如何修复 Boost::Asio Client Http 请求错误？

Question

我正在尝试通过观看 video 来精简 C++ 的 Boost::Asio 网络库，但我坚持使用异步线程发出请求。

代码：

#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 给我这个：

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

在我添加 GrabSomeData 函数之前一切正常，但我完全不知道如何修复它，我们将不胜感激。

PS : 在 subject 的 Boost 网站上有一个示例，但它是面向对象的，所有指针都指向该类，我（认为）它无济于事.

Answer 1

像评论者一样，我无法复制您的消息：它只是编译，

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

现在，我确实看到了其他问题：

1. write_some 可能不会写入所有数据 - 您需要确保组合写入操作

2. 竞争条件：由于您在线程上执行GrabSomeData，因此您 需要同步访问tcp::socket和buffer（ 共享资源）。

   io_context 本身是线程安全的。

   在这种情况下，很容易避免，因为您不需要 在您发送请求之前开始异步操作：

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

3. async_read_some 与写入端有类似的问题。您将需要一个组合读取操作来读取预期的输出，例如 read_until(socket, buf, "\r\n\r\n")，然后根据 Content-Length 标头、Connection: Close 和其他（想想分块编码）读取预期的内容。

   您目前没有存储和访问响应的好方法。使用 streambuf 会容易得多，单个组合读取。

   如果您想真正可靠，请使用 Beast 接收 HTTP/1.1 响应（甚至可以分块），而不必担心何时完成（库会为您完成）：

   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;
   }
   

   哦，不要在线程上这样做（为什么会这样？它实际上只是引入了未定义的行为而没有任何收获）：

简化代码

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;
}

此示例打印：

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=="

注意它确实使用了分块编码，您似乎没有预料到。

Answer 2

感谢@sehe 的回答和建议，花了很长时间，但我已经升级到新的 Windows10 和 Visual Studio 2019，以及 Boost 1_76问题解决了！

这些错误与代码完全无关！