Multicast chat, for sharing links over a lan.

mc.c

/*
 * mc - multicast chat
 *
 *    Usage :
 *      mc [multicast-group [port]]
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <stdio.h>

#define DEFAULT_GROUP "224.11.12.13"
#define DEFAULT_PORT "50000"
#define MAX_PACKET_SIZE 65507

int yes = 1;

void setup_listener(int *sockfd, int argc, char **argv);
void setup_sender(int *sockfd, struct sockaddr_in *addr, int argc, char **argv);
void setup_select(fd_set *readfds, int sockfd);

int main(int argc, char *argv[]) {
    int outsock;
    int insock;
    int ready;
    struct sockaddr_in destination_addr;
    struct sockaddr_in sender_addr;
    fd_set readfds;
    ssize_t bytes_read;
    ssize_t addrlen = sizeof(struct sockaddr_in);
    socklen_t sender_size = sizeof(struct sockaddr_in);
    char running = 1;
    char buffer[65507];

    setup_listener(&insock, argc, argv);
    setup_sender(&outsock, &destination_addr, argc, argv);

    while (running) {
        setup_select(&readfds, insock);

        ready = select(insock + 1, &readfds, NULL, NULL, NULL);
        if (ready) {
            memset(buffer, 0, sizeof(buffer));
            if (FD_ISSET(0, &readfds)) {
                // ready stdin
                bytes_read = read(0, buffer, sizeof(buffer));
		if (bytes_read > 0) {
		    sendto(outsock, buffer, bytes_read, 0,
			(struct sockaddr *) &destination_addr, addrlen);
		} else if (bytes_read < 0) {
		    perror("STDIN read");
		} else {
		    running = 0;
		}
            } else if (FD_ISSET(insock, &readfds)) {
                // ready socket
                bytes_read = recvfrom(insock, buffer, sizeof(buffer), 0,
                    (struct sockaddr *)&sender_addr, &sender_size);
                printf("[%s:%d]: %s", inet_ntoa(sender_addr.sin_addr), ntohs(sender_addr.sin_port), buffer);
            }
        }
    }
    return 0;
}

// 0   1     2
// mc [addr [port]]
void setup_listener(int *sockfd, int argc, char **argv) {
    struct ip_mreq mreq;
    struct sockaddr_in addr;

    char *group = argc >= 2 ? argv[1] : DEFAULT_GROUP;
    char *port = argc >= 3 ? argv[2] : DEFAULT_PORT;

    *sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (*sockfd < 0) {
        perror("in socket");
        exit(1);
    }

    if (setsockopt(*sockfd, SOL_SOCKET, SO_REUSEPORT, &yes, sizeof(int)) < 0) {
	perror("Reuse address");
	exit(5);
    }

    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(atoi(port));
    addr.sin_addr.s_addr = INADDR_ANY;

    if (bind(*sockfd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("bind");
        exit(2);
    }

    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);
    if (setsockopt(*sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) < 0)  {
        perror("Add membership");
        exit(3);
    }
}

void setup_sender(int *sockfd, struct sockaddr_in *addr, int argc, char **argv) {

    char *group = argc >= 2 ? argv[1] : DEFAULT_GROUP;
    char *port = argc >= 3 ? argv[2] : DEFAULT_PORT;

    *sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (*sockfd < 0) {
        perror("out socket");
        exit(1);
    }

    memset(addr, 0, sizeof(struct sockaddr_in));
    addr->sin_family = AF_INET;
    addr->sin_addr.s_addr = inet_addr(group);
    addr->sin_port = htons(atoi(port));
}

void setup_select(fd_set *readfds, int sockfd) {
    FD_ZERO(readfds);
    FD_SET(0, readfds);
    FD_SET(sockfd, readfds);
}