boost::asio read n bytes from socket to streambuf

I have a serialized structure, which is being sent via socket. I need to read it in chunks, since one of its fields contains the size of the data remaining: I need to read first few bytes, find out the length and read the rest. This is what I have got:

    boost::asio::streambuf buffer;
    boost::system::error_code err_code;
    // here I need to read only first 16 bytes
    boost::asio::read(socket, buffer, err_code);
    std::istream is(&buffer);
    boost::archive::binary_iarchive ia(is);
    ia >> my_struct;

I have taken a look at

    boost::asio::async_read(s, boost::asio::buffer(data, size), handler);

but it can only read data to boost::asio::buffer. I am wondering if I could do the same with boost::asio::streambuf? Thank you in advance.

There are overloads for both boost::asio::read() and boost::asio::async_read() that accept instances of boost::asio::basic_streambuf as its buffer:

read(SyncReadStream&, basic_streambuf&);
read(SyncReadStream&, basic_streambuf&, CompletionCondition);

read(SyncReadStream&, basic_streambuf&, boost::system::error_code&);
read(SyncReadStream&, basic_streambuf&, CompletionCondition,
     boost::system::error_code&);

async_read(AsyncReadStream&, basic_streambuf&, ReadHandler);
async_read(AsyncReadStream&, basic_streambuf&, CompletionCondition,
           ReadHandler);

When calling an overload that does not accept a CompletionCondition, it is the equivalent to calling its associated overload with a CompletionCondition of boost::asio::transfer_all(), causing the operation to read streambuf.max_size() bytes.

---

For reading a known amount of bytes into a streambuf, either use:

• the boost::asio::transfer_exactly(n) CompletionCondition to limit the amount of bytes transferred from the composed operation:

  std::size_t n = // ...
  boost::asio::read(socket, streambuf, 
      boost::asio::transfer_exactly(n), error);
  

• Explicitly creating an output sequence that will serve as the buffer, then commiting the bytes read into the streambuf's input sequence:

  std::size_t n = // ...
  std::size_t bytes_transferred = boost::asio::read(socket,
      streambuf.prepare(n), // creates a boost::asio::buffer
      error);
  streambuf.commit(bytes_transferred);
  

---

Here is a complete example demonstrating both of these approaches:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>

// This example is not interested in the handlers, so provide a noop function
// that will be passed to bind to meet the handler concept requirements.
void noop() {}

std::string make_string(boost::asio::streambuf& streambuf)
{
  return {boost::asio::buffers_begin(streambuf.data()), 
          boost::asio::buffers_end(streambuf.data())};
}

int main()
{
  using boost::asio::ip::tcp;
  boost::asio::io_service io_service;

  // Create all I/O objects.
  tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 0));
  tcp::socket server_socket(io_service);
  tcp::socket client_socket(io_service);

  // Connect client and server sockets.
  acceptor.async_accept(server_socket, boost::bind(&noop));
  client_socket.async_connect(acceptor.local_endpoint(), boost::bind(&noop));
  io_service.run();

  // Write to server.
  boost::asio::streambuf write_buffer;
  std::ostream output(&write_buffer);
  output << "abc";
  std::cout << "Writing: " << make_string(write_buffer) << std::endl;
  auto bytes_transferred = boost::asio::write(server_socket, write_buffer);

  // Read from client.
  boost::asio::streambuf read_buffer;
  bytes_transferred = boost::asio::read(client_socket, read_buffer,
      boost::asio::transfer_exactly(bytes_transferred));
  std::cout << "Read: " << make_string(read_buffer) << std::endl;
  read_buffer.consume(bytes_transferred); // Remove data that was read.

  // Write to server.
  output << "def";
  std::cout << "Writing: " << make_string(write_buffer) << std::endl;
  bytes_transferred = boost::asio::write(server_socket, write_buffer);

  // Read from client.
  bytes_transferred = boost::asio::read(client_socket, 
      read_buffer.prepare(bytes_transferred));
  read_buffer.commit(bytes_transferred);      
  std::cout << "Read: " << make_string(read_buffer) << std::endl;
  read_buffer.consume(bytes_transferred); // Remove data that was read.
}

Output:

Writing: abc
Read: abc
Writing: def
Read: def