C / C++ / MFC

// ----------------------------------------------------------------------------
// udp_recv_setup()
//
// This function sets the server socket.  It lets the system determine the port 
// number.  The function returns the server socket number.
// ----------------------------------------------------------------------------
//
int udp_recv_setup()
{
	struct sockaddr_in local;			// socket address for local side 
	int nLocalAddrLen = sizeof(local);	// length of local address
	int nServerSocket = 0;

	//
	// Create the socket.
	//
	nServerSocket = socket(AF_INET, SOCK_DGRAM, 0);

	if ( nServerSocket < 0)
    {
		cout << "%error opening datagram socket \n" ;
		exit(1);
    }

	//
	// Bind the socket to a port, let the system decide the number.
	//
	local.sin_family		= AF_INET;		// Internet family
	local.sin_addr.s_addr	= INADDR_ANY;	// Wild card machine address  (defaults to local)
	local.sin_port			= 0;			// Let system choose the port 

	bind(nServerSocket, (struct sockaddr *) &local,sizeof(local));	//bind the name (address) to a port

	//
	// Get the port name and print it out.
	//
	getsockname( nServerSocket, (struct sockaddr *) &local, &nLocalAddrLen);		
	cout << "socket has port " << local.sin_port << endl;

	return  nServerSocket;
}

-Masaaki Onishi-

For message-oriented sockets, care must be taken not to exceed the maximum packet size of the underlying provider, which can be obtained by using getsockopt to retrieve the value of socket option SO_MAX_MSG_SIZE. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned, and no data is transmitted.