Provided by: libcurl4-doc_8.5.0-2ubuntu10.6_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 is 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 does not.  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 flavors of the multi interface, the select() oriented one and the event based one we call
       multi_socket. You 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 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 then transfers data if there is anything available to  transfer.  It  uses
       the callbacks and everything else you have setup in the individual easy handles. It transfers 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 is 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_poll(3) that helps 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.

       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 reuse 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 makes 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(3) and CURLMOPT_TIMERFUNCTION(3) options
       to  curl_multi_setopt(3).  They are two callback functions that libcurl calls 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(3)
       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(3) 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   have   added  your  initial  set  of  handles,  you  call  curl_multi_socket_action(3)  with
       CURL_SOCKET_TIMEOUT set in the sockfd argument, and you get callbacks invoked that set  you  up  and  you
       then  continue  to  call curl_multi_socket_action(3) accordingly when you get activity on the sockets you
       have 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
        - file:// transfers
        - TELNET transfers

SEE ALSO

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

libcurl 8.5.0                                  September 26, 2023                               libcurl-multi(3)