libcurl-multi(3) libcurl multi interface libcurl-multi(3)NAMElibcurl-multi - how to use the multi interface
DESCRIPTION
This is an overview on how to use the libcurl multi inter-
face in your C programs. There are specific man pages for
each function mentioned in here. There's also the libcurl-
tutorial(3) man page for a complete tutorial to programming
with libcurl and the libcurl-easy(3) man page for an over-
view of the libcurl easy interface.
All functions in the multi interface are prefixed with
curl_multi.
OBJECTIVES
The multi interface introduces several new abilities that
the easy interface refuses to offer. They are mainly:
1. Enable a "pull" interface. The application that uses lib-
curl decides where and when to ask libcurl to get/send data.
2. Enable multiple simultaneous transfers in the same thread
without making it complicated for the application.
3. Enable the application to select() on its own file
descriptors and curl's file descriptors simultaneous easily.
ONE MULTI HANDLE MANY EASY HANDLES
To use the multi interface, you must first create a 'multi
handle' with curl_multi_init(3). This handle is then used as
input to all further curl_multi_* functions.
Each single transfer is built up with an easy handle. You
must create them, and setup the appropriate options for each
easy handle, as outlined in the libcurl(3) man page, using
curl_easy_setopt(3).
When the easy handle is setup for a transfer, then instead
of using curl_easy_perform(3) (as when using the easy inter-
face for transfers), you should instead add the easy handle
to the multi handle using curl_multi_add_handle(3). The
multi handle is sometimes referred to as a 'multi stack'
because of the fact that it may hold a large amount of easy
handles.
Should you change your mind, the easy handle is again
removed from the multi stack using
curl_multi_remove_handle(3). Once removed from the multi
handle, you can again use other easy interface functions
like curl_easy_perform(3) on the handle or whatever you
think is necessary.
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Adding the easy handle to the multi handle does not start
the transfer. Remember that one of the main ideas with this
interface is to let your application drive. You drive the
transfers by invoking curl_multi_perform(3). libcurl will
then transfer data if there is anything available to
transfer. It'll use the callbacks and everything else you
have setup in the individual easy handles. It'll transfer
data on all current transfers in the multi stack that are
ready to transfer anything. It may be all, it may be none.
Your application can acquire knowledge from libcurl when it
would like to get invoked to transfer data, so that you
don't have to busy-loop and call that curl_multi_perform(3)
like crazy. curl_multi_fdset(3) offers an interface using
which you can extract fd_sets from libcurl to use in
select() or poll() calls in order to get to know when the
transfers in the multi stack might need attention. This also
makes it very easy for your program to wait for input on
your own private file descriptors at the same time or
perhaps timeout every now and then, should you want that.
A little note here about the return codes from the multi
functions, and especially the curl_multi_perform(3): if you
receive CURLM_CALL_MULTI_PERFORM, this basically means that
you should call curl_multi_perform(3) again, before you
select() on more actions. You don't have to do it immedi-
ately, 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".
curl_multi_perform(3) stores the number of still running
transfers in one of its input arguments, and by reading that
you can figure out when all the transfers in the multi han-
dles are done. 'done' does not mean successful. One or more
of the transfers may have failed. Tracking when this number
changes, you know when one or more transfers are done.
To get information about completed transfers, to figure out
success or not and similar, curl_multi_info_read(3) should
be called. It can return a message about a current or previ-
ous transfer. Repeated invokes of the function get more mes-
sages until the message queue is empty. The information you
receive there includes an easy handle pointer which you may
use to identify which easy handle the information regards.
When all transfers in the multi stack are done, cleanup the
multi handle with curl_multi_cleanup(3). Be careful and
please note that you MUST invoke separate
curl_easy_cleanup(3) calls on every single easy handle to
clean them up properly.
If you want to re-use an easy handle that was added to the
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libcurl-multi(3) libcurl multi interface libcurl-multi(3)
multi handle for transfer, you must first remove it from the
multi stack and then re-add it again (possibly after having
altered some options at your own choice).
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