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Updated: Wednesday, July 27, 2022
 
 

libcurl-multi (3)

Name

libcurl-multi - how to use the multi interface

Synopsis

Please see following description for synopsis

Description

libcurl-multi(3)            libcurl multi interface           libcurl-multi(3)



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  descrip-
       tors 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 simulta-
       neous 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()  ori-
       ented 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 han-
       dle 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_per-
       form(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 cur-
       rent  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 applica-
       tion 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 suit-
       able 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_per-
       form(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  functional-
       ity  (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 simul-
       taneous 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 time-
       out timer expires.

       You  can  poll  curl_multi_info_read(3) to see if any transfer has com-
       pleted, 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  cur-
       rent restrictions:

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


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | web/curl         |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+

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



NOTES
       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-
       code-downloads.html.

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source       was       downloaded      from       https://curl.se/down-
       load/curl-7.83.1.tar.bz2.

       Further information about this software can be found on the open source
       community website at http://curl.haxx.se/.



libcurl 7.83.1                 December 23, 2021              libcurl-multi(3)