What is the purpose of SO_REUSEADDR?

Question

I am trying to understand a multicast code, and I don't understand the utilities of a little part :

int fd_socket = socket(AF_INET, SOCK_DGRAM, 0);
u_int yes = 1;

setsockopt(fd_socket, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));

I don't understand the utilities of the setsockopt function. I understand that this function permits to modify the socket in the kernel, and SOL_SOCKET is because my modification is about the level of the socket and not of the level of a protocol. I don't understand SO_REUSEADDR.

Answer 1

For UDP sockets, setting the SO_REUSEADDR option allows multiple sockets to be open on the same port.

If those sockets are also joined to a multicast group, any multicast packet coming in to that group and port will be delivered to all sockets open on that port.