curl_multi_perform - reads/writes available data from each easy handle
CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles);
When 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.
When you call curl_multi_perform() and 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.
CURLMcode 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".
NOTE that this only returns errors etc regarding the whole multi stack. There might still have occurred problems on individual transfers even when this function returns OK.
Most applications will use curl_multi_fdset(3) to get the multi_handle’s file descriptors, then it’ll wait for action on them using select(3) and as soon as one or more of them are ready, curl_multi_perform(3) gets called.