libcurl offers a few different APIs to do transfers; where the primary differences are the synchronous easy interface versus the non-blocking multi interface. The multi interface itself can then be further used either by using the event-driven socket interface or the normal perform interface.
Internally however, everything is written for the event-driven interface. Everything needs to be written in non-blocking fashion so that functions are never waiting for data in loop or similar. Unless they are the surface functions that have that expressed functionality.
The function curl_easy_perform() which performs a single transfer
synchronously, is itself just a wrapper function that internally setups and
uses the multi interface itself.