C++ socket option reuse port

Question

I have a problem with a c++ socket.
I'm using CAsyncSocket from MFC that i want to join a multicast group.
Also I need to have multiple listener on this group and here is where i get in trouble.
I found some examples on the web but it doesn't seems to work.
Here is my code:

//create socket on port 17233   
BOOL bRet = Create(17233,SOCK_DGRAM, FD_READ);

//set reuse socket option  
BOOL bMultipleApps = TRUE;
bRet = SetSockOpt(SO_REUSEADDR, (void*)&bMultipleApps, sizeof(BOOL), SOL_SOCKET);

//join multicast group
ip_mreq m_mrMReq;           // Contains IP and interface of the host group
m_mrMReq.imr_multiaddr.s_addr = inet_addr((LPCSTR)"224.30.0.1");    /* group addr */ 
m_mrMReq.imr_interface.s_addr = htons(INADDR_ANY);      /* use default */

int uRes =setsockopt(m_hSocket, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char FAR *)&m_mrMReq, sizeof(m_mrMReq));

There are no errors when i run this.
But when i try to run another instance of the app it fails to create a new socket on that port because the port is in use.

I have done this in C# and it worked fine like this:

IPEndPoint ipep = new IPEndPoint(IPAddress.Any, port);
s.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, 1);
s.Bind(ipep);
IPAddress ip = IPAddress.Parse(mcastGroup);
s.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AddMembership, new MulticastOption(ip, IPAddress.Any));
s.SetSocketOption(SocketOptionLevel.IP,SocketOptionName.MulticastTimeToLive, int.Parse("1"));

So if any body sees a problem with my code or have some tips i will gladly appreciated.

EDIT 1:
Is CAsyncSocket a TCP socket?

EDIT 2: After reading Can two applications listen to the same port?
I think i made a confusion. I need a Multicast UDP port that can be access by multiple application using SO_REUSEADDR

Edit for Clarification:

BOOL bRet = Create(17233,SOCK_DGRAM, FD_READ)

Creates an UDP socket and bind's to to port 17223.
For SetSockOpt(SO_REUSEADDR, (void*)&bMultipleApps, sizeof(BOOL), SOL_SOCKET); to work you need to set it before binding as @Hasturkun said.
The final working code looks like this:

    BOOL bRet = Socket(SOCK_DGRAM, FD_READ);
    if(bRet != TRUE)
    {
        UINT uErr = GetLastError();
        std::cout<<"Error:"<<uErr<<std::endl;
        return FALSE;
    }else{
        std::cout<<"Create sock: OK"<<std::endl;
    }

    //add reuse
    BOOL bMultipleApps = TRUE;      /* allow reuse of local port if needed */
    SetSockOpt(SO_REUSEADDR, (void*)&bMultipleApps, sizeof(BOOL), SOL_SOCKET);

    //bind
    bRet = Bind(17233, NULL);
    if(bRet != TRUE)
    {
        UINT uErr = GetLastError();
        std::cout<<"Error(BIND):"<<uErr<<std::endl;
    }else{
        std::cout<<"BIND sock: OK"<<std::endl;
    }

Thanks,
Gabriel

;

Answer 1

You should be able to separate the creation of the socket from the binding, create the socket using Socket, eg.

BOOL bRet = Socket(SOCK_DGRAM, FD_READ);

Then bind it with Bind after setting the sockopt

BOOL bMultipleApps = TRUE;
bRet = SetSockOpt(SO_REUSEADDR, (void*)&bMultipleApps, sizeof(BOOL), SOL_SOCKET);

bRet = Bind(17233, NULL);

Answer 2

This happens, i guess, because you are binding a client socket to a specific port and address. Maybe on its constructor:

BOOL bRet = Create(17233,SOCK_DGRAM, FD_READ);

You should not binding a client socket to an address. Let windows manage this for you. You should have a option to not create the socket passing a specific port, or if you do not, you should create the socket using another port.

Also, the BOOL bMultipleApps = TRUE; option does not work the way you think it does. It sets a linger option in the socket, but once created and listening, the socket (i mean the socket port) cannot be used in other applications no mather what you do.

Check this out: so-linger-and-closing-socketswinsock

EDIT:

I also dont know that the value of port in your c# code:

IPEndPoint ipep = new IPEndPoint(IPAddress.Any, port);

Are you sure that port does not get a different value every time you run the app?

As I said, try to create the socket in a different port to see what happens. Google for so_linger to know what it means.

EDIT 2:

Take a look at: Can two application listen to the same port?

EDIT 3:

Maybe your c# code:

IPEndPoint ipep = new IPEndPoint(IPAddress.Any, port);

is binding the address to a different NIC. Do you have 2 nics at the same computer? If you do, you can bind the same port in both of them.

EDIT 4:

Example of using UDP sockets: Sending & Receiving UDP Datagrams with MFC's CAsyncSocket

Answer 3

If boost is an option for you, consider using Asio for this. It is very straight-forward and this example shows a simple multicast receiver.

The important part for multiple listeners is:

socket_.set_option(boost::asio::ip::udp::socket::reuse_address(true));

If you're unclear what your application is doing, just run netstat and you'll see e.g. the sockets and how they are bound (IP, port and protocol):

netstat -an

...and look for the port you're interested in. If you run multiple applications listening to the same port you should see multiple entries for the same port with UDP as protocol.