1libcurl-multi(3)            libcurl multi interface           libcurl-multi(3)
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NAME

6       libcurl-multi - how to use the multi interface
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DESCRIPTION

9       This is an overview on how to use the libcurl multi interface in your C
10       programs. There are specific man pages for each function  mentioned  in
11       here.  There's  also  the  libcurl-tutorial(3)  man page for a complete
12       tutorial to programming with libcurl and the libcurl-easy(3)  man  page
13       for an overview of the libcurl easy interface.
14
15       All functions in the multi interface are prefixed with curl_multi.
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OBJECTIVES

18       The  multi  interface  offers several abilities that the easy interface
19       doesn't.  They are mainly:
20
21       1. Enable a "pull" interface. The application that uses libcurl decides
22       where and when to ask libcurl to get/send data.
23
24       2.  Enable  multiple  simultaneous transfers in the same thread without
25       making it complicated for the application.
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27       3. Enable the application to wait for action on its own  file  descrip‐
28       tors and curl's file descriptors simultaneously.
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30       4.  Enable  event-based handling and scaling transfers up to and beyond
31       thousands of parallel connections.
32

ONE MULTI HANDLE MANY EASY HANDLES

34       To use the multi interface, you must first create a 'multi handle' with
35       curl_multi_init(3).  This  handle  is then used as input to all further
36       curl_multi_* functions.
37
38       With a multi handle and the multi interface you can do several simulta‐
39       neous transfers in parallel. Each single transfer is built up around an
40       easy handle. You create all the easy handles you need,  and  setup  the
41       appropriate options for each easy handle using curl_easy_setopt(3).
42
43       There  are  two  flavours of the multi interface, the select() oriented
44       one and the event based one we call multi_socket. You will benefit from
45       reading  through  the  description of both versions to fully understand
46       how they work and differentiate. We start out with  the  select()  ori‐
47       ented version.
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49       When  an  easy  handle is setup and ready for transfer, then instead of
50       using curl_easy_perform(3) like  when  using  the  easy  interface  for
51       transfers,  you  should  add  the  easy handle to the multi handle with
52       curl_multi_add_handle(3). You can add more easy handles to a multi han‐
53       dle at any point, even if other transfers are already running.
54
55       Should  you change your mind, the easy handle is again removed from the
56       multi stack using curl_multi_remove_handle(3). Once  removed  from  the
57       multi  handle,  you  can  again use other easy interface functions like
58       curl_easy_perform(3) on the handle or whatever you think is  necessary.
59       You can remove handles at any point in time during transfers.
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61       Adding the easy handle to the multi handle does not start the transfer.
62       Remember that one of the main ideas with this interface is to let  your
63       application  drive. You drive the transfers by invoking curl_multi_per‐
64       form(3). libcurl will then transfer data if there is anything available
65       to transfer. It'll use the callbacks and everything else you have setup
66       in the individual easy handles. It'll  transfer  data  on  all  current
67       transfers  in  the  multi stack that are ready to transfer anything. It
68       may be all, it may be none. When there's nothing more to do for now, it
69       returns back to the calling application.
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71       Your application extracts info from libcurl about when it would like to
72       get invoked to transfer data or do other work. The most convenient  way
73       is to use curl_multi_poll(3) that will help you wait until the applica‐
74       tion should call libcurl again. The older API to  accomplish  the  same
75       thing  is curl_multi_fdset(3) that extracts fd_sets from libcurl to use
76       in select() or poll() calls in order to get to know when the  transfers
77       in the multi stack might need attention. Both these APIs allow for your
78       program to wait for input on your own private file descriptors  at  the
79       same  time  curl_multi_timeout(3) also helps you with providing a suit‐
80       able timeout period for your select() calls.
81
82       curl_multi_perform(3) stores the number of still running  transfers  in
83       one of its input arguments, and by reading that you can figure out when
84       all the transfers in the multi handles are done. 'done' does  not  mean
85       successful. One or more of the transfers may have failed.
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87       To  get information about completed transfers, to figure out success or
88       not and similar,  curl_multi_info_read(3)  should  be  called.  It  can
89       return a message about a current or previous transfer. Repeated invokes
90       of the function get more messages until the message queue is empty. The
91       information you receive there includes an easy handle pointer which you
92       may use to identify which easy handle the information regards.
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94       When a single transfer is completed, the  easy  handle  is  still  left
95       added to the multi stack. You need to first remove the easy handle with
96       curl_multi_remove_handle(3)     and     then     close     it      with
97       curl_easy_cleanup(3),  or  possibly  set  new  options to it and add it
98       again with curl_multi_add_handle(3) to start another transfer.
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100       When all transfers in the multi stack are done, close the multi  handle
101       with  curl_multi_cleanup(3).  Be  careful and please note that you MUST
102       invoke separate curl_easy_cleanup(3) calls for every single easy handle
103       to clean them up properly.
104
105       If you want to re-use an easy handle that was added to the multi handle
106       for transfer, you must first remove it from the multi  stack  and  then
107       re-add it again (possibly after having altered some options at your own
108       choice).
109

MULTI_SOCKET

111       curl_multi_socket_action(3) function offers a way for  applications  to
112       not  only avoid being forced to use select(), but it also offers a much
113       more high-performance API that will make a significant  difference  for
114       applications using large numbers of simultaneous connections.
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116       curl_multi_socket_action(3)  is  then  used  instead of curl_multi_per‐
117       form(3).
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119       When using this API, you add easy handles to the multi handle  just  as
120       with  the  normal multi interface. Then you also set two callbacks with
121       the CURLMOPT_SOCKETFUNCTION(3) and CURLMOPT_TIMERFUNCTION(3) options to
122       curl_multi_setopt(3). They are two callback functions that libcurl will
123       call with information about what sockets to  wait  for,  and  for  what
124       activity,  and  what  the  current  timeout  time  is - if that expires
125       libcurl should be notified.
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127       The multi_socket API is designed to inform your application about which
128       sockets libcurl is currently using and for what activities (read and/or
129       write) on those sockets your application is expected to wait for.
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131       Your application must make sure to receive all sockets  informed  about
132       in  the  CURLMOPT_SOCKETFUNCTION(3) callback and make sure it reacts on
133       the given activity on them. When a socket has the given  activity,  you
134       call  curl_multi_socket_action(3)  specifying  which  socket and action
135       there are.
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137       The CURLMOPT_TIMERFUNCTION(3) callback is called to set a timeout. When
138       that    timeout    expires,    your   application   should   call   the
139       curl_multi_socket_action(3) function saying it was due to a timeout.
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141       This API is typically used with an event-driven underlying  functional‐
142       ity  (like  libevent,  libev,  kqueue, epoll or similar) with which the
143       application "subscribes" on socket changes.  This  allows  applications
144       and  libcurl to much better scale upward and beyond thousands of simul‐
145       taneous transfers without losing performance.
146
147       When  you've  added   your   initial   set   of   handles,   you   call
148       curl_multi_socket_action(3)  with CURL_SOCKET_TIMEOUT set in the sockfd
149       argument, and you'll get callbacks call that sets you up and  you  then
150       continue  to  call curl_multi_socket_action(3) accordingly when you get
151       activity on the sockets you've been asked to wait on, or if the timeout
152       timer expires.
153
154       You  can  poll  curl_multi_info_read(3) to see if any transfer has com‐
155       pleted, as it then has a message saying so.
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BLOCKING

158       A few areas in the code are still using blocking code, even  when  used
159       from  the multi interface. While we certainly want and intend for these
160       to get fixed in the future, you should be aware of the  following  cur‐
161       rent restrictions:
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163        - Name resolves unless the c-ares or threaded-resolver backends are used
164        - SOCKS proxy handshakes
165        - file:// transfers
166        - TELNET transfers
167

SEE ALSO

169       libcurl-errors(3), libcurl-easy(3), libcurl(3)
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173libcurl 7.69.1                 November 28, 2019              libcurl-multi(3)
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