Pointer to pass to header callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERDATA, void *pointer);
Pass a pointer to be used to write the header part of the received data to.
If CURLOPT_WRITEFUNCTION(3) or CURLOPT_HEADERFUNCTION(3) is used, pointer will be passed in to the respective callback.
If neither of those options are set, pointer must be a valid FILE * and it will be used by a plain fwrite() to write headers to.