libcurl-multi man page on Archlinux

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

       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 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 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_* 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  out‐
       lined 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  interface  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_per‐
       form(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 trans‐
       fers  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 han‐
       dle 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

       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‐

       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 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  cur‐
       rent 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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