How to add Multicast Group under a specific interface (Windows) - windows

Question

I can use the command "netsh interface ip show joins" in cmd to show multicast group under each interface. But I really don't know how to add a group to a
interface, like adding a IP address 239.39.188.188 to "Interface 8: VirtualBox Host-Only Network". The simplest way would be appreciated.
Interface 3: Ethernet
Scope References Last Address
---------- ---------- ---- ---------------------
0 0 Yes 224.0.0.1
Interface 1: Loopback Interface
Scope References Last Address
---------- ---------- ---- ---------------------
0 2 Yes 239.255.255.250
Interface 8: VirtualBox Host-Only Network
Scope References Last Address
---------- ---------- ---- ---------------------
0 0 Yes 224.0.0.1
0 1 Yes 224.0.0.251
239.39.188.188 // this is what I want to add
Btw, I tried with some methods, like opening UDP socket and setting IP_ADD_MEMBERSHIP (How to add my host to Multicast Group...!). Also, I tried with a command on linux "ip maddr [ add | del ] MULTIADDR dev STRING".
After that, I observed that IP_ADD_MEMBERSHIP was set successfully. But as the result, in the above table, I cannot add a group under a specific interface.
For opening UDP socket and setting IP_ADD_MEMBERSHIP part, I coded in linux as belows.
ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr(_outIP.c_str()); // _outIP is destination address(group address), interface is ethernet interface
mreq.imr_interface.s_addr = _interface.length() > 0 ? inet_addr(_interface.c_str()) : htonl(INADDR_ANY);
if (setsockopt(_udpSock,IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(struct ip_mreq)) == -1) {
cout << "Fail to add ip membership!!!!" << endl;
}
else {
cout << "Success to add ip membership!!!!" << endl;
s = sprintf(warnmsg, "Success to add ip membership!!!!");
_ofile->write(warnmsg, s);
}

Answer 1

You must keep the socket with which you joined the group open forever. In other words, your program must not terminate. Add a for (;;) { sleep(1000000); } or so at the end.
When you program terminates, the socket gets closed automatically and your OS (Windows or Linux, it does not matter) leaves the group again.
What happens in the OS is slightly more complex as multiple programs may join the same multicast group, so the OS keeps a reference count and the machine only leaves the group when the group is no longer references by any socket.