在C中发送和接收结构-套接字编程

Question

我有一个结构定义如下：

struct controlMessage
{
   struct iphdr cmIphdr; //this use ip.h fronm linux library
   struct ipPayload cmIpPayload;
};

struct ipPayload
{
  uint8_t mType;
  uint16_t cid;
  char* payload; //this is intended to send different size of content
}

然后，发送方将设置该结构并将controlMessage发送给接收方

 struct controlMessage *cMsg = (struct controlMessage*) malloc (sizeof(struct controlMessage);
 cMsg->cmIphdr.protocol = 200; //this is not important, could be any number
 //etc wth all the other fields in iphdr
 //My question is in the ipPayload struct
 cMsg->cmIpPayload.mType = 82; //this is fine
 cMsg->cmIpPayload.cid = 1; //this is fine

 //Now I want to point my char pointer to a struct and send it as content
 struct controlMsg_payload
 {
   uint16_t somePort;
   //will be more stuffs, but keep it 1 for now for simplicity
 }
 struct controlMsg_payload *payload = (struct controlMsg_payload*) malloc(sizeof(struct controlMsg_payload));
 payload->somePort = 1000; //just assign a static number to it

 cMsg->cmIpPayload.payload = (char*) payload; //not sure if i did it right

 //Now sends it using sendto
 data = sendto(sockfd, cMsg, sizeof(struct controlMessage), 0, (struct sockaddr*) &rcv_addr, sizeof(rcv_addr));
 //remove error check for simplicity

我不确定是否发送正确，但是在接收方，我可以正确检索所有信息，但struct controlMsg_payload中的somePort信息除外。

下面是我在接收方的代码

char* buf = malloc(sizeof(struct controlMessage));
data = recvfrom(sockfd,buf,sizeof(struct controlMessage), 0, struct (sockaddr*) &serv_addr, &serv_len);

 struct iphdr* ip_hdr;
 struct ipPayload* ip_payload;

 ip_hdr = (struct iphdr*) buf;
 ip_payload = (struct ipPayload*) (buf+sizeof(struct iphdr));
 //from here I can print mType and cid correctly from ip_payload and also src & dest IP addr from ip_hdr

 //The only information I did not get correctly is the somePort info below
 struct controlMsg_payload* rcv_load;
 rcv_load = (struct controlMsg_payload*) ip_payload->payload; //get this from the struct ipPayload and cast it as it is char* type, not sure if I did it right
 printf ("my port = %d\n",rcv_load->somePort); //WRONG, not = 1000

我为许多代码感到抱歉，因为没有它很难解释。 基本上，我不会在接收器端读取somePort = 1000。

为了测试目的，我在发送方尝试了最后三行（在接收方），我能够读回1000。因此，我相信这必须是我打包数据并通过它发送/接收数据的方式导致这种情况的网络。

你们能帮我看看吗？ 提前致谢！

Answer 1

您正在发送指向有效负载的指针，而不是有效负载本身。 有效负载指针在接收方没有任何意义。 您需要将有效负载中的所有数据发送到接收器，以使其正常工作。

Answer 2

使用Wireshark查看您实际发送给另一端的内容。 一旦看到正确的数据，就可以开始调试接收器。 在这种情况下，您仅发送标头，而从不发送有效负载。

/* This sends the header... */
data = sendto(sockfd, cMsg, sizeof(struct controlMessage), ...

发送标头后，您需要发送有效负载。

sendto(sockfd, payload, sizeof(struct controlMsg_payload), ...

头中指向有效负载的指针（ char* payload; ）对接收者而言毫无意义。 如果您期望sendto函数跟随指针并发送有效负载，那不是它的工作原理。 sendto实际上只是发送指针的值。

您可以发送指针而只是在接收器上忽略它，或者更好的是您根本无法发送它。