curl_multi_timeout (3) - Linux Man Pages
curl_multi_timeout: how long to wait for action before proceeding
NAMEcurl_multi_timeout - how long to wait for action before proceeding
An application using the libcurl multi interface should call curl_multi_timeout(3) to figure out how long it should wait for socket actions - at most - before proceeding.
Proceeding means either doing the socket-style timeout action: call the curl_multi_socket_action(3) function with the sockfd argument set to CURL_SOCKET_TIMEOUT, or call curl_multi_perform(3) if you're using the simpler and older multi interface approach.
The timeout value returned in the long timeout points to, is in number of milliseconds at this very moment. If 0, it means you should proceed immediately without waiting for anything. If it returns -1, there's no timeout at all set.
An application that uses the multi_socket API SHOULD not use this function, but SHOULD instead use curl_multi_setopt(3) and its CURLMOPT_TIMERFUNCTION option for proper and desired behavior.
Note: if libcurl returns a -1 timeout here, it just means that libcurl currently has no stored timeout value. You must not wait too long (more than a few seconds perhaps) before you call curl_multi_perform() again.