curl_multi_perform (3) - Linux Man Pages
curl_multi_perform: reads/writes available data from each easy handle
NAMEcurl_multi_perform - reads/writes available data from each easy handle
DESCRIPTIONThis function handles transfers on all the added handles that need attention in an non-blocking fashion.
When an application has found out there's data available for the multi_handle or a timeout has elapsed, the application should call this function to read/write whatever there is to read or write right now etc. 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.
If the amount of running_handles is changed from the previous call (or is less than the amount of easy handles you've added to the multi handle), you know that there is one or more transfers less "running". You can then call curl_multi_info_read(3) to get information about each individual completed transfer, and that returned info includes CURLcode and more. If an added handle fails very quickly, it may never be counted as a running_handle.
RETURN VALUECURLMcode type, general libcurl multi interface error code.
Before version 7.20.0: 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". Do note that curl_multi_perform(3) will return CURLM_CALL_MULTI_PERFORM only when it wants to be called again immediately. When things are fine and there is nothing immediate it wants done, it'll return CURLM_OK and you need to wait for "action" and then call this function again.
This function only returns errors etc regarding the whole multi stack. Problems still might have occurred on individual transfers even when this function returns CURLM_OK. Use curl_multi_info_read(3) to figure out how individual transfers did.