libcurl-multi

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.

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  out-
       lined 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_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
       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 han-
       dle 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
       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 signifi-
       cant difference for applications using large  numbers  of  simultaneous
       connections.

       curl_multi_socket_action(3)  is  then  used  instead of curl_multi_per-
       form(3).

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 on non-windows unless c-ares is used
        - GnuTLS SSL connections
        - Active FTP connections
        - HTTP proxy CONNECT operations