Provided by: libcurl4-doc_7.35.0-1ubuntu2_all bug


       libcurl-multi - how to use the multi interface


       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.


       The multi interface offers several abilities that the easy interface  doesn't.   They  are

       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.


       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_*

       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

       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

       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).


       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.


       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