[英]understand boost::asio async_read behavior when there is nothing to read
I am trying to understand what would happen with async_read
when there is nothing to read. 我试图了解什么也没读的情况下,使用async_read
会发生什么。
For example, a client creates a connection to a server, then start async_read()
, but that server does not expect to send anything to this client. 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。 So what would happen? 那会发生什么呢? Should I receive a EOF? 我应该收到EOF吗?
Updata: I think @user786653 is right. Updata:我认为@ user786653是正确的。 I made a simple example (see following). 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";
}
}
Quoting from the latest (1.68.0) documentation : 引用最新(1.68.0) 文档 :
This function is used to asynchronously read a certain number of bytes of data from a stream. 此函数用于从流中异步读取一定数量的数据字节。 The function call always returns immediately. 函数调用总是立即返回。 The asynchronous operation will continue until one of the following conditions is true: 异步操作将继续,直到满足以下条件之一:
- The supplied buffers are full. 提供的缓冲区已满。 That is, the bytes transferred is equal to the sum of the buffer sizes. 即,传输的字节等于缓冲区大小的总和。
- An error occurred. 发生错误。
So nothing will happen until the server closes the connection (resulting in an error). 因此,在服务器关闭连接之前不会发生任何事情(导致错误)。
You can test this out for yourself: 您可以自己测试一下:
#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";
}
}
Start a server that doesn't respond on localhost port 8000 (or change the code). 启动在本地主机端口8000上没有响应的服务器(或更改代码)。 Eg something like nc -l 8000
or python -m SimpleHTTPServer
. 例如nc -l 8000
或python -m SimpleHTTPServer
。 Then run the program and wait. 然后运行程序并等待。 Nothing happens. 什么都没发生。 Now stop the server, on my (Windows) machine this results in: 现在停止服务器,在我的(Windows)机器上,结果是:
Read result: system:10054 - An existing connection was forcibly closed by the remote host
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