Simple listener and sender for UDP multicast

listener.c

//
// Simple listener.c program for UDP multicast
//
// Adapted from:
// http://ntrg.cs.tcd.ie/undergrad/4ba2/multicast/antony/example.html
//
// Changes:
// * Compiles for Windows as well as Linux
// * Takes the port and group on the command line
//

#ifdef _WIN32
    #include <Winsock2.h> // before Windows.h, else Winsock 1 conflict
    #include <Ws2tcpip.h> // needed for ip_mreq definition for multicast
    #include <Windows.h>
#else
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <time.h>
#endif

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define MSGBUFSIZE 256

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


#ifdef _WIN32
    //
    // Initialize Windows Socket API with given VERSION.
    //
    WSADATA wsaData;
    if (WSAStartup(0x0101, &wsaData)) {
        perror("WSAStartup");
        return 1;
    }
#endif

    // 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];
        int addrlen = sizeof(addr);
        int nbytes = recvfrom(
            fd,
            msgbuf,
            MSGBUFSIZE,
            0,
            (struct sockaddr *) &addr,
            &addrlen
        );
        if (nbytes < 0) {
            perror("recvfrom");
            return 1;
        }
        msgbuf[nbytes] = '\0';
        puts(msgbuf);
     }

#ifdef _WIN32
    //
    // Program never actually gets here due to infinite loop that has to be
    // canceled, but since people on the internet wind up using examples
    // they find at random in their own code it's good to show what shutting
    // down cleanly would look like.
    //
    WSACleanup();
#endif

    return 0;
}

sender.c

//
// Simple sender.c program for UDP
//
// Adapted from:
// http://ntrg.cs.tcd.ie/undergrad/4ba2/multicast/antony/example.html
//
// Changes:
// * Compiles for Windows as well as Linux
// * Takes the port and group on the command line
//
// Note that what this program does should be equivalent to NETCAT:
//
//     echo "Hello World" | nc -u 239.255.255.250 1900

#ifdef _WIN32
    #include <Winsock2.h> // before Windows.h, else Winsock 1 conflict
    #include <Ws2tcpip.h> // needed for ip_mreq definition for multicast
    #include <Windows.h>
#else
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <unistd.h> // for sleep()
#endif

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

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, `sender 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 test requires, make these configurable via args
    //
    const int delay_secs = 1;
    const char *message = "Hello, World!";

#ifdef _WIN32
    //
    // Initialize Windows Socket API with given VERSION.
    //
    WSADATA wsaData;
    if (WSAStartup(0x0101, &wsaData)) {
        perror("WSAStartup");
        return 1;
    }
#endif

    // create what looks like an ordinary UDP socket
    //
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        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 = inet_addr(group);
    addr.sin_port = htons(port);

      // now just sendto() our destination!
    //
    while (1) {
        char ch = 0;
        int nbytes = sendto(
            fd,
            message,
            strlen(message),
            0,
            (struct sockaddr*) &addr,
            sizeof(addr)
        );
        if (nbytes < 0) {
            perror("sendto");
            return 1;
        }

     #ifdef _WIN32
          Sleep(delay_secs * 1000); // Windows Sleep is milliseconds
     #else
          sleep(delay_secs); // Unix sleep is seconds
     #endif
     }

#ifdef _WIN32
    //
    // Program never actually gets here due to infinite loop that has to be
    // canceled, but since people on the internet wind up using examples
    // they find at random in their own code it's good to show what shutting
    // down cleanly would look like.
    //
    WSACleanup();
#endif

    return 0;
}

I am not sure this code work reliably on multi-homed workstation. To send multicast UDP you need to send on all interfaces.