** broadcaster.c -- 一個類似 talker.c 的 datagram "client",
#define SERVERPORT 4950 // 所要連線的 port
int main(int argc, char *argv[])
struct sockaddr_in their_addr; // 連線者的位址資訊
//char broadcast = '1'; // 如果上面這行不能用的話,改用這行
fprintf(stderr,"usage: broadcaster hostname message\n");
if ((he=gethostbyname(argv[1])) == NULL) { // 取得 host 資訊
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
// 這個 call 就是要讓 sockfd 可以送廣播封包
if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &broadcast,
sizeof broadcast) == -1) {
perror("setsockopt (SO_BROADCAST)");
their_addr.sin_family = AF_INET; // host byte order
their_addr.sin_port = htons(SERVERPORT); // short, network byte order
their_addr.sin_addr = *((struct in_addr *)he->h_addr);
memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);
if ((numbytes=sendto(sockfd, argv[2], strlen(argv[2]), 0,
(struct sockaddr *)&their_addr, sizeof their_addr)) == -1) {
printf("sent %d bytes to %s\n", numbytes,
inet_ntoa(their_addr.sin_addr));