boost::asio 从套接字读取 n 个字节到 Streambuf

boost::asio read n bytes from socket to streambuf

本文关键字：字节 Streambuf 读取 asio 套接字 boost      更新时间：2023-10-16

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。这是我得到的：

    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;

我看了一下

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

但它只能读取数据来提升：：ASIO：：缓冲区。我想知道我是否可以用 boost：：asio：：streambif 做同样的事情？提前谢谢你。

boost::asio::read() 和 boost::asio::async_read() 都有重载，它们接受 boost::asio::basic_streambuf 实例作为其缓冲区：

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，它等效于调用其关联的重载，CompletionCondition 为 boost::asio::transfer_all() ，导致操作读取 streambuf.max_size() 字节。

要将已知数量的字节读入 streambuf，请使用：

  • boost::asio::transfer_exactly(n)完成条件来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);

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

    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);

下面是演示这两种方法的完整示例：

#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.
}

输出：

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