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libcurl-multi(3) libcurl multi interface libcurl-multi(3)
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-tutorial(3) man page for a complete tutorial to programming with libcurl
and the libcurl-easy(3) man page for an overview 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 libcurl decides where and
when to ask libcurl to get/send data.
2. Enable multiple simultaneous transfers in the same thread without making it com-
plicated 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_per-
form(3) (as when using the easy interface for transfers), you should instead add
the easy handle to the multi handle using curl_multi_add_handle(3). The multi han-
dle 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.
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 trans-
fer 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 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".
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 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 transfers are done.
To get information about completed transfers, to figure out success or not and sim-
ilar, curl_multi_info_read(3) 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 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 multi handle for trans-
fer, you must first remove it from the multi stack and then re-add it again (possi-
bly after having altered some options at your own choice).
libcurl 7.10.1 13 Oct 2001 libcurl-multi(3)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-11-24 00:10 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)