我正在尝试使用 async_read 和 async_write 向服务器发出简单的 tcp 请求并设置超时。

问题是 async_read 在尝试读取直到传输结束时给出错误，在第一个 '\n' 上它返回错误(文件结束)。

逐行读取字符串时(当eots->at(last_request) = '\n')时，它成功读取了整个响应。

if(eots->at(last_request)=="") // read until end
        {
             boost::asio::async_read(
                socket_
                , input_buffer_
                , boost::asio::transfer_at_least(1) // read untill end or error
                , boost::bind(&tcp_client::do_Requests_read_handle, this, boost::asio::placeholders::error)
                );
        }else
        {
            boost::asio::async_read_until(
                socket_
                , input_buffer_
                , eots->at(last_request) // read until current request end of transmission sign/string or error
                , boost::bind(&tcp_client::do_Requests_read_handle, this, _1)
                );
        }

这是预期的行为吗？我做得对吗？

为了测试，我尝试进行 whois 查询 (args whois.iana.org 43 com)。

完整代码:

/*
 * MK async TCP
 * contains basic definitions for extractions and string filtering
 *
 */
#ifndef MK_ASYNC_TCP_HPP
#define MK_ASYNC_TCP_HPP
//
// async_tcp_client.cpp
// ~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2013 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <boost/asio/deadline_timer.hpp>
#include <boost/asio/io_service.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/read_until.hpp>
#include <boost/asio/read.hpp>
#include <boost/asio/streambuf.hpp>
#include <boost/asio/write.hpp>
#include <boost/asio/placeholders.hpp>
//#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <iostream>

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

//
// This class manages socket timeouts by applying the concept of a deadline.
// Some asynchronous operations are given deadlines by which they must complete.
// Deadlines are enforced by an "actor" that persists for the lifetime of the
// tcp_client object:
//
//  +----------------+
//  |                |
//  | check_deadline |<---+
//  |                |    |
//  +----------------+    | async_wait()
//              |         |
//              +---------+
//
// If the deadline actor determines that the deadline has expired, the socket
// is closed and any outstanding operations are consequently cancelled.
//
// Connection establishment involves trying each endpoint in turn until a
// connection is successful, or the available endpoints are exhausted. If the
// deadline actor closes the socket, the connect actor is woken up and moves to
// the next endpoint.
//
//  +---------------+
//  |               |
//  | start_connect |<---+
//  |               |    |
//  +---------------+    |
//           |           |
//  async_-  |    +----------------+
// connect() |    |                |
//           +--->| handle_connect |
//                |                |
//                +----------------+
//                          :
// Once a connection is     :
// made, the connect        :
// actor forks in two -     :
//                          :
// an actor for reading     :       and an actor for
// inbound messages:        :       sending heartbeats:
//                          :
//  +------------+          :          +-------------+
//  |            |<- - - - -+- - - - ->|             |
//  | start_read |                     | start_write |<---+
//  |            |<---+                |             |    |
//  +------------+    |                +-------------+    | async_wait()
//          |         |                        |          |
//  async_- |    +-------------+       async_- |    +--------------+
//   read_- |    |             |       write() |    |              |
//  until() +--->| handle_read |               +--->| handle_write |
//               |             |                    |              |
//               +-------------+                    +--------------+
//
// The input actor reads messages from the socket, where messages are delimited
// by the newline character. The deadline for a complete message is 30 seconds.
//
// The heartbeat actor sends a heartbeat (a message that consists of a single
// newline character) every 10 seconds. In this example, no deadline is applied
// message sending.
//
class tcp_client
{
    public:

      tcp_client(boost::asio::io_service& io_service, std::vector<std::string> * requests , std::vector<std::string>  * responses , std::vector<std::string>  * eots, unsigned int request_timeout = 30, unsigned int connect_timeout = 10)
        : stopped_(false),
          socket_(io_service),
          deadline_(io_service),
          heartbeat_timer_(io_service),
          requests(requests),
          responses(responses),
          eots(eots),
          request_timeout(request_timeout),
          connect_timeout(connect_timeout)
      {
        if(eots->size()==0)
        {
            for(unsigned long i=0 ; i<(requests->size()-1); i++)
            {
                eots->push_back("\n");
            }
            eots->push_back("");
        }
        if(responses->size()==0)
        {
            responses->resize(requests->size());
        }
        if( (eots->size() != requests->size()) || (requests->size() != responses->size()) )
        {
            std::cerr<<std::endl<<"wrong nr of parameters"<<std::endl;
            return;
        }
      }

      // Called by the user of the tcp_client class to initiate the connection process.
      // The endpoint iterator will have been obtained using a tcp::resolver.
      void start(tcp::resolver::iterator endpoint_iter)
      {
        // Start the connect actor.
        start_connect(endpoint_iter);

        // Start the deadline actor. You will note that we're not setting any
        // particular deadline here. Instead, the connect and input actors will
        // update the deadline prior to each asynchronous operation.
        deadline_.async_wait(boost::bind(&tcp_client::check_deadline, this));
      }

      // This function terminates all the actors to shut down the connection. It
      // may be called by the user of the tcp_client class, or by the class itself in
      // response to graceful termination or an unrecoverable error.
      void stop()
      {
        stopped_ = true;
        boost::system::error_code ignored_ec;
        socket_.close(ignored_ec);
        deadline_.cancel();
        heartbeat_timer_.cancel();
      }

    private:
      void start_connect(tcp::resolver::iterator endpoint_iter)
      {
        if (endpoint_iter != tcp::resolver::iterator())
        {
          std::cout << "Trying " << endpoint_iter->endpoint() << "...\n";

          // Set a deadline for the connect operation.
          deadline_.expires_from_now(boost::posix_time::seconds(60));

          // Start the asynchronous connect operation.
          socket_.async_connect(endpoint_iter->endpoint(),
              boost::bind(&tcp_client::handle_connect,
                this, _1, endpoint_iter));
        }
        else
        {
          // There are no more endpoints to try. Shut down the client.
          stop();
        }
      }
      void handle_connect(const boost::system::error_code& ec, tcp::resolver::iterator endpoint_iter)
      {
        if (stopped_)
          return;

        // The async_connect() function automatically opens the socket at the start
        // of the asynchronous operation. If the socket is closed at this time then
        // the timeout handler must have run first.
        if (!socket_.is_open())
        {
          std::cout << "Connect timed out\n";

          // Try the next available endpoint.
          start_connect(++endpoint_iter);
        }

        // Check if the connect operation failed before the deadline expired.
        else if (ec)
        {
          std::cout << "Connect error: " << ec.message() << "\n";

          // We need to close the socket used in the previous connection attempt
          // before starting a new one.
          socket_.close();

          // Try the next available endpoint.
          start_connect(++endpoint_iter);
        }

        // Otherwise we have successfully established a connection.
        else
        {
          std::cout << "Connected to " << endpoint_iter->endpoint() << "\n";
          boost::asio::socket_base::keep_alive option(true);
          socket_.set_option(option);

          //~ // Start the input actor.
          //~ start_read();

          //~ // Start the heartbeat actor.
          //~ start_write();
          deadline_.expires_from_now(boost::posix_time::seconds(this->request_timeout));
          do_Requests_write();


        }
      }


      void handle_Requests_finish()
      {
        if(last_request<requests->size())
        {
            last_request++;
            do_Requests_write();
        }else
        {
            stop();
        }
      }

      void do_Requests_write()
      {
        if (stopped_)
          return;

        // Start an asynchronous operation to send a heartbeat message.
        boost::asio::async_write(
            socket_
            , boost::asio::buffer(requests->at(last_request)+"\n")
            , boost::bind(&tcp_client::do_Requests_write_handle, this, _1)
            );
      }

      void do_Requests_write_handle(const boost::system::error_code& ec)
      {
        if (stopped_)
          return;

        if (!ec)
        {
          do_Requests_read();
        }
        else
        {
          std::cout << "Error do_Requests_write_handle: " << ec.message() << "\n";

          stop();
        }
      }

      void do_Requests_read()
      {
        // Set a deadline for the read operation.
        deadline_.expires_from_now(boost::posix_time::seconds(this->request_timeout));

        // Start an asynchronous operation to read a newline-delimited message.
        if(eots->at(last_request)=="") // read untill end
        {
             boost::asio::async_read(
                socket_
                , input_buffer_
                , boost::asio::transfer_at_least(1) // read untill end or error
                , boost::bind(&tcp_client::do_Requests_read_handle, this, boost::asio::placeholders::error)
                );
        }else
        {
            boost::asio::async_read_until(
                socket_
                , input_buffer_
                , eots->at(last_request) // read untill current request end of transmission sign/string or error
                , boost::bind(&tcp_client::do_Requests_read_handle, this, _1)
                );
        }
      }

      void do_Requests_read_handle(const boost::system::error_code& ec)
      {
        if (stopped_)
          return;

        if (!ec)
        {
          // Extract the newline-delimited message from the buffer.
          //~ std::string line;
          //~ std::istream is(&input_buffer_);
          //~ std::getline(is, line);
          std::istream response_istream(&input_buffer_);
          std::string response;
          response_istream >> response;

          // Empty messages are heartbeats and so ignored.
          std::cout << "Received: " << response << "\n";
          responses->at(last_request)+=response+"\n";
          //~ if (!line.empty())
          //~ {
            //~ std::cout << "Received: " << line << "\n";
          //~ }

          do_Requests_read();
        }
        else
        {
            std::cout<<(std::string)"Error on receive: " + ec.message() + "\n";
            responses->at(last_request)+= (std::string)"Error on receive: " + ec.message() + "\n";
            handle_Requests_finish();
        }
      }

      void check_deadline()
      {
        if (stopped_)
          return;

        // Check whether the deadline has passed. We compare the deadline against
        // the current time since a new asynchronous operation may have moved the
        // deadline before this actor had a chance to run.
        if (deadline_.expires_at() <= deadline_timer::traits_type::now())
        {
          // The deadline has passed. The socket is closed so that any outstanding
          // asynchronous operations are cancelled.
          socket_.close();

          // There is no longer an active deadline. The expiry is set to positive
          // infinity so that the actor takes no action until a new deadline is set.
          deadline_.expires_at(boost::posix_time::pos_infin);
        }

        // Put the actor back to sleep.
        deadline_.async_wait(boost::bind(&tcp_client::check_deadline, this));
      }

    private:
        bool stopped_;
        tcp::socket socket_;
        boost::asio::streambuf input_buffer_;
        deadline_timer deadline_;
        deadline_timer heartbeat_timer_;
        std::vector<std::string> *requests, *responses, *eots;
        unsigned int last_request=0;
        unsigned int request_timeout = 30;
        unsigned int connect_timeout = 10;
};

int main(int argc, char* argv[])
{
    std::vector<std::string> requests, responses, eots;
  try
  {
    if (argc < 4)
    {
      std::cerr << "Usage: tcp_client <host> <port> <query1> <query2> <query3> [..]\n";
      return 1;
    }
    for(int i = 3; i<argc ; i++ )
    {
        requests.push_back(argv[i]);
        eots.push_back("");
        responses.push_back("");
    }

    boost::asio::io_service io_service;
    tcp::resolver r(io_service);
    tcp_client c(io_service,&requests,&responses,&eots);

    c.start(r.resolve(tcp::resolver::query(argv[1], argv[2])));

    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}
#endif // MK_ASYNC_TCP_HPP

最佳答案

async_read()操作以 boost::asio::error::eof 错误代码完成，因为已到达文件末尾，而不是因为到达第一个 \n 时从 input_buffer_ 逐字读取。

whois.iana.org:43 上对 com 的响应是 1830 字节。

$ nc whois.iana.org 43 | wc --bytesenter
comenter
1830

当 boost::asio::streambuf提供给读取操作，它将尝试分配未指定大小的缓冲区，以便可以读取数据。 current implementation将尝试分配一个大小为 512 的缓冲区。因此，如果接收到 1830 个字节，并且每次读取操作读取缓冲区最大值为 512 字节，那么所有接收到的字节将在第 4 次读取操作中被读取。因此，第 5 次读取操作将导致文件结束。

async_read_until() 的完成条件导致行为略有不同。当 streambuf 包含指定的定界符或发生错误时，此操作被视为完成。当 async_read_until() 完成时，streambuf 可能包含分隔符之外的其他数据。如果 streambuf 的附加数据包含分隔符，则随后对 async_read_until() 的调用将满足其完成条件，而无需调用 AsyncReadStream 的 async_read_some( ) 函数。

关于c++ - boost::asio::async_read 在换行符上返回文件结尾错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/22663063/