如何修复 Boost::Asio Client Http 请求错误？

Question

I'm trying to lean Boost ::Asio networking library for C++ by watching this video but I stuck at making request using threads asynchronously.

The code:

#include "stdafx.h"
#include <iostream>
#include <boost/asio.hpp>
#include <boost/asio/ts/buffer.hpp>
#include <boost/asio/ts/internet.hpp>
#include <boost/system/error_code.hpp>

std::vector<char> vBuffrer(20 * 1024);

void GrabSomeData(boost::asio::ip::tcp::socket& socket) {

    socket.async_read_some(boost::asio::buffer(vBuffrer.data(), vBuffrer.size()),
        [&](std::error_code ec, std::size_t length)
        //boost::system::error_code ec
    {
        if (!ec)
        {
            std::cout << "\n\nRead" << length << "bytes\n\n";

            for (int i = 0; i < length; i++)
                std::cout << vBuffrer[i];
            GrabSomeData(socket);
        }
    });


}

int main()
{
    boost::system::error_code ec;
    boost::asio::io_context context;
    boost::asio::io_context::work idleWork(context);
    boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::make_address("13.107.21.200",ec),80);
    boost::asio::ip::tcp::socket socket(context);
    std::thread thrContext = std::thread([&]() {context.run(); });
    std::cout << "Starting " << std::endl;
    socket.connect(endpoint,ec);

    if (!ec)
    {
        std::cout << "Connected ! " << std::endl;
    }
    else {
        std::cout << "Fail to connect ! " << ec.message() << std::endl;

    }

    if (socket.is_open()) {

        GrabSomeData(socket);
        std::string sRequest =
            "GET /index.html HTTP/1.1\r\n"
            "Host: www.example.com\r\n"
            "Connection: close\r\n\r\n";
        socket.write_some(boost::asio::buffer(sRequest.data(), sRequest.size()), ec);

        using namespace std::chrono_literals;
        std::this_thread::sleep_for(2800ms);
        //std::this_thread::sleep_for(1ms);

        context.stop();
        if (thrContext.joinable()) thrContext.join();
    }

    system("pause");

    return 0;
}

Microsoft Visual studio give me this:

Error C2752 'asio_prefer_fn::call_traits<boost::asio::execution::any_executor<boost::asio::execution::context_as_t<boost::asio::execution_context &>,boost::asio::execution::detail::blocking::never_t<0>,boost::asio::execution::prefer_only<boost::asio::execution::detail::blocking::possibly_t<0>>,boost::asio::execution::prefer_only<boost::asio::execution::detail::outstanding_work::tracked_t<0>>,boost::asio::execution::prefer_only<boost::asio::execution::detail::outstanding_work::untracked_t<0>>,boost::asio::execution::prefer_only<boost::asio::execution::detail::relationship::fork_t<0>>,boost::asio::execution::prefer_only<boost::asio::execution::detail::relationship::continuation_t<0>>> &,void (const boost::asio::execution::detail::blocking::possibly_t<0> &,boost::asio::execution::allocator_t<std::allocator<void>>),void>': more than one partial specialization matches the template argument list  boostasiotest   c:\boost\boost_1_75_0\boost\asio\detail\handler_work.hpp    353


Error   C2893   Failed to specialize function template 'enable_if<asio_prefer_fn::call_traits<T,void(P0,P1,PN...),void>::overload==,call_traits<T,void(P0,P1,PN...),void>::result_type>::type asio_prefer_fn::impl::operator ()(T &&,P0 &&,P1 &&,PN &&...) noexcept(<expr>) const'  boostasiotest   c:\boost\boost_1_75_0\boost\asio\detail\handler_work.hpp    353

Everything worked fine till I added GrabSomeData function and I have absolutely no idea how to fix it, any help would be appreciated.

PS: there is an example on the Boost website on this subject , but it is object oriented and all the pointers are referring to the class and I (think) it can't help.

Answer 1

Like the commenter, I cannot repro your message: it just compiles,

• MSVC 19, /std:c++14, Boost 1.75.0: compiler explorer

Now, I do see other issues:

1. write_some may not write all the data - you will want to ensure a composed-write operation

2. a race condition: since you're doing GrabSomeData on a thread, you need to synchronize access to the tcp::socket and buffer (the shared resources).

   io_context itself is thread-safe.

   In this case, it's really easy to avoid, since you don't need to start the async operation until after you sent the request:

    write(socket, boost::asio::buffer(sRequest)); GrabSomeData(socket);

3. async_read_some has a similar problem as the write side. You will want a composed read operation that reads the expected output so read_until(socket, buf, "\r\n\r\n") and then read how much content is expected based on Content-Length header, Connection: Close and others (think chunked encoding).

   You currently have no good way to store and access the response. It would be a lot easier to use a streambuf, a single composed read.

   If you want to be really solid, use Beast to receive a HTTP/1.1 response (which can even be chunked) and not worry about when it's complete (the library does it for you):

    auto GrabSomeData(tcp::socket& socket) { http::response<http::string_body> res; auto buf = boost::asio::dynamic_buffer(vBuffer); http::read(socket, buf, res); return res; }

   Oh, and don't do it on a thread (why was that anyways? it literally just introduced undefined behavior for no gain):

Simplified Code

Live On Coliru

Compiles on MSVC: Godbolt

#include <boost/asio.hpp>
#include <boost/beast/http.hpp>
#include <iostream>
#include <iomanip>
using boost::asio::ip::tcp;

std::vector<char> vBuffer; // TODO FIXME global variable

auto GrabSomeData(tcp::socket& socket) {
    namespace http = boost::beast::http;

    http::response<http::string_body> res;

    auto buf = boost::asio::dynamic_buffer(vBuffer);
    http::read(socket, buf, res);

    return res;
}

int main() try {
    boost::asio::io_context context;

    std::cout << "Starting " << std::endl;
    tcp::endpoint endpoint(boost::asio::ip::make_address("13.107.21.200"), 80);
    tcp::socket   socket(context);
    socket.connect(endpoint);

    std::cout << "Connected" << std::endl;

    std::string const sRequest = "GET /index.html HTTP/1.1\r\n"
        "Host: www.example.com\r\n"
        "Connection: close\r\n"
        "\r\n";

    write(socket, boost::asio::buffer(sRequest));

    auto response = GrabSomeData(socket);

    std::cout << "Response body length: " << response.body().size() << std::endl;
    std::cout << "Response headers: " << response.base() << std::endl;
    std::cout << "Response body: " << std::quoted(response.body()) << std::endl;

    context.run(); // run_for(10s) e.g.
} catch (boost::system::system_error const& se) {
    std::cerr << "Error: " << se.code().message() << std::endl;
}

This sample printed:

Starting 
Connected
Response body length: 194
Response headers: HTTP/1.1 400 Bad Request
Transfer-Encoding: chunked
X-MSEdge-Ref: 0BstXYAAAAACeQ2y+botzQISiBe2U3iGCQ0hHRURHRTE0MDgARWRnZQ==
Date: Sun, 21 Mar 2021 22:39:02 GMT
Connection: close

Response body: "<h2>Our services aren't available right now</h2><p>We're working to restore all services as soon as possible. Please check back soon.</p>0BstXYAAAAACeQ2y+botzQISiBe2U3iGCQ0hHRURHRTE0MDgARWRnZQ=="

NOTE It does indeed use chunked encoding, which you didn't seem to be anticipating.

Answer 2

Thanks @sehe for the answer and recommendations, it took so long but I've upgrade to New Windows10 and Visual Studio 2019 , and Boost 1_76 and the problem solved!

Those errors were totally irrelevant to the code!