Boost Android Asio 连接错误

Question

我正在使用 Boost Asio 进行跨平台网络。我尝试了各种连接方法，但在尝试连接到 Android 上的套接字时，我不断收到“找不到元素”asio.misc 错误。我很困惑，因为这发生在成功解析域名之后。例如，如果我为主机和端口 80 输入 google.com，那么解析器将打印出所有 20 多个谷歌服务器，但稍后它不会连接说 asio.misc “找不到元素”

这是我的代码：

    boost::asio::io_service service;
    boost::system::error_code ec;
    boost::asio::ip::tcp::resolver resolver(service);
    boost::asio::ip::tcp::resolver::query query(host, port);
    boost::asio::ip::tcp::resolver::iterator iter = resolver.resolve(query, ec);
    boost::asio::ip::tcp::resolver::iterator iterEnd;
    boost::asio::ip::tcp::endpoint ep;
    int it = 0;
    while(iter != iterEnd)
    {
        ep = *iter++;
        debug+="\nLooking at resolve with address " + ep.address().to_string();
        it++;
    }

        //Try to connect through asio
        boost::asio::ip::tcp::socket sock( service );
        //asio::connect does all the iterator work for us.
        boost::asio::connect(sock, iter, ec);

        if( ec )
        {
            return new string("Should be connected but I can't");
        }
        else
        {
            debug+= "\n     " + ec.message() + "\n     " + ec.category().name();
            return debug;
        }

Answer 1

调试循环耗尽了迭代器，导致boost::asio::connect() 被一个结束迭代器调用。

boost::asio::ip::tcp::resolver::iterator iter = resolver.resolve(query, ec);

while(iter != iterEnd) // iterate to exhaustion
{
  ep = *iter++;
  // ...      
}

// ...
boost::asio::connect(sock, iter, ec); // use exhausted iterator

要解决此问题，请考虑在通过更改构建调试信息时迭代 iter 的副本：

while(iter != iterEnd)
{
  ep = *iter++;
  // ...      
}

到：

for(tcp::resolver::iterator i = iter, end; i != end; ++i)
{
   ep = *i;
   // ...
}

---

这是一个大致基于原始代码的完整示例，用于演示差异：

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

void connect(const char* address, const char* port, bool exhaust)
{
  using boost::asio::ip::tcp;

  boost::asio::io_service io_service;
  tcp::resolver resolver(io_service);
  tcp::resolver::iterator iterator = resolver.resolve(
      tcp::resolver::query(address, port));

  // If flag is set, exhaust the iterator. 
  if (exhaust)
  {
    std::cout << "Exhausting iterator" << std::endl;
    for (tcp::resolver::iterator end; iterator != end; ++iterator) {}
  }
  // Otherwise, exhaust a copy of the iterator.
  else
  {
    std::cout << "Exhausting copy of iterator" << std::endl;
    for (tcp::resolver::iterator i = iterator, end; i != end; ++i) {}
  }

  tcp::socket socket(io_service);
  boost::system::error_code error;
  connect(socket, iterator, error);
  std::cout << error.category().name() << " -- " 
            << error.message() << std::endl;
}

int main()
{
  connect("google.com", "80", true);  // exhaust
  connect("google.com", "80", false); // do not exhaust
}

产生以下输出：

Exhausting iterator
asio.misc -- Element not found
Exhausting copy of iterator
system -- Success