UDP Multicast not receiving message - c

Question

I am trying to make a simple UDP multicast example where a message is sent from one program and received from the other but right now the output is only:
Connected
Message Sent
and
bind
setup multicast
Can someone please tell me what I am missing so that I can receive the message successfully? Thank you!! Here are the codes in full:
int main(int argc, char *argv[])
{
int udp_socket_info;
struct sockaddr_in udp_server;
char* message="test";
//create socket
udp_socket_info = socket(AF_INET, SOCK_DGRAM, 0);
if (udp_socket_info == -1) {
puts("Could not create socket");
}
//assign local values
udp_server.sin_addr.s_addr = inet_addr("225.0.0.37"); //multicast address
udp_server.sin_family = AF_INET;
udp_server.sin_port = htons( 1100 );
//checks connection
if (connect(udp_socket_info, (struct sockaddr *)&udp_server, sizeof(udp_server)) < 0) {
perror("Connection error");
}
puts("Connected");
//sends message
if( sendto(udp_socket_info , message , strlen(message) , 0, (struct sockaddr *)&udp_server, sizeof(udp_server)) < 0) {
perror("Send failed");
}
puts("Message Sent");
}
and the second program is
int main(int argc, char *argv[])
{
//initialize udp socket and structures
int udp_socket_info;
struct sockaddr_in udp_server;
struct sockaddr addr;
struct ip_mreq mreq;
socklen_t fromlen;
fromlen = sizeof addr;
char incoming_message[100];
//create udp socket
udp_socket_info = socket(AF_INET, SOCK_DGRAM, 0);
if (udp_socket_info == -1) {
puts("Could not create socket");
}
// set up
memset((char*)&udp_server,0,sizeof(udp_server));
udp_server.sin_family=AF_INET;
udp_server.sin_port = htons( 1100 );
udp_server.sin_addr.s_addr = inet_addr("192.168.0.100"); //local address
// bind
if (bind(udp_socket_info,(struct sockaddr *)&udp_server, sizeof(udp_server)) < 0) {
perror("bind error");
exit (1);
}
puts("bind");
// use setsockopt() to join multicast group
mreq.imr_multiaddr.s_addr=inet_addr("225.0.0.37"); //multicast address
mreq.imr_interface.s_addr= htonl(INADDR_ANY); //can use local address here too
if (setsockopt(udp_socket_info, IPPROTO_IP,IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) < 0) {
perror("setsockopt");
exit (1);
}
puts("setup multicast");
//Receive an incoming message
if( recvfrom(udp_socket_info, incoming_message , sizeof(incoming_message), 0, &addr, &fromlen) < 0) {
puts("Received failed");
exit (1);
}
puts("Message received");
puts(incoming_message);
}

Answer 1

You should bind the receiving socket to INADDR_ANY, not a local interface address. Otherwise you run the risk that the sender is out there via a different route and can't reach your socket. On some platforms you can bind it to the multicast address itself.
NB when you get an error it isn't sufficient to print a message of your own devising. The message must contain the errno, or the result of strerror(). For example, call perror().

Answer 2

Your receiver should not bind to a local address. It should instead bind to either INADDR_ANY or the multicast address you intend on joining. Binding to a local address breaks multicast on Linux systems.
Note that if you bind to a multicast address, this means you'll only receive packets for that multicast address. If you want to receive from multiple multicast addresses or if you also want to receive unicast packets then you need to bind to INADDR_ANY.
When joining a multicast group, using INADDR_ANY causes you to join the specified group on the default network interface. It's generally a good idea to explicitly specify an interface.
As EJP mentioned, you should always use perror to print error messages from any system or library call to ensure that a meaningful error message is printed.
Wireshark is an important tool for programs such as this. It helps you ensure that packets are going out and coming in the network interfaces you expect.
Also, if the sender and receive are on different network segments, you'll need to set the TTL via the IP_MULTICAST_TTL socket option. You also need to make sure that any routers between them are configured to pass multicast traffic.