libcurl-multi(5) libcurl multi interface libcurl-multi(5)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-
the-guide document for a complete tutorial to programming
with libcurl and the libcurl(3) man page for an overview
of the libcurl easy interface.
All functions in the multi interface are prefixed with
curl_multi.
PLEASE NOTICE
The multi interface is a rather new member of the libcurl
family. It has not yet been very widely used. It may still
be a few more bugs lurking in there than we are used to.
That said, it might also just work in every aspect you try
it. Please report all bugs and oddities you see.
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
libcurl 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 eas-
ily.
ONE MULTI HANDLE MANY EASY HANDLES
To use the multi interface, you must first create a 'multi
handle' with curl_multi_init. 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 (as when using the easy inter-
face for transfers), you should instead add the easy han-
dle to the multi handle using curl_easy_add_handl. 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_han-
dle. Once removed from the multi handle, you can again use
other easy interface functions like curl_easy_perform on
the handle or whatever you think is necessary.
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. 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
like crazy. curl_multi_fdset 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: if you
receive CURLM_CALL_MULTI_PERFORM, this basicly means that
you should call curl_multi_perform 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 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 handles 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 trans-
fers are done.
To get information about completed transfers, to figure
out success or not and similar, curl_multi_info_read
should be called. It can return a message about a current
or previous transfer. Repeated invokes of the function get
more messages until the message queue is empty. The infor-
mation you receive there includes an easy handle pointer
which you may use to identify which easy handle the infor-
mation regards.
When all transfers in the multi stack are done, cleanup
the multi handle with curl_multi_cleanup. Be careful and
please note that you MUST invoke separate
curl_easy_cleanup 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
multi handle for transfer, you must first remove it from
the multi stack and then re-add it again (possbily after
having altered some options at your own choice).
libcurl 7.10.1 13 Oct 2001 libcurl-multi(5)
