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 offers several abilities that the easy interface doesn't. 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 wait for action 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 han-dle handle
dle 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 interface 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 inter-face interface
face 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 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 imme-diately, immediately,
diately, 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 similar,
curl_multi_info_read(3) should be called. It can return a message about a current or previous trans-fer. transfer.
fer. Repeated invokes of the function get more messages until the message queue is empty. The infor-mation information
mation you receive there includes an easy handle pointer which you may use to identify which easy
handle the information regards.
When a single transfer is completed, the easy handle is still left added to the multi stack. You need
to first remove the easy handle with curl_multi_remove_handle(3) and then close it with
curl_easy_cleanup(3), or possibly set new options to it and add it again with curl_multi_add_han-dle(3) curl_multi_add_handle(3)
dle(3) to start another transfer.
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 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).
MULTI_SOCKET
Since 7.16.0, the curl_multi_socket(3) function offers a way for applications to not only avoid being
forced to use select(), but it also offers a much more high-performing API that will make a signifi-cant significant
cant difference for applications using large numbers of simultaneous connections.
curl_multi_socket(3) (and curl_multi_socket_all(3)) is then used instead of curl_multi_perform(3).
BLOCKING
A few areas in the code are still using blocking code, even when used from the multi interface. While
we certainly want and intend for these to get fixed in the future, you should be aware of the follow-ing following
ing current restrictions:
- Name resolves on non-windows unless c-ares is used
- GnuTLS SSL connections
- Active FTP connections
- HTTP proxy CONNECT operations
- SCP and SFTP connections
- SFTP transfers
- TFTP transfers
- file:// transfers
libcurl 7.16.0 3 Feb 2007 libcurl-multi(3)
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