Multicast listening

I have a problem with my socket program. I have written a program to listen for multicast UDP packets and to respond to them.

If the destination address of the packets is not multicast but unicast to my ip address it works fine. But if I change to a multicast address and subscribe the program doesn’t receive the packets.

What is strange is that if I run wireshark at the same time then the program works fine and see and responds to the multicast packets. Is wireshark activating the IP driver to receive multicast packets somehow??

Its just a little annoying to have to run wireshark just to get my program working.

 My stripped down code is as follows:

int buffer_sfd;

struct sockaddr_in my_sin, buffer_sin;

socklen_t addr_len;

struct ip_mreq stmreq0;

char buf[MAXBUFLEN];

            if ((buffer_sfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {

                        perror("buffer empty socket");

                        exit(1);

            }

            my_sin.sin_family = AF_INET;

            my_sin.sin_port = htons(48640);

            my_sin.sin_addr.s_addr = htonl(INADDR_ANY);

            if(bind(buffer_sfd, (struct sockaddr *)&my_sin, sizeof(my_sin)) < 0) {

                        perror("bind");

                        exit(1);

            }

            stmreq0.imr_multiaddr.s_addr = inet_addr("239.192.12.0");

            stmreq0.imr_interface.s_addr = INADDR_ANY;

            if((numbytes = recvfrom(buffer_sfd, buf, MAXBUFLEN-1, 0, (struct sockaddr *)&buffer_sin, &addr_len)) < 0) {

                        perror("recvfrom");

                        exit(1);

            }

Any help would be most appreciated!