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
       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 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.

       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 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
       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.

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
        - NSS SSL connections
        - HTTP proxy CONNECT operations
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers