Echo server

From Rosetta Code
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,
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,
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_bill(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(w, i, readc, w, i, o, b);
}
 
integer
main(void)
{
dispatch w;
file s;
 
tcpip_listen(s, 12321, 0);
w_watch(w, s, serve, w, s);
w_press(w);
 
return 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.

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.

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.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
 
public class EchoServer {
ServerSocket serverSocket;
public EchoServer(){
}
 
public void start() {
try {
serverSocket = new ServerSocket(12321);
while(true){
Thread clientThread = new Thread(new ClientHandler(serverSocket.accept()));
clientThread.start();
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
System.out.println("closing server socket");
serverSocket.close();
} catch (IOException e) {
e.printStackTrace();
}
}
 
}
 
public static void main(String[] args) {
EchoServer es = new EchoServer();
es.start();
}
}
 
class ClientHandler implements Runnable {
private static int numConnections;
private int connectionId = 0;
Socket clientSocket;
 
public ClientHandler(Socket s) {
connectionId = numConnections++;
System.out.println("handling connection, #" + connectionId);
clientSocket = s;
}
 
public void run() {
PrintWriter out = null;
BufferedReader in = null;
try {
out = new PrintWriter(clientSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String inputLine, outputLine;
while((inputLine = in.readLine()) != null){
outputLine = inputLine;
System.out.println("received: " + outputLine);
out.write(outputLine+"\n");
out.flush();
if (outputLine.equals("exit"))
break;
}
} catch(Exception e) {
e.printStackTrace();
} finally {
out.close();
try {
in.close();
clientSocket.close();
System.out.println("closing connection, #" + connectionId);
} catch (IOException e) {
e.printStackTrace();
}
}
}
}

JavaScript[edit]

Works with: Node.js
var net = require('net');
 
var server = net.createServer(function(conn) {
console.log("Connection from " + conn.remoteAddress + " on port " + conn.remotePort);
conn.setEncoding("utf8");
var buffer = "";
 
conn.on("data", function(data) {
for(var i = 0; i <= data.length; i++) {
var char = data.charAt(i);
buffer += char;
if(char == "\n") {
conn.write(buffer);
buffer = "";
}
}
});
});
 
server.listen(12321, "localhost");


Kotlin[edit]

Translation of: Java
// version 1.1.3
 
import java.io.BufferedReader
import java.io.InputStreamReader
import java.io.PrintWriter
import java.net.ServerSocket
import java.net.Socket
 
class ClientHandler(private val clientSocket: Socket): Runnable {
private val connectionId: Int
 
init {
connectionId = ++numConnections
println("Handling connection, #$connectionId")
}
 
override fun run() {
val pw = PrintWriter(clientSocket.outputStream, true)
val br = BufferedReader(InputStreamReader(clientSocket.inputStream))
while (true) {
val line = br.readLine() ?: break
println("Received: $line")
pw.write("$line\n")
pw.flush()
if (line == "exit") break
}
br.close()
pw.close()
clientSocket.close()
println("Closing connection, #$connectionId")
}
 
private companion object {
var numConnections = 0
}
}
 
fun main(args: Array<String>) {
val serverSocket = ServerSocket(12321)
try {
while (true) {
Thread(ClientHandler(serverSocket.accept())).start()
}
}
finally {
serverSocket.close()
println("Closing server socket")
}
}
Output:

Quick test using telnet:

telnet localhost 12321
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Hello
Hello
Goodbye
Goodbye
exit
exit
Connection closed by foreign host.

Echo server window (closing with ^C):

Handling connection, #1
Received: Hello
Received: Goodbye
Received: exit
Closing connection, #1
^C

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]

Library: LuaSocket

This implementation doesn't rely on coroutines because they're an additional (often confusing) notion that could make the example needlessly hard to understand. Instead it uses a table of not-quite-non-blocking socket client objects (they block for one microsecond), which is iterated over to check on whether each one has either a line to echo or an error to warrant deletion.

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 err
end
return nil
end
 
local delay, clients, newClient = 10^-6, {}
local server = assert(socket.bind("*", 12321))
server:settimeout(delay)
print("Server started")
while 1 do
repeat
newClient = server:accept()
for k, v in pairs(clients) do
if checkOn(v) then table.remove(clients, k) end
end
until newClient
newClient:settimeout(delay)
print(tostring(newClient) .. " connected")
table.insert(clients, newClient)
end

Without the microsecond 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.

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()
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();
};
}
}
}
 

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.

Perl 6[edit]

Works with: rakudo version 2016-03
my $socket = IO::Socket::INET.new:
localhost => 'localhost',
localport => 12321,
listen => 1;
 
while $socket.accept -> $conn {
say "Accepted connection";
start {
while $conn.recv -> $stuff {
say "Echoing $stuff";
$conn.print($stuff);
}
$conn.close;
}
}

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()

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))
 

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();
}
}
 

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

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: