Custom pointer passed to i/o callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_IOCTLDATA, void *pointer);
Pass the pointer that will be untouched by libcurl and passed as the 3rd argument in the ioctl callback set with CURLOPT_IOCTLFUNCTION(3).
By default, the value of this parameter is NULL.
Used with HTTP
Added in 7.12.3
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.