curl_multi_perform(3) libcurl Manual curl_multi_perform(3)
curl_multi_perform - reads/writes available data from each easy handle
CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles);
This function handles transfers on all the added handles that need attention in an non-
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 informa-
tion 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_han-
When running_handles is set to zero (0) on the return of this function, there is no longer
any transfers in progress.
CURLMcode 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
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.
Most applications will use curl_multi_fdset(3) to get the multi_handle's file descriptors,
and curl_multi_timeout(3) to get a suitable timeout period, then it'll wait for action on
the file descriptors using select(3). As soon as one or more file descriptor is ready,
curl_multi_perform(3) gets called.
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_fdset(3), curl_multi_info_read(3),
libcurl 7.9.5 1 March 2002 curl_multi_perform(3)