一，基础概念
数据报文在网络中的点对点传输方式通常有以下三种：
单播(Unicast): 数据报文从一个发送端到一个接收端的通信方式。
组播(Multicast): 数据报文从一个发送端到多个接收端的通信方式。
广播(broadcast): 数据报文从一个发送端到所有接收端的通信方式。
组播的实现需要设置组播地址
在IPv4中组播的地址范围是：224.0.0.0到239.255.255.255。
二，组播的具体实现过程
发送端：
step.01：新建一个socket套接字1用于往组播发送数据报文。
step.02：在sockaddr_in结构体中初始化组播的端口号。
step.03：设置IP_MULTICAST_LOOP等组播选项。
step.04：使用IP_MULTICAST_IF选项来定义组播接口。
step.05：调用sendto()接口往组播中发送数据报文。
接收端：
step.01：新建一个socket套接字2用于从组播接收数据报文。
step.02：设置SO_REUSEADDR选项以允许多个接收端去接收同一端口的数据报文。
step.03：调用bind()接口让套接字2绑定到组播指定的端口号。
step.04：使用IP_ADD_MEMBERSHIP选项，加入指定的组播。
step.05：调用read()接口从组播中接收数据报文。
三，组播属性设置
使用类型为SOCK_DGRAM的套接字，基于UDP协议来实现组播传输过程。
使用setsockopt()接口来设置与组播相关的属性。
setsockopt接口规范：
#include <sys/socket.h>
int setsockopt(int socket,
int level,
int option_name,
const void *option_value,
socklen_t option_len);
#返回值：成功后返回0，否则返回-1并设置对应的错误码setsockopt()接口可以为组播设置以下属性：
IP_ADD_MEMBERSHIP：加入指定的组播。
IP_DROP_MEMBERSHIP：退出指定的组播。
IP_MULTICAST_IF：设置组播中发送数据报文的接口。
IP_MULTICAST_TTL：设置组播中数据报文的生存时间(TTL)。
IP_MULTICAST_LOOP：设置组播中数据报文的副本是否回传。四，完整代码实现
Demo1——C++代码实现发送端：
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
struct in_addr localInterface;
struct sockaddr_in groupSock;
int sd;
char databuf[1024] = "Multicast Data MSG!";
int datalen = sizeof(databuf);
int main (int argc, char *argv[])
{
/*
* Create a datagram socket on which to send.
*/
sd = socket(AF_INET, SOCK_DGRAM, 0);
if (sd < 0) {
perror("opening datagram socket");
exit(1);
}
/*
* Initialize the group sockaddr structure with a
* group address of 225.1.1.1 and port 5555.
*/
memset((char *) &groupSock, 0, sizeof(groupSock));
groupSock.sin_family = AF_INET;
groupSock.sin_addr.s_addr = inet_addr("225.1.1.1");
groupSock.sin_port = htons(5555);
/*
* Disable loopback so you do not receive your own datagrams.
*/
{
char loopch=0;
if (setsockopt(sd, IPPROTO_IP, IP_MULTICAST_LOOP,
(char *)&loopch, sizeof(loopch)) < 0) {
perror("setting IP_MULTICAST_LOOP:");
close(sd);
exit(1);
}
}
/*
* Set local interface for outbound multicast datagrams.
*/
localInterface.s_addr = inet_addr("127.0.0.1");
if (setsockopt(sd, IPPROTO_IP, IP_MULTICAST_IF,
(char *)&localInterface,
sizeof(localInterface)) < 0) {
perror("setting local interface");
exit(1);
}
/*
* Send a message to the multicast group specified by the
* groupSock sockaddr structure.
*/
if (sendto(sd, databuf, datalen, 0,
(struct sockaddr*)&groupSock,
sizeof(groupSock)) < 0)
{
perror("sending datagram message");
}
else
{
printf("sending datagram message ok! n");
printf("The message send to multicast server is: "%s"n", databuf);
}
}接收端：
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
struct sockaddr_in localSock;
struct ip_mreq group;
int sd;
char databuf[1024];
int datalen = sizeof(databuf);
int main (int argc, char *argv[])
{
/*
* Create a datagram socket on which to receive.
*/
sd = socket(AF_INET, SOCK_DGRAM, 0);
if (sd < 0) {
perror("opening datagram socket");
exit(1);
}
/*
* Enable SO_REUSEADDR to allow multiple instances of this
* application to receive copies of the multicast datagrams.
*/
{
int reuse=1;
if (setsockopt(sd, SOL_SOCKET, SO_REUSEADDR,
(char *)&reuse, sizeof(reuse)) < 0) {
perror("setting SO_REUSEADDR");
close(sd);
exit(1);
}
}
/*
* Bind to the proper port number with the IP address
* specified as INADDR_ANY.
*/
memset((char *) &localSock, 0, sizeof(localSock));
localSock.sin_family = AF_INET;
localSock.sin_port = htons(5555);;
localSock.sin_addr.s_addr = INADDR_ANY;
if (bind(sd, (struct sockaddr*)&localSock, sizeof(localSock))) {
perror("binding datagram socket");
close(sd);
exit(1);
}
/*
* Join the multicast group 225.1.1.1 on the local 9.5.1.1
*/
group.imr_multiaddr.s_addr = inet_addr("225.1.1.1");
group.imr_interface.s_addr = inet_addr("127.0.0.1");
if (setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
(char *)&group, sizeof(group)) < 0) {
perror("adding multicast group");
close(sd);
exit(1);
}
/*
* Read from the socket.
*/
datalen = sizeof(databuf);
if (read(sd, databuf, datalen) < 0) {
perror("reading datagram message");
close(sd);
exit(1);
}
else
{
printf("get datagram message ok! n");
printf("The message from multicast server is: "%s"n", databuf);
}
}运行结果：发送端：
sending datagram message ok!
The message send to multicast server is: "Multicast Data MSG!"接收端：
get datagram message ok!
The message from multicast server is: "Multicast Data MSG!"Demo2——Python代码实现发送端：
import socket
from time import sleep, time
MCAST_GRP = '224.1.1.1'
MCAST_PORT = 5007
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 2)
while True:
print "Multicast to group: %sn" % MCAST_GRP
data = 'Hello World: ' + repr(time()) + 'n'
sock.sendto(data, (MCAST_GRP, MCAST_PORT))
sleep(1)接收端：
import socket
import struct
MCAST_GRP = '224.1.1.1'
MCAST_PORT = 5007
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind(('', MCAST_PORT))
mreq = struct.pack("4sl", socket.inet_aton(MCAST_GRP), socket.INADDR_ANY)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
while True:
print sock.recv(10240)运行结果：发送端：
Multicast to group: 224.1.1.1
Multicast to group: 224.1.1.1
Multicast to group: 224.1.1.1
Multicast to group: 224.1.1.1接收端：
Hello World: 1690308503.613114
Hello World: 1690308503.613114
Hello World: 1690308504.615013
Hello World: 1690308504.615013五，参考阅读
https://www.ibm.com/docs/en/i/7.1?topic=designs-examples-using-multicasting-af-inet
https://os.mbed.com/handbook/Socket
https://subingwen.cn/linux/multicast/index.html
声明：文中观点不代表本站立场。本文传送门：https://eyangzhen.com/254190.html
