title: curl callback function threading
body: <p>I have done some googling but haven't found the answer to this as of yet.</p>
  
  <p>When you registrar a function with curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, function) is that function guaranteed to be completed before curl_easy_perform(curl) returns? Or is it possible that function is being ran in parallel and may still be running when the easy_perform returns?</p>