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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  descrip-
       tors 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 simulta-
       neous 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()  ori-
       ented 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 han-
       dle 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_per-
       form(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 applica-
       tion 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 suit-
       able 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_per-
       form(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 functional-
       ity (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  simul-
       taneous 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  com-
       pleted, 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 cur-
       rent restrictions:

        - Name resolves unless the c-ares or threaded-resolver backends are used
        - NSS SSL connections
        - HTTP proxy CONNECT operations
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers


SEE ALSO

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



libcurl                           19 Sep 2014                 libcurl-multi(3)

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