server and client for udp broadcasting, useful for server discovery.

broadcastclient.c

#if defined(_WIN32)
#define BUILD_FOR_WINDOWS
#endif

#ifdef BUILD_FOR_WINDOWS
#pragma warning(disable:4996) 
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#endif

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#ifdef BUILD_FOR_WINDOWS
#include <ws2tcpip.h>
#include <winsock.h>
#pragma comment (lib, "wsock32.lib")
#pragma comment(lib, "Ws2_32.lib")
#else
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h> 
#include <errno.h>
#include <unistd.h>
#endif
#include "common.h"

/////////////////////////////////////////////////////////////
char *program_name = "broadcastclient";
#ifdef BUILD_FOR_WINDOWS
WSADATA wsaData;
#endif
struct sockaddr_in si_me, si_other;
int socket_handle, slen = sizeof(si_other);
char buf[BUFLEN];
int choice = 0;
struct packet packet;
int broadcast = 1;
int conn_termination_request_len;
/////////////////////////////////////////////////////////////
void init_lengths();
void init_socket_lib();
void create_socket();
void fill_in_address();
void set_socket_to_broadcasting_mode();
void send_request();
void receive_response();
void resolve_ip_and_interpret_packet();
void cleanup();
/////////////////////////////////////////////////////////////
int main(int argc, char* argv[])
{
	printf("%s\n", program_name);
#ifdef BUILD_FOR_WINDOWS
	init_socket_lib();
#endif
	create_socket();
	fill_in_address();
	set_socket_to_broadcasting_mode();
	init_lengths();

	while (1) {
		printf("Enter a number ( enter 0 to quit) :");
		scanf("%d", &choice);
		if (choice == 0) {
			strncpy(buf, conn_termination_request, conn_termination_request_len);
			send_request();
			break;
		}
		strncpy(buf, "sendmeserverinfo", BUFLEN);
		printf("Sending request...\n");
		send_request();
		printf("Waiting for response...\n");
		receive_response();
		resolve_ip_and_interpret_packet();
	}
	cleanup();
	return 0;
}
/////////////////////////////////////////////////////////////
void init_socket_lib()
{
#ifdef BUILD_FOR_WINDOWS
	if (WSAStartup(MAKEWORD(2, 0), &wsaData) != 0) {
		printf("\nWSAStartup() failed; WSAGetLastError() returns %d", WSAGetLastError());
		exit(1);
	}
#endif
}

void create_socket()
{
	if ((socket_handle = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsocket() failed; WSAGetLastError() returns %d", WSAGetLastError());
		WSACleanup();
#else
		printf("\nsocket() failed; errno: %d", errno);
#endif
		exit(1);
	}
}

void fill_in_address()
{
	memset((char *)&si_other, 0, sizeof(si_me));
	si_other.sin_family = AF_INET;
	si_other.sin_port = htons(PORT);
	si_other.sin_addr.s_addr = htonl(INADDR_BROADCAST);
}

void set_socket_to_broadcasting_mode()
{
	if (setsockopt(socket_handle, SOL_SOCKET, SO_BROADCAST, (const char *)&broadcast, sizeof broadcast) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsetsockopt (SO_BROADCAST) failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("setsockopt (SO_BROADCAST)");
		close(socket_handle);
#endif
		exit(1);
	}
}

void init_lengths()
{
	conn_termination_request_len = strlen(conn_termination_request);
}

void send_request()
{
	if (sendto(socket_handle, buf, BUFLEN, 0, (struct sockaddr*)&si_other, slen) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsendto() failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("sendto() failed");
		close(socket_handle);
#endif
		exit(1);
	}
}

void receive_response()
{
	if (recvfrom(socket_handle, &packet, sizeof(struct packet), 0, (struct sockaddr*)&si_other, &slen) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\recvfrom() failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("recvfrom() failed");
		close(socket_handle);
#endif
		exit(1);
	}
}

void resolve_ip_and_interpret_packet()
{
	int server_id = packet.server_id;
	char *server_ip = inet_ntoa(si_other.sin_addr);
	int server_port = packet.server_port;
	char *server_name = packet.server_name;
	char *server_time = packet.time_now;
	printf("\nserver_id: %d\n", server_id);
	printf("server_port: %d\n", server_port);
	printf("server_ip: %s\n", server_ip);
	printf("server_name: %s\n", server_name);
	printf("server_time: %s\n", server_time);
}

void cleanup()
{
#ifdef BUILD_FOR_WINDOWS
	closesocket(socket_handle);
	WSACleanup();
#else
	close(socket_handle);
#endif
}

broadcastserver.c

#if defined(_WIN32)
#define BUILD_FOR_WINDOWS
#endif


#ifdef BUILD_FOR_WINDOWS
#pragma warning(disable:4996) 
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#endif

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#ifdef BUILD_FOR_WINDOWS
#include <ws2tcpip.h>
#include <winsock.h>
#pragma comment (lib, "wsock32.lib")
#pragma comment(lib, "Ws2_32.lib")
#else
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h> 
#include <errno.h>
#include <unistd.h>
#endif
#include "common.h"

//////////////////////////////////////////////////
char *program_name = "broadcastserver";
struct sockaddr_in si_me, si_other;
int socket_handle, slen = sizeof(si_other);
char request_buffer[BUFLEN];
time_t tmt;
struct packet response_packet;
int broadcast = 1;
///////////////////////////////////////////////

char *request_origin_ip;
int conn_termination_request_len;
const char* server_name = "fictious server";
int server_id = 1729;
int server_port = 65000;

#ifdef BUILD_FOR_WINDOWS
WSADATA wsaData;
#endif
///////////////////////////////////////////////
void init_lengths();
void init_socket_lib();
void create_socket();
void fill_in_address();
void set_socket_to_broadcasting_mode();
void bind_socket();
void receive_request();
void resolve_ip_of_request();
int check_if_disconnect_request_came();
void prepare_response();
void send_response();
void cleanup();
////////////////////////////////////////////////
int main(int argc, char* argv[])
{
	printf("%s\n", program_name);
#ifdef BUILD_FOR_WINDOWS
	init_socket_lib();
#endif
	create_socket();
	fill_in_address();
	set_socket_to_broadcasting_mode();
	bind_socket();
	init_lengths();
	while (1) {
		printf("Waiting for request...\n");
		receive_request();
		resolve_ip_of_request();
		printf("Request from ip %s came: %s\n", request_origin_ip, request_buffer);
		if (check_if_disconnect_request_came()) {
			break;
		}
		printf("Preparing response...\n");
		prepare_response();
		printf("Sending response...\n");
		send_response();
		printf("Response sent\n");
	}
	cleanup();
	return 0;
}
///////////////////////////////////////////////////////////
void init_lengths()
{
	conn_termination_request_len = strlen(conn_termination_request);
}

void init_socket_lib()
{
#ifdef BUILD_FOR_WINDOWS
	if (WSAStartup(MAKEWORD(2, 0), &wsaData) != 0) {
		printf("\nWSAStartup() failed; WSAGetLastError() returns %d", WSAGetLastError());
		exit(1);
	}
#endif
}

void create_socket()
{
	if ((socket_handle = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsocket() failed; WSAGetLastError() returns %d", WSAGetLastError());
		WSACleanup();
#else
		printf("\nsocket() failed; errno: %d", errno);
#endif
		exit(1);
	}
}

void fill_in_address()
{
	memset((char *)&si_me, 0, sizeof(si_me));
	si_me.sin_family = AF_INET;
	si_me.sin_port = htons(PORT);
	si_me.sin_addr.s_addr = htonl(INADDR_ANY);
}

void set_socket_to_broadcasting_mode()
{
	if (setsockopt(socket_handle, SOL_SOCKET, SO_BROADCAST, (const char *)&broadcast, sizeof broadcast) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsetsockopt (SO_BROADCAST) failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("setsockopt (SO_BROADCAST)");
		close(socket_handle);
#endif
		exit(1);
	}
}

void bind_socket()
{
	if (bind(socket_handle, (struct sockaddr*)&si_me, sizeof(si_me)) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nbind failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("bind");
		close(socket_handle);
#endif
		exit(1);
	}
}

void receive_request()
{
	if (recvfrom(socket_handle, request_buffer, BUFLEN, 0, (struct sockaddr*)&si_other, &slen) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\recvfrom() failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("recvfrom() failed");
		close(socket_handle);
#endif
		exit(1);
	}
}

void resolve_ip_of_request()
{
	request_origin_ip = inet_ntoa(si_other.sin_addr);
}

int check_if_disconnect_request_came()
{
	if (!strncmp(request_buffer, conn_termination_request, conn_termination_request_len)) {
		return 1;
	}
	return 0;
}

void prepare_response()
{
	response_packet.server_id = 1729;
	response_packet.server_port = 65000;
	strncpy(response_packet.server_name, server_name, NAMEMAXLEN);
	tmt = time(NULL);
	strncpy(response_packet.time_now, ctime(&tmt), TIMEMAXLEN);
}

void send_response()
{
	if (sendto(socket_handle, (struct packet *)&response_packet, sizeof(struct packet), 0, (struct sockaddr*)&si_other, slen) == -1) {
#ifdef BUILD_FOR_WINDOWS
		printf("\nsendto() failed; WSAGetLastError() returns %d", WSAGetLastError());
		closesocket(socket_handle);
		WSACleanup();
#else
		perror("sendto() failed");
		close(socket_handle);
#endif
		exit(1);
	}
}

void cleanup()
{
#ifdef BUILD_FOR_WINDOWS
	closesocket(socket_handle);
	WSACleanup();
#else
	close(socket_handle);
#endif

}

common.h

#ifndef _COMMON_H_
#define _COMMON_H_

#define PORT 8888
#define BUFLEN 512
#define TIMEMAXLEN 64
#define NAMEMAXLEN 64

struct packet
{
	int server_id;
	int server_port;
	char server_name[NAMEMAXLEN];
	char time_now[TIMEMAXLEN];
};

const char* conn_termination_request = "quit";
#endif