Question

I have the following issue here: I want to write a server streaming data on a UDP socket on a specific port, and clients should be able to connect to it and receive the data that is being sent out without too much hassle: they just connect, and from the moment they start they should get data using recvfrom from the server.

I have some problems with setting up the network related parts. So, here is a rough piece of code that I try to make work:

int udpSock = socket(AF_INET, SOCK_DGRAM, 0);
if(udpSock == -1)
{
    perror("Could not create audio output socket");
    exit(1);
}

struct sockaddr_in *sin = (struct sockaddr_in*)&gOutgoingAddr;
sin->sin_port = htons(40200);
if(bind(udpSock, (const sockaddr*)sin, sizeof(struct sockaddr_in)) == -1)
{
    perror("Cannot bind audio socket");
    exit(1);
}

int buffer_size = 0;
char* data = get_next_buffer(&buffer_size);
while(buffer_size > 0)
{
    if(sendto(udpSock, (const void*)(data), buffer_size, 0, NULL, 0) == -1)
    {
        perror("sendto failure");
    }
    data = get_next_buffer(&buffer_size);
}

Do not worry about the gOutgoingAddr variable, it is obtained correctly using getifaddrs, it is valid. I am troubled regarding the parametrization of the sendto method, becasue right now the output of the application is:

sendto failure: Destination address required

That’s true, because I don’t have a destination address, since all the examples I have found till now show when the server gets a client connection, and there is the address. But since I don’t have a client yet connected, I’d still want to stream out.

I appreciate all help, I have no idea what Ishould put for the parameters of sendto:

1. The gOutgoingAddress which is the address where I create the socket? I have tried this, but if I use the tcpdump linux command on the specified port, I get nothing.
2. Should I create a multicast socket? This somehow makes no sense…
3. Something else?

Thanks,
frc

Answer 1

You cannot stream out to “nowhere”. Streaming data via UDP is not multicast. That means if you have 100 clients connected, you must send exactly the same data 100-times, once to each of the clients that shall receive it. Multicast was not really part of the initial IPv4 design. It has been added later on and is not widely supported. This is contrary to IPv6, where multicast has been part of the initial design. The only thing you could do it broadcast the traffic within your local network. This will only work if all clients are in your local network segment. To broadcast your server would simply send the data to 255.255.255.255 and to a fixed UDP port. All clients then have to listen on that specific port and will receive the data. Please note, that on most systems you need special permissions for broadcasting (e.g. it is not common that only programs running with root privileges are allowed to broadcast traffic, as broadcasts pollute your network, since all broadcast packets are sent to all clients in the network, whether they care for them or not). Without broadcasts, you have only unicast and unicast means one sender, one receiver. For one sender multiple receiver, you must send out the same data multiple times to multiple addresses.