Proper way to create packets along with sending and receiving them in socket programming using C

Question

I had written a small client-server code where in I was sending integers and characters from my client to server. So I know the basics of socket programming in C like the steps to follow and all. Now I want to create a packet and send it to my server. I thought that I will create a structure

    struct packet  
    { 
    int srcID;
    long int data;
    .....
    .....
    .....
    };
    struct packet * pkt;

Before doing send(), I thought that I will write values inside the packet using

   pkt-> srcID = 01
   pkt-> data = 1 2 3 4 

I need to know whether I am on the right path, and if yes then can I send using

      send(sockfd, &packet, sizeof(packet), 0)

for receiving

    recv(newsockfd, &PACKET, sizeof(PACKET), 0)

I have just started with network programming, so I am not sure whether i am on the right path or not. It would be of great help if anyone can guide me with my question in any form (theoretical,examples etc). Thanks in advance.

Answer 1

The pointer pkt is NOT defined in your application. You have two options: 1) Declare pkt as a normal variable

 struct packet pkt;

 pkt.srcID = 01;
 ....
 send(sockfd, &pkt, sizeof(struct packet), 0);

2) The second approach is useful when your packet contains a header followed by a payload:

 char buffer[MAX_PACKET_SIZE];
 struct packet *pkt = (struct packet *) buffer;
 char *payload = buffer + sizeof(struct packet);
 int packet_size;  /* should be computed as header size + payload size */

 pkt->srcID = 01;
 ...
 packet_size = sizeof(struct packet) /* + payload size */ ;

 send(sockfd, pkt, packet_size, 0);
 .... 

UPDATED (to answer your comment): First, you should know that receiving from a TCP socket MAY NOT provide the whole packet. You need to implement loop (as suggested by Nemo) to read the whole packet. Since you prefer the second option, then you need two loops. The first loop is to read the packet header to extract the payload size and the second loop to read the data. In case of UDP, you don't need to worry about partial receiving. Here is a sample code (without looping) where sockfd is a UDP socket:

char buffer[MAX_PACKET_SIZE];
struct packet *pkt = (struct packet *) buffer;
char *payload = buffer + sizeof(struct packet);
int packet_size;  /* should be computed as header size + payload size */

.....
/* read the whole packet */
if (recv(sockfd, pkt, MAX_PACKET_SIZE, 0) < 0) {
    /* error in receiving the packet. It is up to you how to handle it */
}
/* Now, you can extract srcID as  pkt->srcID */
/* you can get data by processing payload variable */

Remember: * you need to implement serialization as mentioned by other users * UDP is unreliable transport protocol while TCP is a reliable transport protocol.

Answer 2

1. Packet Structure

When you are going to send the data across the network then you need to consider having a fixed size header followed by a variable length payload.

[1 byte header] [Variable byte Payload]

The header should give you the size of data you are going to send so that the receiver will always read a fixed size header and then determine the packet length and read the rest of the bytes.

eg:

int nRet = recv(nSock,(char*)pBuffer,MESSAGE_HEADER_LENGTH,0);  
if (nRet ==MESSAGE_HEADER_LENGTH)               
{
   int nSizeOfPayload = //Get the length from pBuffer;
   char* pData = new Char(nSizeOfPayload );
   int nPayloadLen = recv(nSock, (char*)pData,nSizeOfPayload ,0); 
}

1. Variable length data in payload

If your structure is having string you should always have the size of the string appended before the string.

1. Endianess

If you are sending the packet to two different applications running in different machines you need to agree before hand on how you are representing your bytes i.e whether you are going to send MSB first or LSB first.