curl_multi_perform(3) -- Linux man page
NAMEcurl_multi_perform - reads/writes available data from each easy handle
DESCRIPTIONWhen the app thinks there's data available for the multi_handle, it should call this function to read/write whatever there is to read or write right now. curl_multi_perform() returns as soon as the reads/writes are done. This function does not require that there actually is any data available for reading or that data can be written, it can be called just in case. It will write the number of handles that still transfer data in the second argument's integer-pointer.
RETURN VALUECURLMcode type, general libcurl multi interface error code.
If you receive CURLM_CALL_MULTI_PERFORM, this basically means that you should call curl_multi_perform again, before you select() on more actions. You don't have to do it immediately, but the return code means that libcurl may have more data available to return or that there may be more data to send off before it is "satisfied".
TYPICAL USAGEMost application will use curl_multi_fdset(3) to get the multi_handle's file descriptors, then it'll wait for action on them using select() and as soon as one or more of them are ready, curl_multi_perform(3) gets called.