Boost::Asio. In which thread is the message sent?

问题

I'm trying to understand the internal work of boost::asio library, which is really great. I've written a simple client that sends one message to a server.

The question is - in which thread does it really send the message?

Since I use async_write() method, it returns immediately after calling and doesn't send anything. I've commented the io_service.run() method, and after that the handler was not called (that's okay), but the message was sent! (I've checked it in server logs)

By the way, I don't use -lpthread, just -lboost_system while compiling, so I assume the whole program works in one thread and there are no "hidden" threads.

I cannot make head or tail of where was the message sent.

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

const std::string str("HELLO!");

int main()
{
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::address::from_string("127.0.0.1"), 33333);
    boost::asio::ip::tcp::socket socket(io_service);
    boost::system::error_code ec;
    boost::asio::connect(socket, &endpoint, ec);
    boost::asio::async_write(socket, boost::asio::buffer(str),
                             [](const boost::system::error_code& ec, std::size_t length)
                             {
                                 if (!ec)
                                 {
                                     std::cout << "Sent!" << std::endl;
                                 }
                                 else
                                 {
                                     std::cout << "Err: " << ec.message() << std::endl;
                                 }
    });

    //io_service.run();
    return 0;
}

回答1:

The message would have been sent (or at least queued in the network stack, perhaps to be handled by a kernel thread, or some other implementation-dependent mechanism that you shouldn't need to care about) during the call to async_write. There's no need to defer that, if it can be done without blocking the calling thread, which it typically can be.

The completion handler won't be called until the io_service is run, and is notified that the message has been sent.

来源：https://stackoverflow.com/questions/24776235/boostasio-in-which-thread-is-the-message-sent