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 does
not. 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
curl_multi_init(3). This handle is then used as input to all further
curl_multi_* functions.
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 flavors 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 curl_easy_perform(3) like when using the easy interface for
transfers, you should add the easy handle to the multi handle with
curl_multi_add_handle(3). 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
curl_easy_perform(3) on the handle or whatever you think is necessary.
You can remove handles at any point 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 will use the callbacks and everything
else you have setup in the individual easy handles. it will 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 application.
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 curl_multi_poll(3) that will help you wait until the application
should call libcurl again. The older API to accomplish the same thing is
curl_multi_fdset(3) 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.
curl_multi_timeout(3) also helps you with providing a suitable timeout
period for your select() calls.
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.
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 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
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_handle(3) to start another transfer.
When all transfers in the multi stack are done, close the multi handle
with curl_multi_cleanup(3). 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).
MULTI_SOCKET
curl_multi_socket_action(3) 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.
curl_multi_socket_action(3) is then used instead of
curl_multi_perform(3).
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(3) and CURLMOPT_TIMERFUNCTION(3) options to
curl_multi_setopt(3). 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 notified.
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(3) callback and make sure it reacts on the
given activity on them. When a socket has the given activity, you call
curl_multi_socket_action(3) specifying which socket and action there are.
The CURLMOPT_TIMERFUNCTION(3) callback is called to set a timeout. When
that timeout expires, your application should call the
curl_multi_socket_action(3) 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 have added your initial set of handles, you call
curl_multi_socket_action(3) with CURL_SOCKET_TIMEOUT set in the sockfd
argument, and you will 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 have 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.
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 following current
restrictions:
- Name resolves unless the c-ares or threaded-resolver backends are used
- file:// transfers
- TELNET transfers
SEE ALSO
libcurl-errors(3), libcurl-easy(3), libcurl(3)
libcurl 7.86.0 September 20, 2022 libcurl-multi(3)
curl 7.86.0 - Generated Thu Nov 3 07:28:11 CDT 2022