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 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
       _multi_init&section=3">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
       _easy_setopt&section=3">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 _easy_perform&section=3">curl_easy_perform(3) like when using the easy interface for
       transfers, you should add the easy handle to the multi handle with
       _multi_add_handle&section=3">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 _multi_remove_handle&section=3">curl_multi_remove_handle(3). Once removed from the
       multi handle, you can again use other easy interface functions like
       _easy_perform&section=3">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
       _multi_perform&section=3">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 _multi_poll&section=3">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 _multi_fdset&section=3">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.  _multi_timeout&section=3">curl_multi_timeout(3) also helps you
       with providing a suitable timeout period for your select() calls.

       _multi_perform&section=3">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, _multi_info_read&section=3">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
       _multi_remove_handle&section=3">curl_multi_remove_handle(3) and then close it with
       _easy_cleanup&section=3">curl_easy_cleanup(3), or possibly set new options to it and add it
       again with _multi_add_handle&section=3">curl_multi_add_handle(3) to start another transfer.

       When all transfers in the multi stack are done, close the multi handle
       with _multi_cleanup&section=3">curl_multi_cleanup(3). Be careful and please note that you MUST
       invoke separate _easy_cleanup&section=3">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
       _multi_socket_action&section=3">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.

       _multi_socket_action&section=3">curl_multi_socket_action(3) is then used instead of
       _multi_perform&section=3">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 _SOCKETFUNCTION&section=3">CURLMOPT_SOCKETFUNCTION(3) and _TIMERFUNCTION&section=3">CURLMOPT_TIMERFUNCTION(3) options to
       _multi_setopt&section=3">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 _SOCKETFUNCTION&section=3">CURLMOPT_SOCKETFUNCTION(3) callback and make sure it reacts on
       the given activity on them. When a socket has the given activity, you
       call _multi_socket_action&section=3">curl_multi_socket_action(3) specifying which socket and action
       there are.

       The _TIMERFUNCTION&section=3">CURLMOPT_TIMERFUNCTION(3) callback is called to set a timeout. When
       that timeout expires, your application should call the
       _multi_socket_action&section=3">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
       _multi_socket_action&section=3">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 _multi_socket_action&section=3">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 _multi_info_read&section=3">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
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers

SEE ALSO
       libcurl-errors(3), libcurl-easy(3), libcurl(3)

libcurl 7.77.0                 November 4, 2020               libcurl-multi(3)