How to set up a socket for UDP multicast with 2 network cards present?

I'm trying to get udp multicast data using sockets and c++ (c). I have a server with 2 network cards so I need to bind socket to specific interface. Currently I'm testing on another server that has only one network card.

When I use INADDR_ANY I can see the udp data, when I bind to specific interface I don't see any data. Function inet_addr is not failing (I removed checking for return value for now).

Code is below. On a server with one network card, my IP address is 10.81.128.44. I receive data when I run as: ./client 225.0.0.37 12346

This gives me no data: ./client 225.0.0.37 12346 10.81.128.44

Any suggestions? (Hope the code compiles, I removed comments ...)

   #include <stdlib.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <time.h>
    #include <string.h>
    #include <stdio.h>

    #include <iostream>
    #include <string>

    using namespace std;

    #define HELLO_PORT 12345
    #define HELLO_GROUP "225.0.0.37"
    #define MSGBUFSIZE 256

    int main(int argc, char *argv[])
    {
        string source_iface;
        string group(HELLO_GROUP);
        int port(HELLO_PORT);

        if (!(argc < 2)) group = argv[1];
        if (!(argc < 3)) port = atoi(argv[2]);
        if (!(argc < 4)) source_iface = argv[3];

        cout << "group: " << group << " port: " << port << " source_iface: " << source_iface << endl;

        int fd;
        if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
        {
            perror("socket");
            exit(1);
        }

        u_int yes = 1;
        if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) < 0)
        {
            perror("Reusing ADDR failed");
            exit(1);
        }

        struct sockaddr_in addr;
        memset(&addr, 0, sizeof(addr));