Reading Socket Options

The TTL of a multicast for a socket can be determined by reading the value from the socket options. The following code example shows how the TTL value is read.

int ttl;                          // Allocate space for TTL.
int sizeofttl = sizeof(ttl);        // Create an integer that contains the                                    // size of the TTL value.
getsockopt(    
            sock,
            IPPROTO_IP,
            IP_MULTICAST_TTL,
            (char *)&ttl,
            &sizeofttl);

The ttl parameter in the preceding code example contains the current TTL set value for the multicasts through a socket defined as sock.

Each multicast transmission is sent from a single network interface, even if the host has more than one multicasting-capable interface. The following code example shows how a socket option is available to determine which interface is currently used for transmissions from a specified socket.

unsigned long addr;                 //Allocate space for address.
int sizeofaddr = sizeof(addr);    //Create an integer containing size of
                                //address.
getsockopt(    
            sock,
            IPPROTO_IP,
            IP_MULTICAST_IF,
            (char *)&addr, 
            &sizeofaddr );

The addr parameter contains the local IP address of the current outgoing interface after a getsockopt call.

By default, if a multicast datagram is sent to a group, to which the sending host belongs, a copy of the datagram on the outgoing interface is looped back by IP for local delivery. Any attempt to disable this multicast loop-back results in the call failing with the error message WSAENOPROTOOPT.

By default, if a multicast datagram is sent to a group, to which the sending host belongs, a copy of the datagram on the outgoing interface is looped back by IP for local delivery. This can be modified using socket options. The following code example shows the option available to check the behavior status of the socket.

unsigned long status;                 //Allocate space for the status.
int sizeofstatus = sizeof(status);     //Create an integer
                                    //Contains the size of the status
getsockopt(    
            sock, 
            IPPROTO_IP, 
            IP_MULTICAST_LOOP,
            (char *)&status,
            &sizeofstatus );

The status parameter contains the status of the multicast loop-back after the getsockopt function call.