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       libcurl-multi - how to use the multi interface


       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.


       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.


       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_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.

       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.

       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 program 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.

       A little note here about the return codes from the multi functions, and
       especially    the     curl_multi_perform(3):     if     you     receive
       CURLM_CALL_MULTI_PERFORM,  this  basically  means  that you should call
       curl_multi_perform(3) again, before you select() on more  actions.  You
       don’t have to do it immediately, but the return code means that libcurl
       may have more data available to return or that there may be  more  data
       to send off before it is "satisfied".

       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,  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


       Since 7.16.0, the 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


       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 on non-windows unless c-ares is used
        - GnuTLS SSL connections
        - NSS SSL connections
        - Active FTP connections
        - HTTP proxy CONNECT operations
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers