如何使用 libcurl c++ 从 url 保存 XML 文件

Question

如何使用 libcurl c++ 从 URL 保存 XML 文件，以便稍后使用 pugixml 加载它。我已经试过了，但没用

size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
    size_t written = fwrite(ptr, size, nmemb, stream);
    return written;
}
...
CURL *curl;
FILE *fp;
CURLcode result;
char *c_url = "http://some_url.xml";
char outfilename[FILENAME_MAX] = "sth.xml";
curl_global_init(CURL_GLOBAL_DEFAULT);
curl = curl_easy_init();
if (curl) {
    fp = fopen(outfilename,"wb");
    curl_easy_setopt(curl, CURLOPT_URL, c_url);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
    result = curl_easy_perform(curl);
}
curl_global_init(CURL_GLOBAL_DEFAULT);
...
curl_easy_cleanup(curl);
curl_global_cleanup();
fclose(fp);

Answer 1

通过将 char* 设置为 const 修复错误后，如：

char const* c_url = "http://some_url.xml";

你的代码对我来说工作正常。

然而，您并不总是希望将收到的数据写入磁盘。有时您只是想将其保存在内存中以便即时使用。

在这里，我写了一个函数来将 URL 的目标下载到 std::string 中，您可以随心所欲地使用它。我还使用 智能指针 .

使代码异常安全并且通常更安全

// write the data into a `std::string` rather than to a file.
std::size_t write_data(void* buf, std::size_t size, std::size_t nmemb,
    void* userp)
{
    if(auto sp = static_cast<std::string*>(userp))
    {
        sp->append(static_cast<char*>(buf), size * nmemb);
        return size * nmemb;
    }

    return 0;
}

// To make the function thread safe you can use a smart pointer to
// hold your CURL session pointer.

// A deleter to use in the smart pointer for automatic cleanup
struct curl_dter{void operator()(CURL* curl) const
    { if(curl) curl_easy_cleanup(curl); }};

// A smart pointer to automatically clean up out CURL session
using curl_uptr = std::unique_ptr<CURL, curl_dter>;

// download the URL into a `std::string`.
std::string get_url(std::string const& url)
{
    std::string data;

    if(auto curl = curl_uptr(curl_easy_init()))
    {
        curl_easy_setopt(curl.get(), CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl.get(), CURLOPT_FOLLOWLOCATION, 1L);
        curl_easy_setopt(curl.get(), CURLOPT_WRITEFUNCTION, write_data);
        curl_easy_setopt(curl.get(), CURLOPT_WRITEDATA, &data);

        CURLcode ec;
        if((ec = curl_easy_perform(curl.get())) != CURLE_OK)
            throw std::runtime_error(curl_easy_strerror(ec));

    }

    return data;
}

int main()
{
    curl_global_init(CURL_GLOBAL_DEFAULT);

    auto xml = get_url("http://google.co.uk");

    std::cout << xml << '\n';

    curl_global_cleanup();
}

注意： 我还添加了 CURLOPT_FOLLOWLOCATION 选项，以防文档上有 redirect。

Answer 2

如果有人感兴趣..最后，我最终使用了这个代码：

const char* f = "new_file.xml";
if (curl){
    const char* c_url = "some_url";

    FILE* ofile = fopen(f, "wb");
    if (!ofile) { fprintf(stderr, "Failed to open file: %s\n", strerror(errno)); }

    if (ofile){
        curl_easy_setopt(curl, CURLOPT_URL, c_url);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, ofile);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeData);
        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);

        curl_easy_perform(curl);

        fclose(ofile);
    }
}
pugi::xml_document doc;
doc.load_file(f);

感谢大家的帮助！

如何使用 libcurl c++ 从 url 保存 XML 文件

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