Question

I need to write a simple file transfer protocol in C++ to upload or download a file from a client to a server. Right now both my client and server applications are do-it-all scripts. I want to modularize everything in classes/methods so I can more easily modify things later.

To upload a file, I need the name of user sending file (no authentication though), file name, file size, and file contents. This is what I have on client side:

int sendFile(string fileName, SOCKET s) {
    ifstream file;
    int ibytesent;
    int size;
    char fileSize[packetSize];

    // Open file, set get pointer at end of file
    file.open (fileName.c_str(), ios::binary | ios::ate);

    // Get file size and pass it to buffer
    size = (int) file.tellg();

    //itoa(size, fileSize, packetSize);
    sprintf(fileSize, "%d", size);

    // Set pointer to beginning
    file.seekg(0, ios::beg);

    // Send file size
    if (send(s, fileSize, packetSize, 0) == SOCKET_ERROR) {
        throw "Send failed\n";
    }

    cout << "Size sent: " << size;

    // Buffer the file Name
    char fileNameBuffer[packetSize];

    sprintf(fileNameBuffer, fileName.c_str());

    // Send the file name
    if (send(s, fileNameBuffer, packetSize, 0) == SOCKET_ERROR) {
        throw "Send failed\n";
    }

    cout << "File name sent: " << fileName << endl;

    char buf[packetSize];
    ibytesent = 0;
    // Loop until the whole file is sent
    while(file.good()) {
        // Read next packetSize bytes into buf
        file.read(buf, packetSize);

        // Send the read data
        ibytesent += send(s, buf, packetSize, 0);

        if (ibytessent == SOCKET_ERROR) {
            throw "Send failed\n";
        }
    }

    cout << "File sent.\n";
    //wait for reception of server response.
    ibytesrecv=0;
    /*
    if((ibytesrecv = recv(s,szbuffer,128,0)) == SOCKET_ERROR)
        throw "Receive failed\n";
    else
        cout << "File received.";
     */
    file.close();

    return 1;
}

And then there is a main() method that opens up a socket and connects to the server. My question really is about how to design/implement a proper file transfer protocol. What is a good structure for it? I would also like to know your opinions about how to split (and send) a file into a packet structure rather than the way I do it now. I don’t necessarily need code, but separation of concerns into this and that class, etc.

Please don’t recommend an already existing protocol since that isn’t the point of this exercise.

Thanks

EDIT I kind of found something similar to what I’m looking for here: http://www.eventhelix.com/realtimemantra/PatternCatalog/protocol_layer.htm

Answer 1

Here’s a take on your possible classes:

• ISender and IReceiver interfaces for senders and receivers

• StreamSender and StreamReceiver concrete implementations of ISender and IReceiver respectively (using sockets I guess)

• IDataSource interface to get data from source

• FileReader a concrete implementation of IDataSource to read file off disk to be fed into StreamSender

Example:

ISender sender=new StreamSender(new FileReader("filename.txt"));
sender.setDest(new StreamReceiver(host));
sender.setName(senderName);
sender.send();

Obviously this is just an example off the top of my head. There will be missing pieces.