boost::asio 从socket读取n个字节到streambuf

我有一个序列化结构，它是通过套接字发送的。我需要分块读取它，因为它的一个字段包含剩余数据的大小：我需要读取前几个字节，找出长度并读取其余部分。这就是我所得到的：

    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;

我已经看过了

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

但它只能读取数据到 boost::asio::buffer 。我想知道我是否可以对 boost::asio::streambuf 做同样的事情？先感谢您。

两者都有过载boost::asio::read() and boost::asio::async_read()接受实例boost::asio::basic_streambuf作为其缓冲区：

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);

当调用不接受 CompletionCondition 的重载时，相当于使用 CompletionCondition 调用其关联的重载boost::asio::transfer_all()，导致操作读取streambuf.max_size() bytes.

要将已知数量的字节读取到streambuf中，可以使用：

• the boost::asio::transfer_exactly(n)CompletionCondition 限制从组合操作传输的字节数：

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

• 显式创建一个用作缓冲区的输出序列，然后将读取的字节提交到streambuf输入序列中：

  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);
  

这是一个完整的例子展示这两种方法：

#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