Simple Multicast Sender and Receiver in C over the Loopback interface

r.c

/* receiver run: gcc -o r r.c && ./r */

#include <arpa/inet.h>
#include <assert.h>
#include <net/if.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

/* change to tune into another channel */
#define PORT 10000

int main(int argc, char *argv[]) {
	int fd = socket(AF_INET, SOCK_DGRAM, 0);
	assert(fd >= 0);

	struct ifreq ifr = { .ifr_name = "lo" };
	assert(setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr)) == 0);

	char yes = '\1';
	assert(setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) == 0);
	
	struct sockaddr_in src = {0};
	src.sin_family = AF_INET;
	src.sin_addr.s_addr = htonl(INADDR_ANY);
	src.sin_port = htons(PORT);
	assert(bind(fd, (struct sockaddr *)&src, sizeof(src)) == 0);

	struct ip_mreq mreq = {0};
	mreq.imr_multiaddr.s_addr = inet_addr("239.0.0.1");
	mreq.imr_interface.s_addr = htonl(INADDR_LOOPBACK);
	assert(setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) == 0);

	ssize_t len;
	char buf[100];
	while (1) {
		len = recv(fd, &buf, sizeof(buf), 0);
		printf("%.*s\n", len, buf);
	}

	return 0;
}

s.c

/* sender run: gcc -o s s.c && ./s */

#include <arpa/inet.h>
#include <assert.h>
#include <net/if.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
	char payload[] = "hello world      ";

	int fd = socket(AF_INET, SOCK_DGRAM, 0);
	assert(fd >= 0);

	struct ifreq ifr = { .ifr_name = "lo" };
	assert(setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr)) == 0);

	struct sockaddr_in src = {0};
	src.sin_family = AF_INET;
	src.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
	assert(setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &src, sizeof(src)) == 0);

	char loop = '\1';
	assert(setsockopt(fd, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, 1) == 0);

	struct sockaddr_in dst = {0};
	dst.sin_family = AF_INET;
	dst.sin_addr.s_addr = inet_addr("239.0.0.1");

	while (1) {
		for (int i = 10000; i < 10100; i++) {
			sprintf(&payload[12], "%d", i);
			dst.sin_port = htons(i);
			sendto(fd, &payload, sizeof(payload) - 1, 0, (struct sockaddr *)&dst, sizeof(dst));
		}
		sleep(1);
	}

	return 0;
}