使用libcurl编写HTTP客户端

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本文主要介绍使用C++语言、利用libcurl编写HTTP客户端的方法。

1 概述

libcurl 属于 curl 的一部分，libcurl is a free and easy-to-use client-side URL transfer library.

从 git 上下载 curl 源码，编译安装之后，就可以使用 libcurl 了。当然，也可以直接使用 yum 安装 libcurl。

curl 的源码中，附带了一些 libcurl 的使用示例，示例位置如下：

我们可以参考 libcurl 提供的示例代码，编写 HTTP 客户端（或者其他HTTP程序）。

2 示例程序

这里将展示 GET 和 POST 两种请求方式的 HTTP 客户端示例程序。

2.1 GET请求类型HTTP客户端

示例代码（libcurl_http_client_test1.cpp）如下：

#include <iostream>
#include <curl/curl.h>
#include <string>

using namespace std;

int BufferWriterFunc(char * data, size_t size, size_t nmemb, string * buffer)
{
    int result = 0;
    if (buffer != NULL)
    {
        buffer->append(data, size * nmemb);
        result = size * nmemb;
    }
    return result;
}

int main()
{
    CURL *curl;
    CURLcode res;
    string strUrl = "http://example.com?param1=value1&param2=value2";
    string strResponse;

    curl_global_init(CURL_GLOBAL_DEFAULT);

    // get a curl handle
    curl = curl_easy_init();
    if (curl)
    {
        // set the URL with GET request
        curl_easy_setopt(curl, CURLOPT_URL, strUrl.data());

        // write response msg into strResponse
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, BufferWriterFunc);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &strResponse);

        // perform the request, res will get the return code
        res = curl_easy_perform(curl);
        // check for errors
        if (res != CURLE_OK)
        {
            cout << "curl_easy_perform() failed: " << curl_easy_strerror(res) << endl;
        }
        else
        {
            cout << "curl_easy_perform() success." << endl;
            cout << "strResponse is: " << strResponse << endl;
        }

        // always cleanup
        curl_easy_cleanup(curl);
    }

    curl_global_cleanup();

    return 0;
}

2.2 POST请求类型HTTP客户端

示例代码（libcurl_http_client_test2.cpp）如下：

#include <iostream>
#include <curl/curl.h>
#include <string>

using namespace std;

int BufferWriterFunc(char * data, size_t size, size_t nmemb, string * buffer)
{
    int result = 0;
    if (buffer != NULL)
    {
        buffer->append(data, size * nmemb);
        result = size * nmemb;
    }
    return result;
}

int main()
{
    CURL *curl;
    CURLcode res;
    string strResponse;

    curl_global_init(CURL_GLOBAL_DEFAULT);

    // get a curl handle
    curl = curl_easy_init();

    if (curl)
    {
        // set the URL that is about to receive our POST
        curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");

        // specify the POST data
        curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "param1=value1&param2=value2");
        
        // write response msg into strResponse
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, BufferWriterFunc);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &strResponse);

        // Perform the request, res will get the return code
        res = curl_easy_perform(curl);
        // Check for errors
        if (res != CURLE_OK)
        {
            cout << "curl_easy_perform() failed: " << curl_easy_strerror(res) << endl;
        }
        else
        {
            cout << "curl_easy_perform() success." << endl;
            cout << "strResponse is: " << strResponse << endl;
        }

        // always cleanup
        curl_easy_cleanup(curl);
    }

    curl_global_cleanup();

    return 0;
}

说明：在上述的两个代码中，都是通过使用“curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, BufferWriterFunc); 和 curl_easy_setopt(curl, CURLOPT_WRITEDATA, &strResponse);”，将HTTP服务器返回的消息写入了strResponse中。

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