boost::asio::async_write，写入大于65536字节的数据

Question

我正在尝试使用 async_write() 通过套接字将 jpeg 帧写入客户端。我使用了 boost 异步 TCP 日间服务器< /a> 示例作为起点。

#include <ctime>
#include <iostream>
#include <string>
#include <boost/bind.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/asio.hpp>

using boost::asio::ip::tcp;

std::string make_daytime_string()
{
  using namespace std; // For time_t, time and ctime;
  time_t now = time(0);
  return ctime(&now);
}

class tcp_connection
  : public boost::enable_shared_from_this<tcp_connection>
{
public:
  typedef boost::shared_ptr<tcp_connection> pointer;

  static pointer create(boost::asio::io_service& io_service)
  {
    return pointer(new tcp_connection(io_service));
  }

  tcp::socket& socket()
  {
    return socket_;
  }

  void start()
  {
    message_ = make_daytime_string();

    boost::asio::async_write(socket_, boost::asio::buffer(message_),
        boost::bind(&tcp_connection::handle_write, shared_from_this(),
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }

private:
  tcp_connection(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

  void handle_write(const boost::system::error_code& /*error*/,
      size_t /*bytes_transferred*/)
  {
  }

  tcp::socket socket_;
  std::string message_;
};

class tcp_server
{
public:
  tcp_server(boost::asio::io_service& io_service)
    : acceptor_(io_service, tcp::endpoint(tcp::v4(), 13))
  {
    start_accept();
  }

private:
  void start_accept()
  {
    tcp_connection::pointer new_connection =
      tcp_connection::create(acceptor_.io_service());

    acceptor_.async_accept(new_connection->socket(),
        boost::bind(&tcp_server::handle_accept, this, new_connection,
          boost::asio::placeholders::error));
  }

  void handle_accept(tcp_connection::pointer new_connection,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_connection->start();
      start_accept();
    }
  }

  tcp::acceptor acceptor_;
};

int main()
{
  try
  {
    boost::asio::io_service io_service;
    tcp_server server(io_service);
    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << e.what() << std::endl;
  }

  return 0;
}

我修改了执行async_write()的方法，如下所示：

 void start()
  {
    // fileToVector method reads contents of file to vector;
    std::vector<unsigned char> message_ = fileToVector("/tmp/test");

    boost::asio::async_write(socket_, boost::asio::buffer(message_),
        boost::bind(&tcp_connection::handle_write, shared_from_this(),
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }

当使用客户端从服务器读取大文件时，服务器最多只会写入65536字节。如果我用同步调用 boost::asio::write() 替换 boost::asio::async_write() 调用，则正确的字节数将传输到客户。

所以我想我的问题是，如何使用 boost::asio::async_write() 发送超过 65536 字节？任何帮助将不胜感激。

Answer 1

一个问题是，使用 async_write 函数数据不会立即由该函数发送，而是在 start 方法完成且本地 message_ 变量将被销毁，并且 boost::asio::buffer 不会复制 message_ 的内容。它仅存储对其的引用。结果是不可预测的。可能传输 65536 字节就是这种行为的结果。