[Edited to clarify that bind() may in fact include a multicast address.]
So the application is joining several multicast groups, and receiving messages sent to any of them, to the same port. SO_REUSEPORT allows you to bind several sockets to the same port. Besides the port, bind() needs an IP address. INADDR_ANY is a catch-all address, but an IP address may also be used, including a multicast one. In that case, only packets sent to that IP will be delivered to the socket. I.e. you can create several sockets, one for each multicast group. bind() each socket to the (group_addr, port), AND join group_addr. Then data addressed to different groups will show up on different sockets, and you'll be able to distinguish it that way.
I tested that the following works on FreeBSD:
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/param.h>
#include <unistd.h>
#include <errno.h>
int main(int argc, const char *argv[])
{
const char *group = argv[1];
int s = socket(AF_INET, SOCK_DGRAM, 0);
int reuse = 1;
if (setsockopt(s, SOL_SOCKET, SO_REUSEPORT, &reuse, sizeof(reuse)) == -1) {
fprintf(stderr, "setsockopt: %d\n", errno);
return 1;
}
/* construct a multicast address structure */
struct sockaddr_in mc_addr;
memset(&mc_addr, 0, sizeof(mc_addr));
mc_addr.sin_family = AF_INET;
mc_addr.sin_addr.s_addr = inet_addr(group);
mc_addr.sin_port = htons(19283);
if (bind(s, (struct sockaddr*) &mc_addr, sizeof(mc_addr)) == -1) {
fprintf(stderr, "bind: %d\n", errno);
return 1;
}
struct ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr(group);
mreq.imr_interface.s_addr = INADDR_ANY;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
char buf[1024];
int n = 0;
while ((n = read(s, buf, 1024)) > 0) {
printf("group %s fd %d len %d: %.*s\n", group, s, n, n, buf);
}
}
If you run several such processes, for different multicast addresses, and send a message to one of the addresses, only the relevant process will receive it. Of course, in your case, you probably will want to have all the sockets in one process, and you'll have to use select or poll or equivalent to read them all.