NAME¶
curl_multi_perform - reads/writes available data from each easy handle
SYNOPSIS¶
#include <curl/curl.h>
CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles);
DESCRIPTION¶
This 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.
When
running_handles is set to zero (0) on the return of this function,
there is no longer any transfers in progress.
RETURN VALUE¶
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 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.
TYPICAL USAGE¶
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.
SEE ALSO¶
curl_multi_cleanup(3),
curl_multi_init(3),
curl_multi_fdset(3),
curl_multi_info_read(3),
libcurl-errors(3)