How to check whether a client has joined a multicast group? - c

Question

End goal:
For the parent process of the server to know what clients join or leave the multicast group. So far I've only tried to check on clients joining, for I assume checking on those leaving is a similar matter.
My method(s) so far:
Checking for changes in the socket of the multicast group through select().
After joining, the client executes a sendto() directed to the (parent process of) the server program. The select() in the server is meant to recognize any changes, but apparently doesn't, and thus retval != 0 is never true.
Results so far
I Have tried with many different IP addresses and constants such as INADDR_ANY, but I've only managed as far as sending a message back to the client through the multicast, and this one interpret it as if the server-program had sent it. The most common result is the server-program not receiving any message at all.
Here is my client code:
/* Receiver/client multicast Datagram*/
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_MSG 100
struct sockaddr_in localSock, servSock;
struct ip_mreq group;
int sd, n;
int datalen, mcastport;
char msg[MAX_MSG];
int main(int argc, char *argv[])
{
if(argc!=3) {
printf("usage : %s <address> <port>\n",argv[0]);
exit(0);
}
mcastport = atoi(argv[2]);
/* Create a datagram socket on which to receive. */
sd = socket(AF_INET, SOCK_DGRAM, 0);
if(sd < 0)
{
perror("Opening datagram socket error");
exit(1);
}
else
printf("Opening datagram socket....OK.\n");
/* Enable SO_REUSEADDR to allow multiple instances of this */
/* application to receive copies of the multicast datagrams. */
{
int reuse = 1;
if(setsockopt(sd, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof(reuse)) < 0)
{
perror("Setting SO_REUSEADDR error");
close(sd);
exit(1);
}
else
printf("Setting SO_REUSEADDR...OK.\n");
}
/* Bind to the proper port number with the IP address */
/* specified as INADDR_ANY. */
memset((char *) &localSock, 0, sizeof(localSock));
localSock.sin_family = AF_INET;
localSock.sin_port = htons(mcastport);
localSock.sin_addr.s_addr = INADDR_ANY;
if(bind(sd, (struct sockaddr*)&localSock, sizeof(localSock)))
{
perror("Binding datagram socket error");
close(sd);
exit(1);
}
else
printf("Binding datagram socket...OK.\n");
printf("Enter the group's name you want to join:\n");
scanf("%s", msg);
/* Join the multicast group 226.1.1.1 on the local IP address */
/* interface. Note that this IP_ADD_MEMBERSHIP option must be */
/* called for each local interface over which the multicast */
/* datagrams are to be received. */
group.imr_multiaddr.s_addr = inet_addr(argv[1]);
group.imr_interface.s_addr = inet_addr("127.0.0.1");
if(setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&group, sizeof(group)) < 0)
{
perror("Adding multicast group error");
close(sd);
exit(1);
}
else
printf("Adding multicast group...OK.\n");
/* Initialize the group sockaddr structure with a */
/* group address of 226.1.1.1 and port given by user. */
memset((char *) &servSock, 0, sizeof(servSock));
servSock.sin_family = AF_INET;
servSock.sin_addr.s_addr = inet_addr(argv[1]);
servSock.sin_port = htons(mcastport);
if(sendto(sd, "", 1, 0, (struct sockaddr*) &servSock, sizeof(servSock)) < 0)
{perror("Sending datagram message error");}
else
printf("Sending datagram message...OK\n");
/* Read from the socket. */
if((n=read(sd, msg, MAX_MSG)) < 0)
{
perror("Reading datagram message error");
close(sd);
exit(1);
}
else
{
printf("Reading datagram message...OK.\n");
printf("The message from multicast server is: \"%s\"\n", msg);
}
msg[n] = '\0';
return 0;
}
This is my server program code:
/* Send Multicast Datagram code*/
#include <sys/types.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h> /* for strncpy, memset */
#define MAX_MSG 100
struct in_addr localInterface;
struct sockaddr_in groupSock, cliAddr;
int sd, mcastport, maxJoin, maxJoined = 0, pipefd[2], cliLen, cpid;
char msg[MAX_MSG], groupName[MAX_MSG];
int main (int argc, char *argv[ ])
{
/* check command line args */
if(argc < 2) {
printf("usage : %s <port> \n", argv[0]);
exit(1);
}
mcastport = atoi(argv[1]);
/* Create a datagram socket on which to send. */
sd = socket(AF_INET, SOCK_DGRAM, 0);
if(sd < 0)
{
perror("Opening datagram socket error");
exit(1);
}
else
printf("Opening the datagram socket...OK\n");
/* Initialize the group sockaddr structure with a */
/* group address of 225.1.1.1 and port given by user. */
memset((char *) &groupSock, 0, sizeof(groupSock));
groupSock.sin_family = AF_INET;
groupSock.sin_addr.s_addr = inet_addr("226.1.1.1");
groupSock.sin_port = htons(mcastport);
printf("Create a group: ");
scanf("%s", groupName);
printf("Maximum number of clients that can join the group? ");
scanf("%d", &maxJoin);
/* Disable loopback so you do not receive your own datagrams.
{
char loopch = 0;
if(setsockopt(sd, IPPROTO_IP, IP_MULTICAST_LOOP, (char *)&loopch, sizeof(loopch)) < 0)
{
perror("Setting IP_MULTICAST_LOOP error");
close(sd);
exit(1);
}
else
printf("Disabling the loopback...OK.\n");
}
*/
/* Set local interface for outbound multicast datagrams. */
/* The IP address specified must be associated with a local, */
/* multicast capable interface. */
printf("Setting the local interface...");
localInterface.s_addr = inet_addr("127.0.0.1");
if(setsockopt(sd, IPPROTO_IP, IP_MULTICAST_IF, (char *)&localInterface, sizeof(localInterface)) < 0)
{
perror("error");
exit(1);
}
else
printf("OK\n");
if((cpid = fork()) == 0) //child process --sends messages
{
/* Send a message to the multicast group specified by the*/
/* groupSock sockaddr structure. */
printf("Enter a message to send: \n");
scanf("%s", msg);
if(sendto(sd, msg, strlen(msg)+1, 0, (struct sockaddr*)&groupSock, sizeof(groupSock)) < 0)
{perror("Sending datagram message error");}
else
printf("Sending datagram message...OK\n");
/* Try the re-read from the socket if the loopback is not disable
if(read(sd, databuf, datalen) < 0)
{
perror("Reading datagram message error\n");
close(sd);
exit(1);
}
else
{
printf("Reading datagram message from client...OK\n");
printf("The message is: %s\n", databuf);
}
*/
exit(EXIT_SUCCESS);
}
else //parent process --checks for JOINs and QUITs
{
fd_set rfds;
struct timeval tv;
int retval, status;
while (waitpid(cpid, &status, WNOHANG) != cpid)
{
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(sd, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(sd+1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if (retval == -1)
perror("select()");
else if (retval != 0)
{
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
cliLen = sizeof(cliAddr);
int n;
if((n = recvfrom(sd, msg, MAX_MSG, 0, (struct sockaddr *) &cliAddr,&cliLen)) == -1)
perror("Some bullshit happened");
msg[n] = '\0';
printf("Client IP:port is: %s:%d", inet_ntoa(cliAddr.sin_addr), (int) ntohs(cliAddr.sin_port));
} else
{printf("no data.\n");}
}
exit(0);
}
return 0;
}
Possible solutions I haven't tried:
Maybe I shouldn't use the multicast group to send information client->server, but rather another type of connection? I'm just guessing here. I know you guys don't like doing anyone's work.
I have been at this "simple" issue for several hours and tried reading everywhere I could, including this question which seems very similar, but I haven't managed to solve it in any way. I'm completely striking out here.

Answer 1

If the client sends to the multicast group and the server expects to read that, the server needs to join the multicast group as well.
It isn't a great solution, as all the other client members will also receive that multicast.
It would make more sense for the client to first receive a multicast from the server, and then to respond to the server's address, which is provided via the result arguments of recvfrom().

Answer 2

Two points that may help you:
There is no need to use multicasting if your communication is only local (you use 127.0.0.1 in your code)
Multicast groups are joined by network interfaces, not by processes. Once a network interface has joined a multicast interface, a second joining of the same network interface to the same multicast group will change nothing. Even when it's another process on the same host that requests the second
joining.