Provided by: libcurl4-doc_7.81.0-1ubuntu1.20_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 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 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 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
       will  use  the  callbacks  and  everything  else  you  have setup in the individual easy handles. it will
       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_poll(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.

       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(3) and CURLMOPT_TIMERFUNCTION(3) 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(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 will 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
       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 7.81.0                                  December 23, 2021                               libcurl-multi(3)