libcurl-multi
libcurl-multi(5) libcurl multi interface libcurl-multi(5)
NAME
libcurl-multi - how to use the multi interface
DESCRIPTION
This is an overview on how to use the libcurl multi interface 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 tuto-
rial 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 easily.
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 out-
lined in the libcurl(3) man page.
When the easy handle is setup for a transfer, then instead of using
curl_easy_perform (as when using the easy interface for transfers), you
should instead add the easy handle 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_handle. Once removed from the multi
handle, you can again use other easy interface functions like
curl_easy_perform or whatever you think is necessary.
Adding the easy handles to the multi handle does not start any trans-
fer. 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 any-
thing 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 a mad man! 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 trans-
fers 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.
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 suc-
cessful. One or more of the transfers may have failed.
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.
When all transfers in the multi stack are done, cleanup the multi han-
dle 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.
libcurl 7.9.5 20 March 2001 libcurl-multi(5)
