User buffer size to receive multicast packets?

Question

The below code is from Git. It joins a multicast group and receives packets.

Here we loop and receive the data in a buffer called msgbuf:

while (1) 
{
    char msgbuf[MSGBUFSIZE];
    const int addrlen = sizeof(addr);
    const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);

How do I choose the size for the buffer msgBuf? Does it just have to be the max packet size? Or do I need to store multiple packets whilst I process the first?

Full code:

int main(int argc, char *argv[])
{
    if (argc != 3) {
       printf("Command line args should be multicast group and port\n");
       printf("(e.g. for SSDP, `listener 239.255.255.250 1900`)\n");
       return 1;
    }

    char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
    int port = atoi(argv[2]); // 0 if error, which is an invalid port

    if(port <= 0)
    {
        perror("Invalid port");
        return 1;
    }


    // create what looks like an ordinary UDP socket
    //
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) 
    {
        perror("socket");
        return 1;
    }

    // allow multiple sockets to use the same PORT number
    //
    u_int yes = 1;
    if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (char*) &yes, sizeof(yes)) < 0)
    {
       perror("Reusing ADDR failed");
       return 1;
    }

        // set up destination address
    //
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY); // differs from sender
    addr.sin_port = htons(port);

    // bind to receive address
    //
    if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0) 
    {
        perror("bind");
        return 1;
    }

    // use setsockopt() to request that the kernel join a multicast group
    //
    struct ip_mreq mreq;
    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);

    if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)) < 0)
    {
        perror("setsockopt");
        return 1;
    }

    // now just enter a read-print loop
    //
    while (1) 
    {
        char msgbuf[MSGBUFSIZE];
        const int addrlen = sizeof(addr);
        const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);

        if (nbytes < 0) 
        {
            perror("recvfrom");
            return 1;
        }
        msgbuf[nbytes] = '\0';
        puts(msgbuf);
     }

    return 0;
}

Answer 1

Unlike TCP which combines packets into a stream, UDP respects packet boundaries so recvfrom only gets one packet at a time.

So MSGBUFSIZE only needs to be as big as a single packet. If you're not using jumbo packets that would be 1500, otherwise it would be 9000.