Send() message include some field in TCP socket (C Linux)

Question

System include some clients and server. Client send message to server have 3 types of message:

• Connection setup: User type command-line: remote-chat <IP addr server>: <port addr server> <IP address client>: <port addr client>, a link TCP connection will be established between the client and server chat. After the TCP connection is set, the client sends the connection setup information, including some fields:
  • The message identifier: is a 32-bit integer. With Connection setup message, this field equal to 0.
  • Field IP address of client 2 (client 1 call), Port address of client 2.

Server receive this message. Then create a connection to client 2.

• Data exchange: used to exchange data between client and server. Include some fields:
  • The message identifier: is a 32-bit integer. With Data exchange message this field = 1
  • The data length: an integer indicating the length of the text message.
  • The data text field: contains text messages to exchange.

When receiving data sent from client 1, chat server transfer text messages from client 1 to client 2 (client 2 also using data exchange message).

So my question is: how do I send the message with some field? I already know send string by function send(). Here I have to send message Connection Setup or Data exchange with some field then I whether used pack in client side then unpack in server side or not? Please give me some solution for this problem?

I think use:

typedef struct _ConnectionSetup_Message
{
    int  message_ID;    // 0 ConnectionSetup message
    unsigned int Port;
    unsigned shor IP;
} HELLO_Message;

typedef struct _DataExchange_Message
{
    int  message_Length;
    int  message_ID;    // 1 for DataExchange message
    char *text;
} DataExchange_Message;

Answer 1

You should try it yourself, its very educational. Some short advice, though:

1. Always put the message_ID first in the message, so you can recognize incoming messages by reading the first 4 bytes.
2. At the sending end, encode the fields of the structure in a char *buffer and send it with send(fd, buf, mlen, 0), e.g.,. (int *) buffer = htonl(hello_msg.message_ID); mlen = mlen + sizeof(hello_msg.message_ID);
3. At the receiving end do the revers. hello_msg.message_ID = ntohl(*(int *)buffer);
4. For the rest of the fields, keep count of the offset in the send/receive buffer.

EDIT For a c-language example you might look at e.g. https://stackoverflow.com/questions/5759043/sending-message-over-tcp-ip-using-sockets

Answer 2

Use serialization for your structures. And be aware never send pointers, but fully serialized data(strings). For more information read https://stackoverflow.com/a/1654822/2294017 and Serialize and send a data structure using Boost?. Also http://www.ocoudert.com/blog/2011/07/09/a-practical-guide-to-c-serialization/ can be handy if you need to dive deeper.

Answer 3

Maybe you can try to send an identifier for the kind of message and then send the struct with a len of sizeof(your_struct), and read it on the other side the same way.

for exemple:

server side:

 write(socket, &flag, sizeof(flag));
 write(socket, &your_struct, sizeof(DataExchange_Message));

and then on the client side:

read(socket, &flag, sizeof(flag));

//and according to the flag you can do what you want because you know what you are going to receive 
read(socket, &your_struct, sizeof(your_struct));