Reading Socket Options (Windows CE 5.0)
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.
See Also
Creating an IP Multicast Application
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