Provided by: libcurl4-doc_7.58.0-2ubuntu3.24_all bug

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
       simultaneously.

       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 simultaneous 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() oriented 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 handle 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_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. 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
       application  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 suitable 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_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).  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 functionality (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  simultaneous  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 completed,  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
       current restrictions:

        - Name resolves unless the c-ares or threaded-resolver backends are used
        - SOCKS proxy handshakes
        - file:// transfers
        - TELNET transfers

SEE ALSO

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