Question

我正在使用带有C ++的cURL-library从这个url中检索一个非常大的JSON字符串：https://www.bitstamp.net/api/order_book/?group=1（长度约为170 000个字符）

我的问题是cURL没有将整个字符串传递给我的回调函数。我用标准的回调函数测试了它，它将整个字符串打印到stdout，所以我可以肯定地说这不是接收数据的问题。但是一旦调用了我的回调函数，传递的数据的大小就是7793或15776个字符。（我想不出任何可能与这两个数字的变化相关的变化）

我首先认为CURL_MAX_WRITE_SIZE值可能是原因因此我将其从16384更改为131072，但它也没有帮助：

#ifdef CURL_MAX_WRITE_SIZE
    #undef CURL_MAX_WRITE_SIZE
    #define CURL_MAX_WRITE_SIZE 131072
#endif

这是我的代码的简要概述：

class EXCHANGE
{
   public:

   enum { BASE_TYPE }; //to check if template-types are derived from this class

   virtual size_t orderbookDataProcessing(void* buffer, unsigned int size, unsigned int nmemb) = 0; 

   template <typename TTYPE> //only for classes derived from EXCHANGE
   static size_t orderbookDataCallback(void* buffer, unsigned int size, unsigned int nmemb, void* userdata)
   {
       //check if TTYPE is derived from EXCHANGE (BASE_TYPE is declared in EXCHANGE)
       enum { IsItDerived = TTYPE::BASE_TYPE };   

       TTYPE* instance = (TTYPE*)userdata;

       if(0 != instance->orderbookDataProcessing(buffer, size, nmemb))
       {
           return 1;
       }
       else
       {
           return (size*nmemb);
       }
   }
}

class BITSTAMP : public EXCHANGE
{
   public:

   size_t orderbookDataProcessing(void* buffer, unsigned int size, unsigned int nmemb) 
   {   
       string recvd_string; //contains the received string

       //copy the received data into a new string
       recvd_string.append((const char*)buffer, size*nmemb);

       //do stuff with the recvd_string

       return 0;
   }
}


//main.cpp
///////////////////////////////////

CURL* orderbook_curl_handle = curl_easy_init();

curl_easy_setopt(orderbook_curl_handle, CURLOPT_URL, "https://www.bitstamp.net/api/order_book/?group=1") //URL to receive the get-request 

curl_easy_setopt(orderbook_curl_handle, CURLOPT_HTTPGET, 1) //use get method 

curl_easy_setopt(orderbook_curl_handle, CURLOPT_NOSIGNAL, 1)  //required for multi-threading ?

curl_easy_setopt(orderbook_curl_handle, CURLOPT_WRITEDATA, &bitstamp_USD)  //instance passed to the callback-function

curl_easy_setopt(orderbook_curl_handle, CURLOPT_WRITEFUNCTION, (size_t(*)(void*, unsigned int, unsigned int, void*))&EXCHANGE::orderbookDataCallback<BITSTAMP>) //callback function to recv the data  

curl_easy_setopt(orderbook_curl_handle, CURLOPT_SSL_VERIFYPEER, FALSE) //do not verify ca-certificate

curl_easy_perform(orderbook_curl_handle);

Answer 1

CURL在收到数据时调用您的写回调函数。如果您的数据太大（而您的数据太大），则会通过多次调用写回调给您。

您必须从每个写回调中收集数据，直到CURL告诉您响应完成。在您的情况下，这应该是curl_easy_perform返回时。

cURL处理大型JSON响应

1 个答案: