Some interesting code that will send a UDP packet to all available broadcast/point-to-point IPv4 addresses

sendall.c

#include <sys/socket.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <ifaddrs.h>
#include <stdlib.h>
#include <netdb.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>

int main(int argc, const char** argv)
{
	int on = 1;
	char* sendstring = "HELLO";
	int sendlen = 6;

	struct ifaddrs *ifap;

	if (getifaddrs(&ifap) == 0)
	{
		struct ifaddrs* intf = ifap;
		struct sockaddr_in intfaddr, sendaddr;
		
		int sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

		if (sockfd < 0)
		{
			perror("socket");
			return -1;
		}

		if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, (char*) &on, sizeof(on)) < 0)
		{
			perror("setsockopt -> SO_BROADCAST");
			return -1;
		}

		while (intf)
		{
			memset(&intfaddr, 0, sizeof(struct sockaddr_in));
			memset(&sendaddr, 0, sizeof(struct sockaddr_in));

			intfaddr.sin_family = AF_INET;
			intfaddr.sin_port = htons(3456);
			
			if (intf->ifa_addr != NULL && ((struct sockaddr_in*) intf->ifa_addr)->sin_family == AF_INET)
				intfaddr.sin_addr = ((struct sockaddr_in*) intf->ifa_addr)->sin_addr;

			sendaddr.sin_family = AF_INET;
			sendaddr.sin_port = htons(3456);

			if (intf->ifa_flags & IFF_BROADCAST)
			{
				if (intf->ifa_broadaddr != NULL && ((struct sockaddr_in*) intf->ifa_broadaddr)->sin_family == AF_INET)
					sendaddr.sin_addr = ((struct sockaddr_in*) intf->ifa_broadaddr)->sin_addr;
			}
				else
			if (intf->ifa_flags & IFF_POINTOPOINT)
			{
				if (intf->ifa_dstaddr != NULL && ((struct sockaddr_in*) intf->ifa_dstaddr)->sin_family == AF_INET)
					sendaddr.sin_addr = ((struct sockaddr_in*) intf->ifa_dstaddr)->sin_addr;
			}

			if (sendaddr.sin_addr.s_addr != 0)
			{
				printf("interface %s\n", intf->ifa_name);
				
				char sendipaddr[32], intfipaddr[32];
				inet_ntop(AF_INET, &sendaddr.sin_addr, sendipaddr, 32);
				inet_ntop(AF_INET, &intfaddr.sin_addr, intfipaddr, 32);
				
				printf("\t%s -> %s\n", intfipaddr, sendipaddr);

				if (sockfd > 0)
				{
					int sent = sendto(sockfd, &sendstring, strlen(sendstring), 0, (struct sockaddr*) &sendaddr, sizeof(sendaddr));
				
					if (sent < 0)
						perror("sendto");
				}
			}

			intf = intf->ifa_next;
		}
		
		freeifaddrs(ifap);
	}

	return 0;
}