CURLINFO_PRIVATE man page
CURLINFO_PRIVATE — get the private pointer
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_PRIVATE, char **private);
Pass a pointer to a char pointer to receive the pointer to the private data associated with the curl handle (set with the CURLOPT_PRIVATE(3)). Please note that for internal reasons, the value is returned as a char pointer, although effectively being a 'void *'.
Added in 7.10.3
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_getinfo(3), curl_easy_setopt(3), CURLOPT_PRIVATE(3),