如何发起“事件"当我的 Boost::asio tcp 服务器刚开始运行时(又名 io_service.run() )?

2021-12-24 00:00:00 c++ c++11 boost boost-asio

基于 boost::asio 客户端/服务器关系，只有当服务器线程处于等待连接"状态时，我才必须从服务器程序启动客户端程序.

我的问题是如何知道那个状态?

Based on an boost::asio client/server relationship, I have to launch the client program from the server program only when the server thread is in a "waiting to be connected" state.

My question is how to have the knowledge of that state ?

作为示例，使用 asio 示例/序列化link，并替换掉main函数带有该代码的 server.cpp:

As a sample use the asio example/serialization link, and replace the main function of server.cpp with that code:

#include <conio.h>
#include <concrt.h> // wait function
#include <future>
#include <thread>

void server_thread( std::promise<bool>& run )
{ 
    boost::asio::io_service io_service;
    s11n_example::server server(io_service, 123);
    // too early to run.set_value( true );
    io_service.run();
    // too late to run.set_value( true );
}

int main(int argc, char* argv[])
{
    std::promise<bool> run;
    std::thread thrd( server_thread, boost::ref( run ) );
    thrd.detach(); 

    bool launched = run.get_future().get();
    // server is waiting for connection
    // launch the client
    if( launched )
    {
        int rc = system( "start client.exe localhost 123" );
        if( rc )
            std::cerr << "system failed returning " << rc << std::endl ;
    }
    else
        std::cerr << "server_thread failure" << std::endl ;

    std::cout << "hit a key to exit"  ;
    while( !_kbhit() )
        Concurrency::wait( 100 );

    return 0;
}

谢谢，

推荐答案

简而言之，s11n_example::server 处于这样一种状态:在构造函数调用完成后，传入的连接将立即排队.

In short, s11n_example::server is in a state where incoming connections will be queued immediately after the constructor call has completed.

通过定义状态和操作之间的区别可能更容易理解这一点.状态决定了操作系统可以对对象做什么；应用程序启动执行操作的操作，并且可能取决于状态.例如，当套接字处于 open 状态时，操作系统会将数据排队；read 操作检索排队的数据.这同样适用于接受者.当接受者处于 listen 状态时，操作系统会将连接排队；accept 操作将完成连接，将其从队列中移除.

It may be easier to understand this by defining a distinction between state and operations. A state determines what the OS can do with the object; an application initiates operations that perform actions and may depend on state. For example, when a socket in an open state, the OS will queue data; a read operation retrieves the queued data. The same applies to the acceptors. When an acceptor is in a listen state, the OS will queue connections; an accept operation will complete the connection, removing it from the queue.

一个acceptor的[states]和transitions()如下:

     .----> [closed] ------.     [closed]:    socket not open
     |                     |     [opened]:    socket open but not listening for
     |                     V                  connections
  close() <------.      open()   [listening]: incoming connections will be
     ^           |         |                  queued until accepted(), causing
     |           |         V                  the connection to be established
[listening]      '---- [opened]
     ^                     |
     |                     |
     '------ listen() <----'

各种重载构造函数将导致 acceptor 在关闭、打开或侦听状态下开始其生命周期.对于 s11n_example::server，acceptor 是用端点构造的，所以 这个 重载将导致接受器在构造后处于侦听状态.这相当于做:

The various overloaded constructors will result in the acceptor starting its lifetime in a closed, opened, or listening state. In the case of s11n_example::server, the acceptor is constructed with an endpoint, so this overload will result in the acceptor being in a listening state post-construction. It is the equivalent of doing:

using boost::asio::ip::tcp;
tcp::endpoint endpoint_(tcp::v4(), 123);
tcp::acceptor acceptor_(io_service); // closed state
acceptor.open(endpoint_.protocol()); // opened state
acceptor.bind(endpoint);
acceptor.listen();                   // listening state

因此，promise 可以在 server 构建之后，io_service.run() 之前设置:

Therefore, the promise can be set after server is constructed and before io_service.run(): 

void server_thread(std::promise<bool>& run)
{ 
    boost::asio::io_service io_service;
    s11n_example::server server(io_service, 123);
    // The server's acceptor is in a listening state, so connection attempts
    // will be queued even without the io_service event loop running.  The
    // server also has an outstanding asynchronous accept operation.
    run.set_value(true);
    // Run the service, this will start an asynchronous loop that accepts 
    // connections.
    io_service.run();
}

需要注意的一个微妙之处是 Boost.Asio 的接受器不提供:

One subtlety to note is that Boost.Asio's acceptors do not provide:

  • 用于接受连接的基于反应器的操作.因此，无法检测到连接何时准备好接受(即连接已排队等待接受).
  • 一种检测 acceptor 是否处于监听状态的高级方法.然而，这可以通过查询接受者的 native_handle.例如，使用 getockopt() 获取SOL_SOCKET/SO_ACCEPTCONN的值.
  • Reactor based operations for accepting connections. Thus, it is not possible to detect when a connection is ready to be accepted (i.e. a connection is queued and waiting to be accepted).
  • An higher-level way to detect if the acceptor is in a listening state. Nevertheless, this may be accomplished by querying the acceptor's native_handle. For example, using getsockopt() to get the value of SOL_SOCKET/SO_ACCEPTCONN.

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