理解boost::asio-async_read在无需读取内容时的行为

understand boost::asio async_read behavior when there is nothing to read

本文关键字：读取 asio-async boost read 理解      更新时间：2023-10-16

我正试图了解async_read在没有可读取内容时会发生什么。

例如，客户端创建与服务器的连接，然后启动async_read()，但该服务器不希望向该客户端发送任何内容。那么会发生什么呢？我应该接受EOF吗？

更新数据：我认为@user786653是对的。我做了一个简单的例子(见下文(。

#include <iostream>
#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/asio.hpp>
class test{
public:
test(boost::asio::io_service& io_service):_socket(io_service){
}
void handle_connect(){
std::cout<<"entering test::handle_connect"<<std::endl;
char reply[128];
boost::asio::async_read(_socket, boost::asio::buffer(reply, sizeof(reply)),
[](boost::system::error_code ec, std::size_t /*length*/){
std::cout<<"Read result:"<< ec<<" - "<<ec.message()<<std::endl;
});
}
boost::asio::ip::tcp::socket & socket(){
return _socket;
}
private:
boost::asio::ip::tcp::socket _socket;
};

int main() {
try {
boost::asio::io_service io_service;
boost::asio::ip::tcp::socket s(io_service);
boost::asio::ip::tcp::resolver resolver(io_service);
boost::asio::ip::tcp::resolver::query query("127.0.0.1", "8000");
boost::asio::ip::tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
boost::asio::ip::tcp::endpoint endpoint = *endpoint_iterator;
test t(io_service);
t.socket().async_connect(endpoint,boost::bind(&test::handle_connect, &t));
io_service.run();
} catch (std::exception& e) {
std::cerr << "Exception: " << e.what() << "n";
}
}

引用最新(1.68.0(文档：

此函数用于从流中异步读取特定字节数的数据。函数调用总是立即返回。异步操作将继续，直到以下条件之一成立：

  • 提供的缓冲区已满。也就是说，传输的字节等于缓冲区大小之和
  • 出现错误

因此，在服务器关闭连接之前不会发生任何事情(导致错误(。

你可以自己测试一下：

#include <iostream>
#include <boost/asio.hpp>
int main() {
try {
boost::asio::io_context io_context;
boost::asio::ip::tcp::socket s(io_context);
boost::asio::ip::tcp::resolver resolver(io_context);
boost::asio::connect(s, resolver.resolve("localhost", "8000"));
char reply[128];
async_read(s, boost::asio::buffer(reply, sizeof(reply)), [](boost::system::error_code ec, std::size_t /*length*/) {
std::cout << "Read result: " << ec << " - " << ec.message() << "n";
});
io_context.run();
} catch (std::exception& e) {
std::cerr << "Exception: " << e.what() << "n";
}
}

在localhost端口8000上启动一个没有响应的服务器(或更改代码(。例如nc -l 8000或python -m SimpleHTTPServer之类的东西。然后运行程序并等待。什么也没发生。现在停止服务器，在我的(Windows(机器上这会导致：

Read result: system:10054 - An existing connection was forcibly closed by the remote host
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