廣播
前面介紹的數據包發送方式只有一個接收方，稱為單播
如果發送給局域網中的所有主機，稱為廣播
只有用戶數據報（使用UDP協議）套接字才能廣播
廣播地址：
以192.168.1.0（255.255.255.0）網段為例，最大的主機地址192.168.1.255代表該網段的廣播地址
發送該地址的數據包被所有主機接收
255.255.255.255在所有網段中都代表廣播地址
廣播發送
創建用戶數據報套接字
缺省創建的套接字不允許廣播數據包，需要設置屬性--setsockopt可以設置套接字屬性
接收方指定為廣播地址
指定端口信息
發送數據包
設置
套接口的選項:
#include <
sys/types.h>
#include <sys/socket.h>
int setsockopt(int sockfd, int level, int optname,const void *optval, socklen_t optlen);
sockfd：標識一個套接口的描述字。
level：選項定義的層次；支持SOL_SOCKET、IPPROTO_TCP、IPPROTO_IP和IPPROTO_IPV6。
optname：需設置的選項。
optlen：optval緩沖區長度。
成功返回0，失敗返回-1並設置errno
廣播發送代碼：
sockfd=socket()
......
int on=1
setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&on,sizeof(on));
.........
sendto(....);
廣播接收：
創建用戶數據報套接字
綁定ip地址（廣播ip或0.0.0.0）和端口
綁定的端口必須和發送方的指定的端口相同
等待接收數據
實例如下：
send.c
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #define err_log(errlog) do{perror(errlog); exit(1);}while(0) #define N 128 // ./send 192.168.0.255 10000 int main(int argc, const char *argv[]) { int sockfd; struct sockaddr_in broadcastaddr; struct sockaddr_in clientaddr; socklen_t addrlen = sizeof(clientaddr); char buf[N] = {}; if(argc < 3) { fprintf(stderr, "usage:%s serverip port.\n", argv[0]); return -1; } if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { err_log("fail to socket"); } broadcastaddr.sin_family = AF_INET; broadcastaddr.sin_addr.s_addr = inet_addr(argv[1]); broadcastaddr.sin_port = htons(atoi(argv[2])); int on = 1; if(setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on)) < 0) { err_log("fail to setsockopt"); } while(1) { printf("Input > "); fgets(buf, N, stdin); buf[strlen(buf)-1] = '\0'; if(sendto(sockfd, buf, N, 0, (struct sockaddr *)&broadcastaddr, addrlen) < 0) { err_log("fail to sendto"); } if(strncmp(buf, "quit", 4) == 0) { break; } printf("%s\n", buf); } close(sockfd); return 0; }
recv.c
#include <stdio.h> #include <string.h> #include <stdlib.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #define err_log(errlog) do{perror(errlog); exit(1);}while(0) #define N 128 int main(int argc, const char *argv[]) { int sockfd; struct sockaddr_in broadcastaddr; struct sockaddr_in clientaddr; socklen_t addrlen = sizeof(clientaddr); char buf[N] = {}; if(argc < 3) { fprintf(stderr, "usage:%s serverip port.\n", argv[0]); return -1; } if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { err_log("fail to socket"); } broadcastaddr.sin_family = AF_INET; broadcastaddr.sin_addr.s_addr = inet_addr(argv[1]); broadcastaddr.sin_port = htons(atoi(argv[2])); if(bind(sockfd, (struct sockaddr*)&broadcastaddr, sizeof(broadcastaddr)) < 0) { err_log("fail to bind"); } while(1) { if(recvfrom(sockfd, buf, N, 0, (struct sockaddr*)&clientaddr, &addrlen) < 0) { err_log("fail to recvfrom"); } printf("From clientaddr:%s\n", buf); strcat(buf, " from server..."); } close(sockfd); return 0; }
編譯運行測試
./send 192.168.1.255 10000
分別在兩台局域網內的機器上測試
./recv 192.168.1.255 10000 收到
./recv 192.168.1.255 10000 收到
（2）組播（多播）
單播方式只能發送給一個接收方
廣播方式發給所有的主機。過多的廣播會大量占用網絡帶寬，造成廣播風暴，影響正常通信
組播（多播）是一種折中的方式只有加入某個多播組的主機才能收到數據
多播方式既可以發送給多個主機，又能避免廣播那樣帶來帶來過多的負載（每台主機要到傳輸層才能判斷廣播包是否要處理）
組播發送：
創建用戶數據報套接字
接收方地址指定為組播地址
指定端口信息
發送數據包
組播接收：
創建用戶數據報套接字
加入多播組
綁定ip地址（加入組的組ip或0.0.0.0）和端口---綁定的端口必須和發送方指定的端口相同
等待接收數據
加入多播組
struct ip_mreq
{
struct in_addr imr_multiaddr;
struct in_addr imr_interface;
}
struct ip_mreq req;
bzero(&req,sizeof(req)) req.imr_multiaddr.s_addr = inet_addr(argv[1]); req.imr_interface.s_addr = htonl(INADDR_ANY); if(setsockopt(sockfd, IPPROTO_IP,IP_ADD_MEMBERSHIP, &req, sizeof(req)) < 0) { err_log("fail to setsockopt"); }
send.c
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #define err_log(errlog) do{perror(errlog); exit(1);}while(0) #define N 128 // ./send 192.168.0.255 10000 int main(int argc, const char *argv[]) { int sockfd; struct sockaddr_in groupcastaddr; struct sockaddr_in clientaddr; socklen_t addrlen = sizeof(clientaddr); char buf[N] = {}; if(argc < 3) { fprintf(stderr, "usage:%s serverip port.\n", argv[0]); return -1; } if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { err_log("fail to socket"); } groupcastaddr.sin_family = AF_INET; groupcastaddr.sin_addr.s_addr = inet_addr(argv[1]); groupcastaddr.sin_port = htons(atoi(argv[2])); while(1) { printf("Input > "); fgets(buf, N, stdin); buf[strlen(buf)-1] = '\0'; if(sendto(sockfd, buf, N, 0, (struct sockaddr *)&groupcastaddr, addrlen) < 0) { err_log("fail to sendto"); } if(strncmp(buf, "quit", 4) == 0) { break; } printf("%s\n", buf); } close(sockfd); return 0; }
recv.c
#include <stdio.h> #include <string.h> #include <stdlib.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #define err_log(errlog) do{perror(errlog); exit(1);}while(0) #define N 128 int main(int argc, const char *argv[]) { int sockfd; struct sockaddr_in groupcastaddr; struct sockaddr_in clientaddr; socklen_t addrlen = sizeof(clientaddr); char buf[N] = {}; if(argc < 3) { fprintf(stderr, "usage:%s serverip port.\n", argv[0]); return -1; } if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { err_log("fail to socket"); } groupcastaddr.sin_family = AF_INET; groupcastaddr.sin_addr.s_addr = inet_addr(argv[1]); groupcastaddr.sin_port = htons(atoi(argv[2])); if(bind(sockfd, (struct sockaddr*)&groupcastaddr, sizeof(groupcastaddr)) < 0) { err_log("fail to bind"); } struct ip_mreq req; req.imr_multiaddr.s_addr = inet_addr(argv[1]); // req.imr_interface.s_addr = inet_addr("192.168.0.196"); req.imr_interface.s_addr = htonl(INADDR_ANY); if(setsockopt(sockfd, IPPROTO_IP,IP_ADD_MEMBERSHIP, &req, sizeof(req)) < 0) { err_log("fail to setsockopt"); } while(1) { if(recvfrom(sockfd, buf, N, 0, (struct sockaddr*)&clientaddr, &addrlen) < 0) { err_log("fail to recvfrom"); } printf("From clientaddr:%s\n", buf); } close(sockfd); return 0; }