在C中发送和接收结构-套接字编程

Question

I have a struct defined as follow:

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
}

Then the sender will set up the struct and send the controlMessage to receiver

 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

I'm not sure if I send it correctly, but in the receiver side, I could retrieve all information correctly, except for the somePort info in the struct controlMsg_payload.

Below is my code in the receiver side

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

I'm sorry for a lot of codes since it's hard to explain without it. Basically, I don't read somePort = 1000 back in the receiver side.

I tried the same last three lines (in the receiver side) in the sender side for testing purposes, I'm able to read back 1000. So I believe that it has to be the way I package the data and send/receive it over the network that causes this.

Could you guys please help me take a look? Thanks in advance!

Answer 1

You are sending the pointer to the payload, not the payload itself. The payload pointer does not make any sense on the receiver side. You need to send all data in the payload to the receiver for this to work.

Answer 2

Use Wireshark to look at what you have actually sent to the other side. Once you see the correct data, then you can start debugging your receiver. In this case, you are sending the header only and never sending the payload.

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

After sending the header, you need to send the payload.

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

The pointer to the payload ( char* payload; ) in the header is meaningless to the receiver. If you are expecting the sendto function to follow the pointer and send the payload, that's not how it works. sendto is literally just sending the value of the pointer.

You could send the pointer and just ignore it on the receiver, or better yet you could not send it at all.