Sending a StringStream of Binary Data from Cereal with ENet

Question

I have been working on wrapping ENet into a set of easy to use functions for a few weeks now and seem to have a bit of an issue.

I have a std::stringstream and am attempting to send the contents to a remote machine using ENet then reconstruct the std::stringstream on the remote machine.

The reason I need to use a std::stringstream is due to the fact that I'm serializing my data with the Cereal Serialization Library which requires a stream.

With Azoth's help he has identified that I need to be using std::istringstream and std::ostringstream. Previously I was only using std::stringstream which was causing an exception.

However now an exception is being thrown within Cereal at portable_binary.hpp line 156:

throw Exception("Failed to read " + std::to_string(size) + " bytes from input stream! Read " + std::to_string(readSize));

Here's what I'm doing:

void Send(ENetHost* Host)
{
    std::ostringstream SData;
    {
        cereal::PortableBinaryOutputArchive Archive(SData);
        Archive(PacketData);
    }

    std::string Out = SData.str();

    ENetPacket* Packet = enet_packet_create(Out.c_str(), Out.size(), ENET_PACKET_FLAG_RELIABLE);
    enet_host_broadcast(Host, 0, Packet);
}

A Cereal Portable Binary Data Archive is constructed to hold a single vector. The std::ostringstream is sent off to the host using ENet.

This part seems to work okay, I can print the information out before and after and it appears to be the same, albeit some weird symbols, but they print out the same on both ends.

Now a std::istringstream is created on the host with the data we received.

NetPacket(enet_uint8 const* Data)
{
    std::istringstream SData(reinterpret_cast<char const*>(Data));
    {
        cereal::PortableBinaryInputArchive Archive(SData);
        Archive(PacketData);
    }
}

At this point I receive the exception at line:

Archive(PacketData)

I have a feeling the data is being changed somehow when it's sent through ENet and/or I'm not pulling the data out of the std::ostringstream correctly and/or not putting the data back into the std::istringstream correctly.

Thank you very much for your time I greatly appreciate it.

Answer 1

Disclaimer: I'm not familiar with enet.

You are getting this error because you aren't constructing the std::stringstream properly upon receiving the packet. A send/receive pair should look something like:

my_send_function()
{
  std::ostringstream os;
  {    
    cereal::PortableBinaryOutputArchive ar(os);
    ar( whatever_needs_to_be_serialized );
  } // the binary archives will flush their output 
    // immediately, but it's better to
    // use cereal archives in an RAII matter all the time

  std::string data = os.str();

  create_packet(data.c_str(), data.size());
  // send out
}

And then on the receiving end, something like this:

my_receive_function( uint8_t const * data ) // data came from some packet
{
  MyDataType d;

  std::istringstream is(reinterpet_cast<char const *>(data));
  // this is safe to do since we generated the data using c_str(), which added
  // a null terminator to the data
  {
    cereal::PortableBinaryInputArchive ar(is);
    ar( d );
  }
}

The basic idea here: use cereal and some ostringstream to generate a string (which is really just an array of bytes), send those raw bytes over the network, pull them into an istringstream, and then have cereal parse that.