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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 descriptors and curl's file descriptors simultane-
       ous easily.

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.

       Each  single  transfer is built up with an easy handle. You must create them, and setup the appropriate options
       for each easy handle, as outlined in the libcurl(3) man page, using curl_easy_setopt(3).

       When the easy handle is setup for a transfer, then instead of using curl_easy_perform(3)  (as  when  using  the
       easy   interface  for  transfers),  you  should  instead  add  the  easy  handle  to  the  multi  handle  using
       curl_multi_add_handle(3). The multi handle is sometimes referred to as a ?multi stack? because of the fact that
       it may hold a large amount of easy handles.

       Should you change your mind, the easy handle is again removed from the multi stack using curl_multi_remove_han-
       dle(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.

       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.

       Your application can acquire knowledge from libcurl when it would like to get invoked to transfer data, so that
       you don't have to busy-loop and call that  curl_multi_perform(3)  like  crazy.  curl_multi_fdset(3)  offers  an
       interface  using  which you can extract 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. This also makes it very easy for your  pro-
       gram  to  wait for input on your own private file descriptors at the same time or perhaps timeout every now and
       then, should you want that.

       curl_multi_perform(3) stores the number of still running transfers in one of its input arguments, and by  read-
       ing that you can figure out when all the transfers in the multi handles are done. 'done' does not mean success-
       ful. 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  func-
       tion  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, cleanup the multi handle with curl_multi_cleanup(3). Be careful
       and  please  note that you MUST invoke separate curl_easy_cleanup(3) calls on 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).

       The API is then 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 then make sure to receive all sockets informed about in the CURLMOPT_SOCKETFUNCTION call-
       back 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.

BLOCKING
       A few areas in the code are still using blocking code, even when used from the multi interface. While  we  cer-
       tainly  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
        - NSS SSL connections
        - HTTP proxy CONNECT operations
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers



libcurl 7.16.0                    3 Feb 2007                  libcurl-multi(3)