BSD Socket API 的多播概念问题

Question

我对套接字编程和多播消息等主题相对较新，并试图更好地理解这个概念。 假设一台 PC 通过地址 224.0.0.0 和端口 4010 发送多播消息。 接收方部分设置了一个 UDP 套接字来侦听多播消息。 我的问题是，如果我从 recvfrom() 函数打印出 IP 地址，这应该是地址多播地址 224.0.0.0 还是来自发送者的本地 IP 地址？我已将具体问题放在打印值部分的注释中。还有做法有什么问题吗？

/*Sender IP is 11.12.13.14
Receiver local IP is 20.30.40.50*/


int sockUnicast;
int sockMulticast; /*For multicast*/

struct sockaddr_in local_addr;
struct sockaddr_in mcast_addr;

bool issockUnicastConnected = FALSE;
bool issockMulticastConnected = FALSE;

void myinit()
{
    sockUnicast = 0;
    sockMulticast = 0;
    
    /*Setup socket for unicast communicatin and bind it to any available port*/
    sockUnicast = socket(AF_INET,SOCK_DGRAM,0);
    
    if(sockUnicast >=0)
    {
        memset(&local_addr, 0, sizeof(local_addr));
        local_addr.sin_family = AF_INET;
        local_addr.sin_addr.s_addr = htonl(INADDR_ANY);
        local_addr.sin_port = htons(4000);
        
        if(bind(sockUnicast, (struct sockaddr*)&local_addr, sizeof(local_addr)) >=0)
        {
            issockUnicastConnected = TRUE;
        }
        
        /*Setup socket to listen on Multicast*/
        sockMulticast = socket(AF_INET,SOCK_DGRAM,0);
        
        memset(&mcast_addr, 0, sizeof(mcast_addr));
        mcast_addr.sin_family = AF_INET;
        mcast_addr.sin_addr.s_addr = htonl(INADDR_ANY);
        mcast_addr.sin_port = htons(4010);
        
        if(sockMulticast>=0)
        {
            if((bind(sockMulticast, (struct sockaddr*)&mcast_addr, sizeof(mcast_addr)) >= 0))
            {
                struct ip_mreq mreq;
                mreq.imr_multiaddr.s_addr = inet_addr("224.0.0.0");
                mreq.imr_interface.s_addr = htonl(INADDR_ANY);
                
                if (setsockopt(sockMulticast, IPPROTO_IP, IP_ADD_MEMBERSHIP , (void*)&mreq, sizeof(mreq))>= 0)
                {
                    issockMulticastConnected = TRUE;
                }
            }
        }
        else
        {
            soc_close(sockMulticast);
        }
    else
    {
        soc_close(sockUnicast);
    }
}


void mymain()
{
   char buffer[2048];
   struct sockaddr_in src_addr;
   INT addrlen = sizeof(src_addr);
while(1)
{
    
     if(issockUnicastConnected)
     {
        ssize_t rec_len = recvfrom(sockUnicast,buffer,sizeof(buffer),0, (struct  sockaddr*)&src_addr, &addrlen);
        char ip_str[32];
        inet_ntop(AF_INET, &(src_addr.sin_addr), ip_str, 32);
        
        /*I'm sending a package from the sender (11.12.13.14) as Unicast to Address 20.30.40.50 and Port 4000*/
        /*Thats OK and I see it printed out*/
        
        print("Unicast IP address is:: %s\n", ip_str);   
        print("PORT is: %d\n", ntohs(src_addr.sin_port));
        
        if (rec_len > 0)
        {
          /*do something with data received on unicast*/;
        }
    }
    
     if(issockMulticastConnected)
     {
        ssize_t rec_len_multi = recvfrom(sockMulticast,buffer,sizeof(buffer),0, (struct  sockaddr*)&src_addr, &addrlen);
        char ip_str[32];
        inet_ntop(AF_INET, &(src_addr.sin_addr), ip_str, 32);
        
        /*Now If I send a a package as a multicast message with IP 224.0.0.0 and to PORT 4010 from the sender (11.12.13.14) shouldn't I see the multicast IP and Port printed out here? I get 0.0.0.0 */
        
        print("Multicast IP address is:: %s\n", ip_str);
        print("PORT is: %d\n", ntohs(src_addr.sin_port));

        if(rec_len_multi > 0)
        {
            /*do something with data received on multicast*/
        }
    }  
  }
}

Answer 1

recvfrom

函数的第五个参数将始终填充为发送者的IP地址，无论使用的是单播还是多播。