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

Provided by: libcurl4-doc_7.47.0-1ubuntu2.19_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 simultaneous easily.

       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_wait(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. 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 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 and
CURLMOPT_TIMERFUNCTION 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 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 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) which 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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