Ubuntu Manpage: libcurl-multi - how to use the multi interface

Provided by: libcurl4-doc_7.68.0-1ubuntu2.25_all

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
       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 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 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 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 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've added your initial set of handles, you call curl_multi_socket_action(3) 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.

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