NAME

     libcurl-multi - how to use the multi interface


DESCRIPTION

     This  is  an overview on how to use the libcurl multi inter­
     face in your C programs. There are specific  man  pages  for
     each  function  mentioned in here. There's also the libcurl-
     the-guide document for a complete  tutorial  to  programming
     with  libcurl and the libcurl(3) man page for an overview of
     the libcurl easy interface.

     All functions in  the  multi  interface  are  prefixed  with
     curl_multi.


PLEASE NOTICE

     The  multi  interface  is a rather new member of the libcurl
     family. It has not yet been very widely used. It  may  still
     be  a  few  more  bugs lurking in there than we are used to.
     That said, it might also just work in every aspect  you  try
     it. Please report all bugs and oddities you see.


OBJECTIVES

     The  multi  interface  introduces several new abilities that
     the easy interface refuses to offer. 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  select()  on  its own file
     descriptors and curl's file descriptors simultaneous easily.


ONE MULTI HANDLE MANY EASY HANDLES

     To  use  the multi interface, you must first create a 'multi
     handle' with curl_multi_init. This handle is  then  used  as
     input to all further curl_multi_* functions.

     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.

     When  the  easy handle is setup for a transfer, then instead
     of using curl_easy_perform (as when using the easy interface
     for  transfers),  you  should instead add the easy handle to
     the multi handle using curl_easy_add_handl. 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.
     Once removed from the multi handle, you can again use  other
     easy  interface functions like curl_easy_perform or whatever
     you think is necessary.

     Adding the easy handles to the multi handle does  not  start
     any  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. 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
     like  a  mad man! curl_multi_fdset 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  per­
     haps timeout every now and then, should you want that.

     curl_multi_perform stores the number of still running trans­
     fers 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  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.

     When  all transfers in the multi stack are done, cleanup the
     multi handle with curl_multi_cleanup. Be careful and  please
     note  that  you MUST invoke separate curl_easy_cleanup calls
     on every single easy handle to clean them up properly.











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