Custom pointer passed to the seek callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKDATA, void *pointer);
Data pointer to pass to the seek callback function. If you use the CURLOPT_SEEKFUNCTION(3) option, this is the pointer you'll get as input.
If you don't set this, NULL is passed to the callback.
HTTP, FTP, SFTP
Added in 7.18.0