1libcurl-multi(3) libcurl multi interface libcurl-multi(3)
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6 libcurl-multi - how to use the multi interface
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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 All functions in the multi interface are prefixed with curl_multi.
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18 The multi interface offers several abilities that the easy interface
19 doesn't. They are mainly:
20 1. Enable a "pull" interface. The application that uses libcurl decides
22 where and when to ask libcurl to get/send data.
23 2. Enable multiple simultaneous transfers in the same thread without
25 making it complicated for the application.
26 3. Enable the application to wait for action on its own file descrip‐
28 tors and curl's file descriptors simultaneously.
29 4. Enable event-based handling and scaling transfers up to and beyond
31 thousands of parallel connections.
32
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 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 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.
48 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 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.
60 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.
70 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 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.
86 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.
93 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.
99 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 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
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.
115 curl_multi_socket_action(3) is then used instead of curl_multi_per‐
117 form(3).
118 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.
126 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.
130 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.
136 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.
140 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 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 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.
156
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:
162 - Name resolves unless the c-ares or threaded-resolver backends are used
164 - SOCKS proxy handshakes
165 - file:// transfers
166 - TELNET transfers
167
169 libcurl-errors(3), libcurl-easy(3), libcurl(3)
170libcurl 7.71.1 April 26, 2020 libcurl-multi(3)