Question

The problem is it only connects to one client instead of two. Can anyone help me figure out why?

Server:

#include <SFML/System.hpp>
#include <SFML/Network.hpp>
#include <iostream>

void sendInfo(void *UserData)
{
    sf::IPAddress* ip = static_cast<sf::IPAddress*>(UserData);
    // Print something...
    while(true){
        // Create the UDP socket
        sf::SocketUDP Socket;

        // Create bytes to send
        char Buffer[] = "sending info.";

        // Send data to "192.168.0.2" on port 4567
        if (Socket.Send(Buffer, sizeof(Buffer), *ip, 4444) != sf::Socket::Done)
        {
            // Error...
        }
    }
}

void receiveInfo(void *userData)
{
    // Print something...
    while(true){
        // Create the UDP socket
        sf::SocketUDP Socket;

        // Bind it (listen) to the port 4567
        if (!Socket.Bind(4444))
        {
            // Error...
        }

        char Buffer[128];
        std::size_t Received;
        sf::IPAddress Sender;
        unsigned short Port;
        if (Socket.Receive(Buffer, sizeof(Buffer), Received, Sender, Port) != sf::Socket::Done)
        {
            // Error...
        }

        // Show the address / port of the sender
        std::cout << Buffer << std::endl;

        Socket.Close();

    }
}

int main()
{
    sf::IPAddress client[2];
    int connected = 0;
    while(connected < 2){
        // Create the UDP socket
        sf::SocketUDP Socket;

        // Bind it (listen) to the port 4567
        if (!Socket.Bind(4444))
        {
            // Error...
        }

        char Buffer[128];
        std::size_t Received;
        sf::IPAddress Sender;
        unsigned short Port;
        if (Socket.Receive(Buffer, sizeof(Buffer), Received, Sender, Port) != sf::Socket::Done)
        {
            // Error...
        }

        // Show the address / port of the sender
        client[connected] = Sender;

        Socket.Close();

        sf::Thread* send = new sf::Thread(&sendInfo, &client[connected]);
        sf::Thread* receive = new sf::Thread(&receiveInfo, &client[connected]);
        // Start it !
        send->Launch();
        receive->Launch();
        connected++;
    }

    while(true){

    }

    return EXIT_SUCCESS;
}

Client:

#include <SFML/System.hpp>
#include <SFML/Network.hpp>
#include <iostream>

void sendInfo(void *UserData)
{
    // Print something...
    while(true){
        // Create the UDP socket
        sf::SocketUDP Socket;

        // Create bytes to send
        char Buffer[] = "client sending info.";

        // Send data to "192.168.0.2" on port 4567
        if (Socket.Send(Buffer, sizeof(Buffer),  "127.0.0.1", 4444) != sf::Socket::Done)
        {
            // Error...
        }
    }
}

void receiveInfo(void *userData)
{
    // Print something...
    while(true){
        // Create the UDP socket
        sf::SocketUDP Socket;

        // Bind it (listen) to the port 4567
        if (!Socket.Bind(4444))
        {
            // Error...
        }

        char Buffer[128];
        std::size_t Received;
        sf::IPAddress Sender;
        unsigned short Port;
        if (Socket.Receive(Buffer, sizeof(Buffer), Received, Sender, Port) != sf::Socket::Done)
        {
            // Error...
        }

        // Show the address / port of the sender
        std::cout << Buffer << std::endl;

        Socket.Close();

    }
}

int main()
{
    // Create the UDP socket
    sf::SocketUDP Socket;

    // Create bytes to send
    char Buffer[] = "Client Joined.";

    // Send data to "192.168.0.2" on port 4567
    if (Socket.Send(Buffer, sizeof(Buffer), "127.0.0.1", 4444) != sf::Socket::Done)
    {
        // Error...
    }

    sf::Thread* send = new sf::Thread(&sendInfo);
    sf::Thread* receive = new sf::Thread(&receiveInfo);
    // Start it !
    send->Launch();
    receive->Launch();


    while(true){

    }

    return EXIT_SUCCESS;
}

Answer 1

First things first: is this a chat server or a ‘more typical’ server?

If this is a chat server, then you either need to have a list of sockets that are connected to the clients (you can connect UDP sockets using the connect() call, very convenient, and it also helps reduce the chances of spoofed peers) or a list of all the client addresses that you can supply to sendto() or sendmsg().

More ‘typical’ servers won’t try to send messages to any client except the one that most recently made a request: those servers typically don’t save anything from the clients, and instead will use recvfrom() or recvmsg() to get the peer’s address for use in later sendto() or sendmsg() calls.

Also, most protocols only rely on one well known port; the server uses one specific port by convention, but clients select whatever port is open and free. FTP relies heavily on well-known ports on the client-side as well, and as a result is a gigantic pain to tunnel through Network Address Translation firewalls.

It isn’t just academic: both your client and your server are attempting to bind() to port 4444. This means you need at least two IP addresses on a single machine to test, or use virtualization software to run an entirely separate machine on the same hardware, or just have two machines available. It’s more work than it needs to be, and there’s no reason for the clients to care about their local port numbers:

Server:

    // Bind it (listen) to the port 4567
    if (!Socket.Bind(4444))
    {
        // Error...
    }

Client:

    // Bind it (listen) to the port 4567
    if (!Socket.Bind(4444))
    {
        // Error...
    }

Poof! These two will never run on the same host without significant tricks. I expect your “it connects to one” is probably just the server or the client connecting to itself, but without some code to fill in those // Error blocks, it’d be tough to tell for sure.

(And while we’re here, I’d like to take an aside to talk about comments; comments that simply re-state what the code does aren’t very useful. You’ll note that most of your comments are in fact wrong, referring to wrong IPs or ports. Some just don’t add any information:

    // Create the UDP socket
    sf::SocketUDP Socket;

I know we’re taught to add comments, but sadly we’re not always taught what kind of comments to add. The only comment in both programs that I’d recommend even keeping would be this one, slightly amended:

    // udp doesn't require listen or accept
    if (!Socket.Bind(4444))

It isn’t obvious from reading the code, and won’t be wrong when the port number is read out of an environment variable, command line parameter, configuration file, or registry. (It might be too redundant in a team of people familiar with the sockets API, but might be gold for a programmer not that familiar with the differences between UDP and TCP.)

Good function names, variable names, etc., will win over comments almost every time. End of aside. 🙂

And now, the more minor nit-picking: your thread handlers are doing some tasks like this:

while(1) {
    socket s;
    bind s;
    r = recv s;
    print r;
    close s;
}

This needless creation, binding, and closing, is all wasted energy, both the computer’s energy and (much more importantly) your energy. Consider the following two re-writings:

recv_thread() {
    socket s;
    bind s;
    while (1) {
        r = recv s;
        print r;
    }
    close s;
}

or

recv_thread(s) {
    while (1) {
        r = recv s;
        print r;
    }
}
/* ... */
socket s;
bind s;
sf::Thread* rt = new sf::Thread(&recv_thread);
rt->Launch(s);

The first option is a simple refactoring of your existing code; it keeps the socket creation and destruction in the thread function, but moves the loop invariants out of the loop. The code inside the loop now does only what is necessary.

The second option is a more drastic reworking: it moves the socket creation to the main thread, where error-handling is probably much easier, and the thread function only does exactly what the remote peer needs that thread to do. (If you wanted to change from UDP to TCP, the second option would be by far the much easier easier one to change — your threading code might not need any modifications at all.)

I hope this helps. 🙂