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(3) function with the sockfd argument set to
CURL_SOCKET_TIMEOUT and the easy argument set to CURL_EASY_TIMEOUT, or
simply calling 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.
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.