libcurl-multi - how to use the multi interface
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.
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 simultaneously.
4. Enable event-based handling and scaling transfers up to and beyond thousands
of parallel connections.
ONE MULTI HANDLE MANY EASY HANDLES¶
To use the multi interface, you must first create a 'multi handle' with
. This handle is then used as input to all further
With a multi handle and the multi interface you can do several simultaneous
transfers in parallel. Each single transfer is built up around an easy handle.
You create all the easy handles you need, and setup the appropriate options
for each easy handle using curl_easy_setopt(3)
There are two flavours of the multi interface, the select() oriented one and the
event based one we call multi_socket. You will benefit from reading through
the description of both versions to fully understand how they work and
differentiate. We start out with the select() oriented version.
When an easy handle is setup and ready for transfer, then instead of using
like when using the easy interface for transfers,
you should add the easy handle to the multi handle with
. You can add more easy handles to a multi
handle at any point, even if other transfers are already running.
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
on the handle or whatever you think is necessary.
You can remove handles at any point in time during transfers.
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. When
there's nothing more to do for now, it returns back to the calling
Your application extracts info from libcurl about when it would like to get
invoked to transfer data or do other work. The most convenient way is to use
that will help you wait until the application should
call libcurl again. The older API to accomplish the same thing is
that extracts 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. Both these APIs allow for your program to
wait for input on your own private file descriptors at the same time
also helps you with providing a suitable timeout
period for your select() calls.
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
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 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
and then close it with
, or possibly set new options to it and add it
again with curl_multi_add_handle(3)
to start another transfer.
When all transfers in the multi stack are done, close the multi handle with
. Be careful and please note that you MUST
invoke separate curl_easy_cleanup(3)
calls for 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).
function offers a way for applications to not
only avoid being forced to use select(), but it also offers a much more
high-performance API that will make a significant difference for applications
using large numbers of simultaneous connections.
is then used instead of
When using this API, you add easy handles to the multi handle just as with the
normal multi interface. Then you also set two callbacks with the
CURLMOPT_SOCKETFUNCTION and CURLMOPT_TIMERFUNCTION options to
. They are two callback functions that libcurl will
call with information about what sockets to wait for, and for what activity,
and what the current timeout time is - if that expires libcurl should be
The multi_socket API is designed to inform your application about which sockets
libcurl is currently using and for what activities (read and/or write) on
those sockets your application is expected to wait for.
Your application must make sure to receive all sockets informed about in the
CURLMOPT_SOCKETFUNCTION callback and make sure it reacts on the given activity
on them. When a socket has the given activity, you call
specifying which socket and action there
The CURLMOPT_TIMERFUNCTION callback is called to set a timeout. When that
timeout expires, your application should call the
function saying it was due to a timeout.
This API is typically used with an event-driven underlying functionality (like
libevent, libev, kqueue, epoll or similar) with which the application
"subscribes" on socket changes. This allows applications and libcurl
to much better scale upward and beyond thousands of simultaneous transfers
without losing performance.
When you've added your initial set of handles, you call
with CURL_SOCKET_TIMEOUT set in the sockfd
argument, and you'll get callbacks call that sets you up and you then continue
to call curl_multi_socket_action(3)
accordingly when you get activity
on the sockets you've been asked to wait on, or if the timeout timer expires.
You can poll curl_multi_info_read(3)
to see if any transfer has
completed, as it then has a message saying so.
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 following current restrictions:
- Name resolves unless the c-ares or threaded-resolver backends are used
- SOCKS proxy handshakes
- file:// transfers
- TELNET transfers