C++ network programing in linux: Server Questions

Question

I am learning how to network program using c/c++ and I have created a server(TCP) that is suppose to respond in specific ways to messages from a client in order to do this I created a class that the server class passes the message to and returns a string to report back to the client.

Here is my problem sometimes it reports the correct string back other times if just repeats what I sent to the message handler. Which no where in the code do I have it return what was passed in. So I am wondering am I handling getting the message correctly?

Secondly, I am unsure of how to keep a connection open in a while loop to continually pass messages back and forth. You can see how I did it in the code below but I am pretty sure this is incorrect, any help on this would be great. Thanks!

if (!fork())
    { // this is the child process
        close(sockfd); // child doesn't need the listener

        while ((numbytes = recv(new_fd, buf, MAXDATASIZE-1, 0)) > 0)
        {
            //numbytes = recv(new_fd, buf, MAXDATASIZE-1, 0);

            buf[numbytes-1] = '\0';
            const char* temp = ash.handleMessage(buf).c_str();
            int size_of_temp = ash.handleMessage(buf).length();
            send(new_fd, temp, size_of_temp, 0);

            //send(new_fd, temp, size_of_temp+1, 0);
        }
    }//end if

Please excuse my ghetto code

Handles the message

Class Method handler uses

Answer 1

If your learning about sockets you should also know that you can't assume that what you send() as a "complete message", will be delivered as a complete message.

If you send() some big data from your client you might need to use multiple recv() on the server (or vice versa) to read it all. This is a big difference of how files usually work...

If you'r designing your own protocol you can opt to also send the messages length, like [LEN][message]. An easy example would be if the strings you send are limited to 256 bytes you can start with send()ing a short representing the strings length,

Or easier, decide that you use line-feeds (newline - \n) to terminate messages. The the protocol would look like

"msg1\nmsg2\n"

then you would have to recv(), and append the data, until you get a newline. This is all I can muster right now, there are a lot of great examples on the internet, but I would recommend getting the source of some "real" program and look at how it handles its network.

Answer 2

You are calling handleMessage twice. You didn't post the code, but it looks like you're returning a string. It might be better to do:

        string temp = ash.handleMessage(buf);
        int size_of_temp = temp.length();

This would avoid repeating any action that takes place in handleMessage.