client {$client_count} ({$client_name}) connected\n";
  $pid = pcntl_fork();
  if($pid == -1) die('Could not fork');
  if($pid){
    pcntl_waitpid(-1, $status, WNOHANG);
    continue;
  }

  //In a child process
  while(true){
    if($input = socket_read($client, 1024)){
      socket_write($client, $input);
    } else {
      socket_shutdown($client);
      socket_close($client);
      echo "Client {$client_count} ({$client_name}) disconnected\n";
      exit();
    }
  }
}

PicoLisp[edit]

(setq Port (port 12321))

(loop
   (setq Sock (listen Port))           # Listen
   (NIL (fork) (close Port))           # Accepted
   (close Sock) )                      # Parent: Close socket and continue

# Child:
(prinl (stamp) " -- (Pid " *Pid ") Client connected from " *Adr)

(in Sock
   (until (eof)                        # Echo lines
      (out Sock (prinl (line))) ) )

(prinl (stamp) " -- (Pid " *Pid ") Client disconnected")
(bye)                                  # Terminate child

PureBasic[edit]

NewMap RecData.s()
OpenWindow(0, 100, 200, 200, 100, "Echo Server", #PB_Window_SystemMenu | #PB_Window_MinimizeGadget )
InitNetwork()
CreateNetworkServer(1, 12321)

Repeat
   Event = NetworkServerEvent()
   ClientID = EventClient()
 
   If Event = #PB_NetworkEvent_Connect    ; When a new client has been connected...
      AddMapElement(RecData(), Str(ClientID))
      
   ElseIf Event = #PB_NetworkEvent_Data
      *Buffer = AllocateMemory(20000)
      count = ReceiveNetworkData(ClientID, *Buffer, 20000)
      For i = 1 To count
         RecData(Str(ClientID)) + Mid( PeekS(*Buffer, count), i , 1)
         If Right( RecData(Str(ClientID)), 2) = #CRLF$
            SendNetworkString (ClientID, RecData(Str(ClientID)))
            Debug  IPString(GetClientIP(ClientID)) + ":" + Str(GetClientPort(ClientID)) + "  " + RecData(Str(ClientID))
            RecData(Str(ClientID)) = ""   
         EndIf
      Next
      FreeMemory(*Buffer)
      
   ElseIf Event = #PB_NetworkEvent_Disconnect  ; When a client has closed the connection...
      DeleteMapElement(RecData(), Str(ClientID))
   EndIf

   Event = WaitWindowEvent(10)
Until Event = #PB_Event_CloseWindow

Python[edit]

Works with: Python version 2.3 or above
import SocketServer

HOST = "localhost"
PORT = 12321

# this server uses ThreadingMixIn - one thread per connection
# replace with ForkMixIn to spawn a new process per connection

class EchoServer(SocketServer.ThreadingMixIn, SocketServer.TCPServer):
    # no need to override anything - default behavior is just fine
    pass

class EchoRequestHandler(SocketServer.StreamRequestHandler):
    """
    Handles one connection to the client.
    """
    def handle(self):
        print "connection from %s" % self.client_address[0]
        while True:
            line = self.rfile.readline()
            if not line: break
            print "%s wrote: %s" % (self.client_address[0], line.rstrip())
            self.wfile.write(line)
        print "%s disconnected" % self.client_address[0]


# Create the server
server = EchoServer((HOST, PORT), EchoRequestHandler)

# Activate the server; this will keep running until you
# interrupt the program with Ctrl-C
print "server listening on %s:%s" % server.server_address
server.serve_forever()
Works with: Python version 3.5 or above
#!/usr/bin/env python
# $ printf 'echo\r\n' | nc localhost 12321
# echo
import asyncio
import logging
import os

logger = logging.getLogger('echoserver')

async def echo_handler(reader, writer):
  address = writer.get_extra_info('peername')
  logger.debug('accept: %s', address)
  message = await reader.readline()
  writer.write(message)
  await writer.drain()
  writer.close()

if __name__ == '__main__':
  logging.basicConfig()
  logger.setLevel(logging.DEBUG)
  loop = asyncio.get_event_loop()
  factory = asyncio.start_server(
    echo_handler,
    os.environ.get('HOST'),
    os.environ.get('PORT', 12321)
  )
  server = loop.run_until_complete(factory)
  try:
    loop.run_forever()
  except KeyboardInterrupt:
    pass
  server.close()
  loop.run_until_complete(server.wait_closed())
  loop.close()
Works with: Python version 2 and 3

Using only the low-level socket and threading modules. Supports timing out inactive clients

    #!usr/bin/env python
    import socket
    import threading

    HOST = 'localhost'
    PORT = 12321
    SOCKET_TIMEOUT = 30

    # This function handles reading data sent by a client, echoing it back
    # and closing the connection in case of timeout (30s) or "quit" command
    # This function is meant to be started in a separate thread
    # (one thread per client)
    def handle_echo(client_connection, client_address):
        client_connection.settimeout(SOCKET_TIMEOUT)
        try:
            while True:
                data = client_connection.recv(1024)
                # Close connection if "quit" received from client
                if data == b'quit\r\n' or data == b'quit\n':
                    print('{} disconnected'.format(client_address))
                    client_connection.shutdown(1)
                    client_connection.close()
                    break
                # Echo back to client
                elif data:
                    print('FROM {} : {}'.format(client_address,data))
                    client_connection.send(data)
        # Timeout and close connection after 30s of inactivity
        except socket.timeout:
            print('{} timed out'.format(client_address))
            client_connection.shutdown(1)
            client_connection.close()

    # This function opens a socket and listens on specified port. As soon as a
    # connection is received, it is transfered to another socket so that the main
    # socket is not blocked and can accept new clients.
    def listen(host, port):
        # Create the main socket (IPv4, TCP)
        connection = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        connection.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        connection.bind((host, port))
        # Listen for clients (max 10 clients in waiting)
        connection.listen(10)
        # Every time a client connects, allow a dedicated socket and a dedicated
        # thread to handle communication with that client without blocking others.
        # Once the new thread has taken over, wait for the next client.
        while True:
            current_connection, client_address = connection.accept()
            print('{} connected'.format(client_address))
            handler_thread = threading.Thread( \
                target = handle_echo, \
                args = (current_connection,client_address) \
            )
            # daemon makes sure all threads are killed if the main server process
            # gets killed
            handler_thread.daemon = True
            handler_thread.start()

    if __name__ == "__main__":
        try:
            listen(HOST, PORT)
        except KeyboardInterrupt:
            print('exiting')
            pass

Racket[edit]

An example echo server from the front page of the Racket website:

#lang racket
(define listener (tcp-listen 12321))
(let echo-server ()
  (define-values [I O] (tcp-accept listener))
  (thread (λ() (copy-port I O) (close-output-port O)))
  (echo-server))

Raku[edit]

(formerly Perl 6)

Works with: rakudo version 2018.03
my $socket = IO::Socket::INET.new:
    :localhost<localhost>,
    :localport<12321>,
    :listen;

while $socket.accept -> $conn {
    say "Accepted connection";
    start {
        while $conn.recv -> $stuff {
            say "Echoing $stuff";
            $conn.print($stuff);
        }
        $conn.close;
    }
}

Async version:

react {
    whenever IO::Socket::Async.listen('0.0.0.0', 12321) -> $conn {
        whenever $conn.Supply.lines -> $line {
            $conn.print( "$line\n" ) ;
        }
    }
}

REALbasic[edit]

This example uses the built-in ServerSocket class to handle multiple users.

Class EchoSocket
Inherits TCPSocket
  Sub DataAvailable()
    If Instr(Me.LookAhead, EndofLine.Windows) > 0 Then
      Dim data As String = Me.ReadAll
      Dim lines() As String = Split(data, EndofLine.Windows)
      For i As Integer = 0 To Ubound(lines)
        Me.Write(lines(i) + EndOfLine.Windows)
        Print(lines(i))
      Next
    End If
  End Sub
End Class

Class EchoServer
Inherits ServerSocket
  Function AddSocket() As TCPSocket
    Return New EchoSocket
  End Function
End Class

Class App
Inherits ConsoleApplication
  Function Run(args() As String) As Integer
    Listener = New EchoServer
    Listener.Port = 12321
    Listener.Listen()
    While True
      DoEvents() 'pump the event loop
    Wend
  End Function
  Private Listener As EchoServer
End Class

REBOL[edit]

server-port: open/lines tcp://:12321
forever [
    connection-port: first server-port
    until [
        wait connection-port
        error? try [insert connection-port first connection-port]
    ]
    close connection-port
]
close server-port

Ruby[edit]

require 'socket'
server = TCPServer.new(12321)

while (connection = server.accept)
  Thread.new(connection) do |conn|
    port, host = conn.peeraddr[1,2]
    client = "#{host}:#{port}"
    puts "#{client} is connected"
    begin
      loop do
        line = conn.readline
        puts "#{client} says: #{line}"
        conn.puts(line)
      end
    rescue EOFError
      conn.close
      puts "#{client} has disconnected"
    end
  end
end

Ruby 1.9.2 introduced an alternate method to create TCP server sockets. The Socket.tcp_server_loop method encapsulates the guts of the server into a block.

Works with: Ruby version 1.9.2
require 'socket'

Socket.tcp_server_loop(12321) do |conn, addr|
  Thread.new do
    client = "#{addr.ip_address}:#{addr.ip_port}"
    puts "#{client} is connected"
    begin
      loop do
        line = conn.readline
        puts "#{client} says: #{line}"
        conn.puts(line)
      end
    rescue EOFError
      conn.close
      puts "#{client} has disconnected"
    end
  end
end

Rust[edit]

use std::net::{TcpListener, TcpStream};
use std::io::{BufReader, BufRead, Write};
use std::thread;

fn main() {
    let listener = TcpListener::bind("127.0.0.1:12321").unwrap();
    println!("server is running on 127.0.0.1:12321 ...");
    
    for stream in listener.incoming() {
        let stream = stream.unwrap();
        thread::spawn(move || handle_client(stream));
    }
}

fn handle_client(stream: TcpStream) {
    let mut stream = BufReader::new(stream);
    loop {
        let mut buf = String::new();
        if stream.read_line(&mut buf).is_err() {
            break;
        }
        stream
            .get_ref()
            .write(buf.as_bytes())
            .unwrap();
    }
}

Scala[edit]

import java.io.PrintWriter
import java.net.{ServerSocket, Socket}

import scala.io.Source

object EchoServer extends App {
  private val serverSocket = new ServerSocket(23)
  private var numConnections = 0

  class ClientHandler(clientSocket: Socket) extends Runnable {
    private val (connectionId, closeCmd) = ({numConnections += 1; numConnections}, ":exit")

    override def run(): Unit =
      new PrintWriter(clientSocket.getOutputStream, true) {
        println(s"Connection opened, close with entering '$closeCmd'.")
        Source.fromInputStream(clientSocket.getInputStream).getLines
          .takeWhile(!_.toLowerCase.startsWith(closeCmd))
          .foreach { line =>
            Console.println(s"Received on #$connectionId: $line")
            println(line)  // Echo
          }
        Console.println(s"Gracefully closing connection, #$connectionId")
        clientSocket.close()
    }

    println(s"Handling connection, $connectionId")
  }

  while (true) new Thread(new ClientHandler(serverSocket.accept())).start()
}

Scheme[edit]

Works with: Guile

Based on the Guile Internet Socket Server Example.

; Needed in Guile for read-line
(use-modules (ice-9 rdelim))

; Variable used to hold child PID returned from forking
(define child #f)

; Start listening on port 12321 for connections from any address
(let ((s (socket PF_INET SOCK_STREAM 0)))
  (setsockopt s SOL_SOCKET SO_REUSEADDR 1)
  (bind s AF_INET INADDR_ANY 12321)
  (listen s 5) ; Queue size of 5

  (simple-format #t "Listening for clients in pid: ~S" (getpid))
  (newline)

; Wait for connections forever
  (while #t
    (let* ((client-connection (accept s))
        (client-details (cdr client-connection))
        (client (car client-connection)))
; Once something connects fork
      (set! child (primitive-fork))
      (if (zero? child)
      (begin
; Then have child fork to avoid zombie children (grandchildren aren't our responsibility)
        (set! child (primitive-fork))
        (if (zero? child)
          (begin
; Display some connection details
          (simple-format #t "Got new client connection: ~S" client-details)
          (newline)
          (simple-format #t "Client address: ~S"
            (gethostbyaddr (sockaddr:addr client-details)))
          (newline)
; Wait for input from client and then echo the input back forever (or until client quits)
          (do ((line (read-line client)(read-line client))) ((zero? 1))
            (display line client)(newline client))))
; Child exits after spawning grandchild.
        (primitive-exit))
; Parent waits for child to finish spawning grandchild
      (waitpid child)))))

Seed7[edit]

The code below uses the library listener.s7i. The function waitForRequest returns requests from new and existing connections.

$ include "seed7_05.s7i";
  include "socket.s7i";
  include "listener.s7i";

const proc: main is func
  local
    var listener: aListener is listener.value;
    var file: existingConnection is STD_NULL;
    var file: newConnection is STD_NULL;
  begin
    aListener := openInetListener(12321);
    listen(aListener, 10);
    while TRUE do
      waitForRequest(aListener, existingConnection, newConnection);
      if existingConnection <> STD_NULL then
        if eof(existingConnection) then
          writeln("Close connection " <& numericAddress(address(existingConnection)) <&
                  " port " <& port(existingConnection));
          close(existingConnection);
        else
          write(existingConnection, gets(existingConnection, 1024));
        end if;
      end if;
      if newConnection <> STD_NULL then
        writeln("New connection " <& numericAddress(address(newConnection)) <&
                " port " <& port(newConnection));
      end if;
    end while;
  end func;

Tcl[edit]

This code is single-threaded. It uses non-blocking I/O to perform the transfers, sitting on top of the event multiplexer system call (e.g., select() on Unix) to decide when to take new connections or service a particular socket. This makes this into a very lightweight echo server in terms of overall system resources.

# How to handle an incoming new connection
proc acceptEcho {chan host port} {
    puts "opened connection from $host:$port"
    fconfigure $chan -blocking 0 -buffering line -translation crlf
    fileevent $chan readable [list echo $chan $host $port]
}

# How to handle an incoming message on a connection
proc echo {chan host port} {
    if {[gets $chan line] >= 0} {
        puts $chan $line
    } elseif {[eof $chan]} {
        close $chan
        puts "closed connection from $host:$port"
    }
    # Other conditions causing a short read need no action
}

# Make the server socket and wait for connections
socket -server acceptEcho -myaddr localhost 12321
vwait forever

Alternative version[edit]

A more succinct version (though one harder to adapt to other kinds of services, but closer to the standard unix echo daemon since it has no line-buffering) is to use an asynchronous binary copy.

# How to handle an incoming new connection 
proc acceptEcho {chan host port} {
    puts "opened connection from $host:$port"
    fconfigure $chan -translation binary -buffering none
    fcopy $chan $chan -command [list done $chan $host $port]
}

# Called to finalize the connection
proc done {chan host port args} {
    puts "closed connection from $host:$port"
    close $chan
}

# Make the server socket and wait for connections
socket -server acceptEcho -myaddr localhost 12321
vwait forever

X86 Assembly[edit]

; x86_64 Linux NASM

global _start

%define af_inet 2
%define sock_stream 1
%define default_proto 0
%define sol_sock 1
%define reuse_addr 2
%define reuse_port 15
%define server_port 9001
%define addr_any 0
%define family_offset 0
%define port_offset 2
%define addr_offset 4
%define unused_offset 8
%define addr_len 16
%define buffer_len 64
%define max_connections 3


section .text

; rdi - 16 bit value to be byte swapped
; return - byte swapped value
htn_swap16:

  xor rax, rax
  mov rdx, 0x000000ff

  mov rsi, rdi
  and rsi, rdx
  shl rsi, 8
  or rax, rsi
  shl rdx, 8

  mov rsi, rdi
  and rsi, rdx
  shr rsi, 8
  or rax, rsi
  ret

; return - server socket
create_server_socket:

  mov rax, 41
  mov rdi, af_inet
  mov rsi, sock_stream
  mov rdx, default_proto
  syscall
  push rax

  mov rax, 54
  mov rdi, qword [rsp]
  mov rsi, sol_sock
  mov rdx, reuse_addr
  mov qword [rsp - 16], 1
  lea r10, [rsp - 16]
  mov r8, 4
  syscall

  mov rax, 54
  mov rdi, qword [rsp]
  mov rsi, sol_sock
  mov rdx, reuse_port
  mov qword [rsp - 16], 1
  lea r10, [rsp - 16]
  mov r8, 4
  syscall


  pop rax
  ret

; rdi - socket
; rsi - port
; rdx - connections
; return - void
bind_and_listen:

  push rdi
  push rdx

  mov rdi, rsi
  call htn_swap16

  lea rsi, [rsp - 16]
  mov word [rsi + family_offset], af_inet
  mov word [rsi + port_offset], ax
  mov dword [rsi + addr_offset], addr_any
  mov qword [rsi + unused_offset], 0

  mov rax, 49
  mov rdi, qword [rsp + 8]
  mov rdx, addr_len
  syscall

  mov rax, 50
  pop rsi
  pop rdi
  syscall
  ret

; rdi - server socket
; return - client socket
accept:

  mov rax, 43
  lea rsi, [rsp - 16]
  lea rdx, [rsp - 24]
  syscall
  ret

; rdi - client socket
; return - void
echo:

  push rdi
  mov rax, 0
  lea rsi, [rsp - 104]
  mov rdx, buffer_len
  syscall

  pop rdi
  mov rdx, rax 
  lea rsi, [rsp - 112]
  mov rax, 1
  syscall
  ret


_start:

  call create_server_socket
  mov r14, rax

  mov rdi, rax
  mov rsi, server_port
  mov rdx, max_connections
  call bind_and_listen

accept_connection:

  mov rdi, r14
  call accept

  mov r15, rax
  mov rax, 57
  syscall

  test rax, rax
  jz handle_connection

  ; close client socket
  mov rax, 3
  mov rdi, r15
  syscall
  jmp accept_connection
    
handle_connection:

  mov rdi, r15
  call echo

  close_client:
    mov rax, 3
    mov rdi, r15
    syscall

  close_server:
    mov rax, 3
    mov rdi, r14
    syscall

  exit:
    mov rax, 60
    xor rdi, rdi
    syscall

Wren[edit]

Translation of: C

An embedded program so we can ask the C host to call the relevant library functions for us and also handle simultaneous connections from multiple clients using a multi-process approach.

/* echo_server.wren */

var MAX_ENQUEUED = 20
var BUF_LEN = 256
var PORT_STR = "12321"

var AF_UNSPEC = 0
var SOCK_STREAM = 1
var AI_PASSIVE = 1

foreign class AddrInfo {
    foreign static getAddrInfo(name, service, req, pai)

    construct new() {}

    foreign family

    foreign family=(f)

    foreign sockType

    foreign sockType=(st)

    foreign flags

    foreign flags=(f)

    foreign protocol

    foreign addr

    foreign addrLen
}

foreign class AddrInfoPtr {
    construct new() {}

    foreign deref

    foreign free()
}

class Socket {
    foreign static create(domain, type, protocol)

    foreign static bind(fd, addr, len)

    foreign static listen(fd, n)

    foreign static accept(fd, addr, addrLen)
}

foreign class SockAddrPtr {
    construct new() {}

    foreign size
}

class SigAction {
    foreign static cleanUpProcesses()
}

foreign class Buffer {
    construct new(size) {}
}

class Posix {
    foreign static read(fd, buf, nbytes)

    foreign static write(fd, buf, n)

    foreign static fork()

    foreign static close(fd)
}

// Child process.
var echoLines = Fn.new { |csock|
    var buf = Buffer.new(BUF_LEN)
    var r
    while ((r = Posix.read(csock, buf, BUF_LEN)) > 0) {
        Posix.write(csock, buf, r)
    }
}

// Parent process.
var takeConnectionsForever = Fn.new { |ssock|
    while (true) {
        var addr = SockAddrPtr.new()
        var addrSize = addr.size

        /* Block until we take one connection to the server socket */
        var csock = Socket.accept(ssock, addr, addrSize)

        /* If it was a successful connection, spawn a worker process to service it. */
        if (csock == -1) {
            System.print("Error accepting socket.")
        } else if (Posix.fork() == 0) {
            Posix.close(ssock)
            echoLines.call(csock)
            return
        } else {
            Posix.close(csock)
        }
    }
}

/* Look up the address to bind to. */
var hints = AddrInfo.new()
hints.family = AF_UNSPEC
hints.sockType = SOCK_STREAM
hints.flags = AI_PASSIVE
var addrInfoPtr = AddrInfoPtr.new()
if (AddrInfo.getAddrInfo("", PORT_STR, hints, addrInfoPtr) != 0) {
    Fiber.abort("Failed to get pointer to addressinfo.")
}

/* Make a socket. */
var res = addrInfoPtr.deref
var sock = Socket.create(res.family, res.sockType, res.protocol)
if (sock == -1) Fiber.abort("Failed to make a socket.")

/* Arrange to clean up child processes (the workers). */
SigAction.cleanUpProcesses()

/* Associate the socket with its address. */
if (Socket.bind(sock, res.addr, res.addrLen) != 0) {
    Fiber.abort("Failed to bind socket.")
}

addrInfoPtr.free()

/* State that we've opened a server socket and are listening for connections. */
if (Socket.listen(sock, MAX_ENQUEUED) != 0) {
    Fiber.abort("Failed to listen for connections.")
}

/* Serve the listening socket until killed */
takeConnectionsForever.call(sock)


which we now embed in the following C program, build and run:

/* gcc echo_server.c -o echo_server -lwren -lm  */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <sys/wait.h>
#include <signal.h>
#include "wren.h"

/* Clean up dead processes. */
void wait_for_zombie(int s) {
    while (waitpid(-1, NULL, WNOHANG) > 0) ;
}

/* C <=> Wren interface functions */

void C_addrInfoAllocate(WrenVM* vm) {
    wrenSetSlotNewForeign(vm, 0, 0, sizeof(struct addrinfo));
}

void C_addrInfoPtrAllocate(WrenVM* vm) {
    wrenSetSlotNewForeign(vm, 0, 0, sizeof(struct addrinfo*));
}

void C_sockAddrPtrAllocate(WrenVM* vm) {
    wrenSetSlotNewForeign(vm, 0, 0, sizeof(struct sockaddr*));
}

void C_bufferAllocate(WrenVM* vm) {
    size_t bufsize = (size_t)wrenGetSlotDouble(vm, 1);
    wrenSetSlotNewForeign(vm, 0, 0, bufsize);
}

void C_getAddrInfo(WrenVM* vm) {
    const char *name = wrenGetSlotString(vm, 1);
    if (strcmp(name, "") == 0) name = NULL;
    const char *service = wrenGetSlotString(vm, 2);
    const struct addrinfo *req = (const struct addrinfo *)wrenGetSlotForeign(vm, 3);
    struct addrinfo** ppai = (struct addrinfo**)wrenGetSlotForeign(vm, 4);
    int status = getaddrinfo(name, service, req, ppai);
    wrenSetSlotDouble(vm, 0, (double)status);
}

void C_family(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)(pai->ai_family));
}

void C_setFamily(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    int f = (int)wrenGetSlotDouble(vm, 1);
    pai->ai_family = f;
}

void C_sockType(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)(pai->ai_socktype));
}

void C_setSockType(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    int type = (int)wrenGetSlotDouble(vm, 1);
    pai->ai_socktype = type;
}

void C_flags(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)(pai->ai_flags));
}

void C_setFlags(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    int flags = (int)wrenGetSlotDouble(vm, 1);
    pai->ai_flags = flags;
}

void C_protocol(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)(pai->ai_protocol));
}

void C_addr(WrenVM* vm) {
    wrenEnsureSlots(vm, 2);
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenGetVariable(vm, "main", "SockAddrPtr", 1);
    struct sockaddr **ppsa = (struct sockaddr**)wrenSetSlotNewForeign(vm, 0, 1, sizeof(struct sockaddr*));
    *ppsa = pai->ai_addr;
}

void C_addrLen(WrenVM* vm) {
    struct addrinfo* pai = (struct addrinfo*)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)(pai->ai_addrlen));
}

void C_deref(WrenVM* vm) {
    wrenEnsureSlots(vm, 2);
    struct addrinfo** ppai = (struct addrinfo**)wrenGetSlotForeign(vm, 0);
    wrenGetVariable(vm, "main", "AddrInfo", 1);
    struct addrinfo *pai = (struct addrinfo*)wrenSetSlotNewForeign(vm, 0, 1, sizeof(struct addrinfo));
    *pai = **ppai;
}

void C_free(WrenVM* vm) {
    struct addrinfo* pai = *(struct addrinfo**)wrenGetSlotForeign(vm, 0);
    freeaddrinfo(pai);
}

void C_create(WrenVM* vm) {
    int domain   = (int)wrenGetSlotDouble(vm, 1);
    int type     = (int)wrenGetSlotDouble(vm, 2);
    int protocol = (int)wrenGetSlotDouble(vm, 3);
    int fd = socket(domain, type, protocol);
    wrenSetSlotDouble(vm, 0, (double)fd);
}

void C_bind(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    __CONST_SOCKADDR_ARG *psa = (__CONST_SOCKADDR_ARG *)wrenGetSlotForeign(vm, 2);
    socklen_t len = (socklen_t)wrenGetSlotDouble(vm, 3);
    int status = bind(fd, *psa, len);
    wrenSetSlotDouble(vm, 0, (double)status);
}

void C_listen(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    int n  = (int)wrenGetSlotDouble(vm, 2);
    int status = listen(fd, n);
    wrenSetSlotDouble(vm, 0, (double)status);
}

void C_accept(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    __SOCKADDR_ARG *psa = (__SOCKADDR_ARG *)wrenGetSlotForeign(vm, 2);
    socklen_t len = (socklen_t)wrenGetSlotDouble(vm, 3);
    int status = accept(fd, *psa, &len);
    wrenSetSlotDouble(vm, 0, (double)status);
}

void C_size(WrenVM* vm) {
    struct sockaddr** ppaddr = (struct sockaddr**)wrenGetSlotForeign(vm, 0);
    wrenSetSlotDouble(vm, 0, (double)sizeof(**ppaddr));
}

void C_cleanUpProcesses(WrenVM* vm) {
    struct sigaction sa;
    sa.sa_handler = wait_for_zombie;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if (sigaction(SIGCHLD, &sa, NULL) == -1 ) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }
}

void C_read(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    void *buf = (void *)wrenGetSlotForeign(vm, 2);
    size_t nbytes = (size_t)wrenGetSlotDouble(vm, 3);
    ssize_t res = read(fd, buf, nbytes);
    wrenSetSlotDouble(vm, 0, (double)res);
}

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

void C_fork(WrenVM* vm) {
    __pid_t pid = fork();
    wrenSetSlotDouble(vm, 0, (double)pid);
}

void C_close(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    int status = close(fd);
    wrenSetSlotDouble(vm, 0, (double)status);
}

WrenForeignClassMethods bindForeignClass(WrenVM* vm, const char* module, const char* className) {
    WrenForeignClassMethods methods;
    methods.allocate = NULL;
    methods.finalize = NULL;
    if (strcmp(module, "main") == 0) {
        if (strcmp(className, "AddrInfo") == 0) {
            methods.allocate = C_addrInfoAllocate;
        } else if (strcmp(className, "AddrInfoPtr") == 0) {
            methods.allocate = C_addrInfoPtrAllocate;
        } else if (strcmp(className, "SockAddrPtr") == 0) {
            methods.allocate = C_sockAddrPtrAllocate;
        } else if (strcmp(className, "Buffer") == 0) {
            methods.allocate = C_bufferAllocate;
        }
    }
    return methods;
}

WrenForeignMethodFn bindForeignMethod(
    WrenVM* vm,
    const char* module,
    const char* className,
    bool isStatic,
    const char* signature) {
    if (strcmp(module, "main") == 0) {
        if (strcmp(className, "AddrInfo") == 0) {
            if ( isStatic && strcmp(signature, "getAddrInfo(_,_,_,_)") == 0) return C_getAddrInfo;
            if (!isStatic && strcmp(signature, "family") == 0)               return C_family;
            if (!isStatic && strcmp(signature, "family=(_)") == 0)           return C_setFamily;
            if (!isStatic && strcmp(signature, "sockType") == 0)             return C_sockType;
            if (!isStatic && strcmp(signature, "sockType=(_)") == 0)         return C_setSockType;
            if (!isStatic && strcmp(signature, "flags") == 0)                return C_flags;
            if (!isStatic && strcmp(signature, "flags=(_)") == 0)            return C_setFlags;
            if (!isStatic && strcmp(signature, "protocol") == 0)             return C_protocol;            
            if (!isStatic && strcmp(signature, "addr") == 0)                 return C_addr;
            if (!isStatic && strcmp(signature, "addrLen") == 0)              return C_addrLen;
        } else if (strcmp(className, "AddrInfoPtr") == 0) {
            if (!isStatic && strcmp(signature, "deref") == 0)                return C_deref;
            if (!isStatic && strcmp(signature, "free()") == 0)               return C_free;
        } else if (strcmp(className, "Socket") == 0) {
            if ( isStatic && strcmp(signature, "create(_,_,_)") == 0)        return C_create;
            if ( isStatic && strcmp(signature, "bind(_,_,_)") == 0)          return C_bind;
            if ( isStatic && strcmp(signature, "listen(_,_)") == 0)          return C_listen;
            if ( isStatic && strcmp(signature, "accept(_,_,_)") == 0)        return C_accept;
        } else if (strcmp(className, "SockAddrPtr") == 0) {
            if (!isStatic && strcmp(signature, "size") == 0)                 return C_size;
        } else if (strcmp(className, "SigAction") == 0) {
            if ( isStatic && strcmp(signature, "cleanUpProcesses()") == 0)   return C_cleanUpProcesses;
        } else if (strcmp(className, "Posix") == 0) {
            if ( isStatic && strcmp(signature, "read(_,_,_)") == 0)          return C_read;
            if ( isStatic && strcmp(signature, "write(_,_,_)") == 0)         return C_write;
            if ( isStatic && strcmp(signature, "fork()") == 0)               return C_fork;
            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;
}

int main(int argc, char **argv) {
    WrenConfiguration config;
    wrenInitConfiguration(&config);
    config.writeFn = &writeFn;
    config.errorFn = &errorFn;
    config.bindForeignClassFn = &bindForeignClass;
    config.bindForeignMethodFn = &bindForeignMethod;
    WrenVM* vm = wrenNewVM(&config);
    const char* module = "main";
    const char* fileName = "echo_server.wren";
    char *script = readFile(fileName);
    WrenInterpretResult result = wrenInterpret(vm, module, script);
    switch (result) {
        case WREN_RESULT_COMPILE_ERROR:
            printf("Compile Error!\n");
            break;
        case WREN_RESULT_RUNTIME_ERROR:
            printf("Runtime Error!\n");
            break;
        case WREN_RESULT_SUCCESS:
            break;
    }
    wrenFreeVM(vm);
    free(script);
    return 0;
}
Output:

Quick test using 2 terminals and telnet as the client (also tested on 3 terminals - not shown here):

/* start server on terminal 1 */
$ ./echo_server

/* start telnet on terminal 2 and type 'hello' */
$ telnet localhost 12321
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
hello
hello
^]
telnet> quit
Connection closed.

/* press ctrl-c on terminal 1 to kill the server */
^C

zkl[edit]

const PORT=12321;
pipe:=Thread.Pipe(); // how server tells thread to connect to user
 
fcn echo(socket){		// a thread, one per connection
   text:=Data();
   while(t:=socket.read()){
      text.append(t);
      if(text.find("\n",text.cursor)){ text.readln().print(); }
   }
   // socket was closed
}
 
   // Set up the server socket.
server:=Network.TCPServerSocket.open(PORT);
println("Listening on %s:%s".fmt(server.hostname,server.port));
server.listen(echo.launch);  // Main event loop
Output:

The next three windows overlap in time

//start the server
$ zkl bbb
Listening on Octavius:12321
hohothis is a test
a different terminal
^\Quit
//on another terminal, run the REPL
$ zkl
zkl: var s=Network.TCPClientSocket.connectTo("localhost",12321);
TCPClientSocket
zkl: s.write("hoho")
4
zkl: s.write("this is a test\n")
15
zkl: 
//and on a third terminal
$ zkl
zkl: var s=Network.TCPClientSocket.connectTo("localhost",12321);
TCPClientSocket
zkl: s.write("a different terminal\n")
21
zkl: