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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 doesn't.  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
       simultaneous easily.

ONE MULTI HANDLE MANY EASY HANDLES
       To use the multi interface, you must first create a 'multi handle' with curl_multi_init(3). This han-dle handle
       dle 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, using curl_easy_setopt(3).

       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 inter-face interface
       face functions like curl_easy_perform(3) on the handle or whatever you think is necessary.

       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  trans-fer. transfer.
       fer.  Repeated invokes of the function get more messages until the message queue is empty. The infor-mation information
       mation 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_han-dle(3) curl_multi_add_handle(3)
       dle(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).

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 dif-ference difference
       ference 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  inter-face. interface.
       face.  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_SOCKET-FUNCTION CURLMOPT_SOCKETFUNCTION
       FUNCTION  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.

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 follow-ing following
       ing 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



libcurl 7.16.0                                   3 Feb 2007                                 libcurl-multi(3)

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