如何制作 libcurl HTTP webrequest？

Question

我正在尝试从 url 获取 json 数据：

sub.domain.com/rest/getdiscounts?supermarketid=5323

我想获取从该 url 返回的数据。在终端我可以这样做：

curl --user user:password "http://sub.domain.com/rest/getdiscounts?supermarketid=5323"

到目前为止，我已经尝试了多种方法，例如：

static string lastResponse;

size_t server_response_stream(void *ptr, size_t size, size_t nmemb, void *)
{
  lastResponse += (const char*)ptr;
  return size * nmemb;
}

bool get_request(string data1, string& errorDescription)
{
  CURL *curl = curl_easy_init();
  if (curl == NULL)
  {
    errorDescription = "Unable to initialise Curl";
    cout << errorDescription << std::endl;
    return false;
  }
  const string url = "http://sub.domain.com/rest/";
  curl_easy_setopt(curl, CURLOPT_URL, (url + "getdiscounts").c_str());
  curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
  curl_easy_setopt(curl, CURLOPT_USERPWD, USPA);

  char* dc1 = curl_easy_escape(curl, data1.c_str(), 0);
  string arg = "supermarketid=" + (string)dc1;
  const char* dt = arg.c_str();

  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, dt);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)strlen(dt));

  curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

  lastResponse = "";
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, server_response_stream);

  CURLcode res = curl_easy_perform(curl);

  res = curl_easy_perform(curl);

  if (res != CURLE_OK)
  {
    errorDescription = "Curl call to server failed!";
    cout << errorDescription << std::endl;
    return false;
  }

  curl_easy_cleanup(curl);
  return true;
}

我使用this question 和this link 作为来源。

编辑：感谢您的帮助！我又从头开始了，它现在可以工作了：

static string responseStr;
size_t write_callback(void *ptr, size_t size, size_t nmemb, void *)
{
  responseStr = (const char*)ptr;
  return size * nmemb;
}

string get_request()
{
  CURL *curl;
  CURLcode res;

  curl = curl_easy_init();
  if (curl)
  {
    curl_easy_setopt(curl, CURLOPT_URL, "http://sub.domain.com/rest/getdiscounts?supermarketid=5323");
    curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
    curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_easy_setopt(curl, CURLOPT_USERPWD, CREDENTIALS);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);

    res = curl_easy_perform(curl);

    if (res != CURLE_OK)
    {
      cout << "curl_easy_perform() failed!" << std::endl;
      curl_easy_strerror(res);
      curl_easy_cleanup(curl);
      return "";
    }

    else if (res == CURLE_OK)
    {
      curl_easy_cleanup(curl);
      return responseStr;
    }
  }
}

Answer 1

CURLOPT_WRITEFUNCTION callback 函数server_response_stream 的第一个参数ptr 指向接收到的数据。该数据不是以空值结尾的。

字符串 += operator 必须接收一个指向空终止字符串的指针。

调用lastResponse += (const char*)ptr; 导致未定义的行为。字节必须以不同的方式附加到字符串中。您可以逐个字符地附加字符串，使用参数size 和nmemb 来确定缓冲区的大小。