All functions in the share interface are prefixed with curl_share.
You tell the shared object what data you want it to share by using curl_share_setopt(3). Currently you can only share DNS and/or COOKIE data.
Since you can use this share from multiple threads, and libcurl has no internal thread synchronization, you must provide mutex callbacks if you're using this multi-threaded. You set lock and unlock functions with curl_share_setopt(3) too.
Then, you make an easy handle to use this share, you set the CURLOPT_SHARE option with curl_easy_setopt(3), and pass in share handle. You can make any number of easy handles share the same share handle.
To make an easy handle stop using that particular share, you set CURLOPT_SHARE to NULL for that easy handle. To make a handle stop sharing a particular data, you can CURLSHOPT_UNSHARE it.
When you're done using the share, make sure that no easy handle is still using it, and call curl_share_cleanup(3) on the handle.