如何使用 Boost::Asio 访问 Web 服务？

Question

我想知道是否有可能使用 boost asio 库访问网络服务。

我尝试了从 boost asio 文档中获得的以下代码（在 IOS，C++11 中）。 但它抛出了以下内容。

try
{
    boost::asio::io_service io_service;
    std::string address = "http://www.thomas-bayer.com/axis2/services/BLZService/";

    // Get a list of endpoints corresponding to the server name.
    tcp::resolver resolver(io_service);
    tcp::resolver::query query(address,boost::asio::ip::resolver_query_base::numeric_service);


    tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

    endpoint_iterator->host_name() = "www.thomas-bayer.com/axis2/services/BLZService/";

    std::cout<<"Print Query --"<<std::endl;

    // Try each endpoint until we successfully establish a connection.
    tcp::socket socket(io_service);

    boost::asio::connect(socket, endpoint_iterator);

    // Form the request. We specify the "Connection: close" header so that the
    // server will close the socket after transmitting the response. This will
    // allow us to treat all data up until the EOF as the content.
    boost::asio::streambuf request;
    std::ostream request_stream(&request);
    request_stream << "POST: HTTP/1.0\r\n";
    request_stream << "Host: " << address << "\r\n";
    request_stream << "Accept: */*\r\n";
    request_stream << "Connection: close\r\n\r\n";

    // Send the request.
    boost::asio::write(socket, request);

    // Read the response status line. The response streambuf will automatically
    // grow to accommodate the entire line. The growth may be limited by passing
    // a maximum size to the streambuf constructor.

    boost::asio::streambuf response;
    boost::asio::read_until(socket, response, "\r\n");

    // Check that response is OK.
    std::istream response_stream(&response);
    std::string http_version;
    response_stream >> http_version;
    unsigned int status_code;
    response_stream >> status_code;
    std::string status_message;
    std::getline(response_stream, status_message);

    if (!response_stream || http_version.substr(0, 5) != "HTTP/")
    {
        std::cout << "Invalid response\n";
        return;
    }
    if (status_code != 200)
    {
        std::cout << "Response returned with status code " << status_code << "\n";
        return;
    }

    // Read the response headers, which are terminated by a blank line.
    boost::asio::read_until(socket, response, "\r\n\r\n");

    // Process the response headers.
    std::string header;
    while (std::getline(response_stream, header) && header != "\r")
        std::cout << header << "\n";
    std::cout << "\n";

    // Write whatever content we already have to output.
    if (response.size() > 0)
        std::cout << &response;

    // Read until EOF, writing data to output as we go.
    boost::system::error_code error;
    while (boost::asio::read(socket, response,
                             boost::asio::transfer_at_least(1), error))
        std::cout << &response;
    if (error != boost::asio::error::eof)
        throw boost::system::system_error(error);
}
catch (std::exception& e)
{
    std::cout << "Exception: " << e.what() << "\n";
}

例外：连接：无法分配请求的地址

或者

异常：解决：找不到主机（权威）

代码有什么问题？还是我做错了？

谢谢

Answer 1

名称解析失败，因为您混淆了什么是请求、URL、协议、主机名和 IP 地址。

在FQDN 上提出请求。除非您知道，否则您需要提供服务。在这种情况下，服务遵循协议¹，`http:// 通常在端口 80 上提供：

std::string const address = "www.thomas-bayer.com";
tcp::resolver::query query(address, "80", boost::asio::ip::resolver_query_base::numeric_service);

请注意，在大多数系统上，您可以等效地使用：

std::string const address = "www.thomas-bayer.com";
tcp::resolver::query query(address, "http");

看到http:// 和www.thomas-bayer.com 部分去了哪里？

现在/axis2/services/BLZService/ 是查询路径，就像您在 GET 请求中所写的那样：

request_stream << "GET /axis2/services/BLZService/ HTTP/1.1\r\n";

注意事项：

1. POST 不是标头（所以没有冒号！）它是 HTTP“动词”（GET、POST、DELETE、PUT...）

2. “主机”标头是主机名：

    request_stream << "Host: " << address << "\r\n";
   

   是正确的 iff address 确实是主机的逻辑名称

3. 这样设置主机名：

    endpoint_iterator->host_name() = "www.thomas-bayer.com/axis2/services/BLZService/";
   

   是我以前从未见过的东西，我不确定它应该实现什么。也许只是错了？

¹ 按照惯例，可能是其他

修复

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

void test() try {
    boost::asio::io_service io_service;
    std::string const address = "www.thomas-bayer.com";

    // Get a list of endpoints corresponding to the server name.
    tcp::resolver resolver(io_service);
    tcp::resolver::query query(address, "80", boost::asio::ip::resolver_query_base::numeric_service);

    tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

    std::cout << "Print Query --" << std::endl;

    // Try each endpoint until we successfully establish a connection.
    tcp::socket socket(io_service);

    boost::asio::connect(socket, endpoint_iterator);

    // Form the request. We specify the "Connection: close" header so that the
    // server will close the socket after transmitting the response. This will
    // allow us to treat all data up until the EOF as the content.
    boost::asio::streambuf request;
    std::ostream request_stream(&request);
    request_stream << "GET /axis2/services/BLZService HTTP/1.0\r\n";
    request_stream << "Host: " << address << "\r\n";
    request_stream << "Accept: */*\r\n";
    request_stream << "Connection: close\r\n\r\n";

    // Send the request.
    boost::asio::write(socket, request);

    // Read the response status line. The response streambuf will automatically
    // grow to accommodate the entire line. The growth may be limited by
    // passing a maximum size to the streambuf constructor.
    boost::asio::streambuf response;
    boost::asio::read_until(socket, response, "\r\n");

    // Check that response is OK.
    std::istream response_stream(&response);
    std::string http_version, status_message;
    unsigned int status_code;
    std::getline(response_stream >> http_version >> status_code, status_message);

    if (!response_stream || http_version.substr(0, 5) != "HTTP/") {
        std::cout << "Invalid response\n";
        return;
    }
    if (status_code != 200) {
        std::cout << "Response returned with status code " << status_code << "\n";
        return;
    }

    // Read the response headers, which are terminated by a blank line.
    boost::asio::read_until(socket, response, "\r\n\r\n");

    // Process the response headers.
    std::string header;
    while (std::getline(response_stream, header) && header != "\r")
        std::cout << header << "\n";
    std::cout << "\n";

    // Write whatever content we already have to output.
    if (response.size() > 0)
        std::cout << &response;

    // Read until EOF, writing data to output as we go.
    boost::system::error_code error;
    while (boost::asio::read(socket, response, boost::asio::transfer_at_least(1), error))
        std::cout << &response;
    if (error != boost::asio::error::eof)
        throw boost::system::system_error(error);
} catch (std::exception &e) {
    std::cout << "Exception: " << e.what() << "\n";
}

int main() { test(); }