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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 tu-
       torial 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	simultaneously.

       4.  Enable  event-based handling	and scaling transfers up to and	beyond
       thousands of parallel connections.

       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 in time 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'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. 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  re-
       turn  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

       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
       curl_multi_setopt(3). They are two callback functions that libcurl will
       call with information about what	sockets	to wait	for, and for what  ac-
       tivity,	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 ap-
       plication "subscribes" on socket	changes. This allows applications  and
       libcurl	to much	better scale upward and	beyond thousands of simultane-
       ous transfers without losing performance.

       When  you've  added   your   initial   set   of	 handles,   you	  call
       curl_multi_socket_action(3)  with CURL_SOCKET_TIMEOUT set in the	sockfd
       argument, and you'll 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've been asked to wait on, or	if the timeout
       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.

       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
	- SOCKS	proxy handshakes
	- file:// transfers
	- TELNET transfers

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

libcurl	7.74.0		       November	04, 2020	      libcurl-multi(3)


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