How to get a new socket connection mid-session?

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I'm using C-style TCP sockets with send() and recv(). I have a connection running between user A and user B, where user A acts as a server and user B acts as a client.

I want to have a passive user C, which does not communicate anything, but receives data from user A. However, the new passive user C can join the session at any time. A might send C different packets than what it would send B.. I imagine it would be best for A-C to communicate on a different port than A-B

How can this connection be made (without threading, or the like) in an arbitrary point of communication?

edit still unsolved.

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You could setup a listener that detects new connections, and mirror traffic to all open sockets. I recently wrote what I mean in C#: (i'll see whether I can quickly turn that into a C sample)

This example only accepts a fixed nr of incoming connections at the start, but it is dead easy to change that.

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

public class Demo
{
    static IList<Socket> StartServer(int numberOfClients)
    {
        using(Socket main = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
        {
            main.Bind(new IPEndPoint(IPAddress.Any, 9050));
            main.Listen(numberOfClients);

            var clients = Enumerable
                .Range(1,numberOfClients)
                .Select(i => {
                        Console.WriteLine("Waiting for 1 more client...");
                        Socket client = main.Accept();
                        Console.WriteLine("Connected to {0}", client.RemoteEndPoint);    
                        return client; })
                .ToList();
            main.Close();

            return clients;
        }
    }

    public static void Main()
    {
        var clients = StartServer(4);

        while(clients.Count()>1) // still a conversation
        {
            var copyList = clients.ToList();
            Console.WriteLine("Monitoring {0} sockets...", copyList.Count);
            Socket.Select(copyList, null, null, 10000000);

            foreach(Socket client in copyList)
            {
                byte[] data = new byte[1024];
                int recv = client.Receive(data);

                if (recv == 0)
                {
                    Console.WriteLine("Client {0} disconnected.", client.RemoteEndPoint);
                    client.Close();
                    clients.Remove(client);
                }
                else
                    foreach (var other in clients.Except(new [] {client}))
                        other.Send(data, recv, SocketFlags.None);
            }
        }
        Console.WriteLine("Last client disconnected, bye");
    }
}

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You can simply open 2 sockets on the server A, and bind them on 2 different ports. Then use select function on the 2 created socket file descriptors.

Select will return first time when one of the 2 clients make a connect. Remember that on server side, after accepting a connect, you should set the returned new file descriptor (with FD_SET) in order to make select listen to events that will happen on the new socket( which returned from accept).