Boost ASIO SSL number of bytes received

Question

I would like to create client/server communication programs pair using Boost ASIO + SSL. So I started off with the examples provided by boost, and I learned how that works, and I'm almost ready to develop my communication protocol, except that there's one problem.

So starting from this example, I'm modifying the handle_read() callback function after the handshake. The following is my code. My only modification is: Add another callback function called startComm(), which will start the communication.

void handle_read(const boost::system::error_code& error,
                 size_t bytes_transferred)
{
    if (!error)
    {
        std::cout << "Reply: ";
        std::cout.write(reply_, bytes_transferred);
        std::cout << "\n";

        boost::asio::async_write(socket_,
                                 boost::asio::buffer(std::string("Now?")),
                                 boost::bind(&SSLClient::startComm, this,
                                             boost::asio::placeholders::error,
                                             boost::asio::placeholders::bytes_transferred));
    }
    else
    {
        std::cout << "Read failed: " << error.message() << "\n";
    }
}

void startComm(const boost::system::error_code& error,
                 size_t bytes_transferred)
{
    if (!error)
    {
        std::cout << "Reply: ";
        std::cout.write(reply_, bytes_transferred); //problem here, bytes transferred should contain the number of received chars not number of written chars
        std::cout << "\n";
    }
    else
    {
        std::cout << "Read failed: " << error.message() << "\n";
    }


}

In the async_write() above, there's an argument boost::asio::placeholders::bytes_transferred which parametrizes my callback function to provide the number of bytes that were sent to the server. Now I would like to know the number of bytes the server responded with. How can I do that in my simple example?

Thanks. If you require any additional details, please ask.

Answer 1

The write call sends data.

Since it doesn't, at all, receive data the number of bytes received is by definition 0.

If you want to receive data, use (async_)read and it will tell you the number of bytes received.

These call backs use the same placeholder (bytes_transferred) but it carries different meaning depending on the direction of the transfer that has been completed.

Here's a solution that technically does what you want: define an extra parameter of startComm and bind it (not with a placeholder).

void handle_read(const boost::system::error_code &error, size_t bytes_received) {
    if (!error) {
        std::cout << "Reply: ";
        std::cout.write(reply_, bytes_received);
        std::cout << "\n";

        boost::asio::async_write(socket_, boost::asio::buffer(std::string("Now?")),
                                 boost::bind(&SSLClient::startComm, 
                                     this, 
                                     boost::asio::placeholders::error,
                                     bytes_received,
                                     boost::asio::placeholders::bytes_transferred));
    } else {
        std::cout << "Read failed: " << error.message() << "\n";
    }
}

void startComm(const boost::system::error_code &error, size_t had_received, size_t bytes_sent) {
    if (!error) {
        std::cout << "Reply: ";
        std::cout.write(reply_, had_received);
        std::cout << "\n";
    } else {
        std::cout << "Write failed: " << error.message() << "\n";
    }
}

Note that I still think you might mistakenly expect async_write to receive a reply, which (obviously?) isn't the case