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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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23 .TH libcurl-multi 3 "19 Sep 2014" "libcurl" "libcurl multi interface"
24 .SH NAME
25 libcurl-multi \- how to use the multi interface
26 .SH DESCRIPTION
27 This is an overview on how to use the libcurl multi interface in your C
28 programs. There are specific man pages for each function mentioned in
29 here. There's also the \fIlibcurl-tutorial(3)\fP man page for a complete
30 tutorial to programming with libcurl and the \fIlibcurl-easy(3)\fP man page
31 for an overview of the libcurl easy interface.
32
33 All functions in the multi interface are prefixed with curl_multi.
34 .SH "OBJECTIVES"
35 The multi interface offers several abilities that the easy interface doesn't.
36 They are mainly:
37
38 1. Enable a "pull" interface. The application that uses libcurl decides where
39 and when to ask libcurl to get/send data.
40
41 2. Enable multiple simultaneous transfers in the same thread without making it
42 complicated for the application.
43
44 3. Enable the application to wait for action on its own file descriptors and
45 curl's file descriptors simultaneously.
46
47 4. Enable event-based handling and scaling transfers up to and beyond
48 thousands of parallel connections.
49 .SH "ONE MULTI HANDLE MANY EASY HANDLES"
50 To use the multi interface, you must first create a 'multi handle' with
51 \fIcurl_multi_init(3)\fP. This handle is then used as input to all further
52 curl_multi_* functions.
53
54 With a multi handle and the multi interface you can do several simultaneous
55 transfers in parallel. Each single transfer is built up around an easy
56 handle. You create all the easy handles you need, and setup the appropriate
57 options for each easy handle using \fIcurl_easy_setopt(3)\fP.
58
59 There are two flavours of the multi interface, the select() oriented one and
60 the event based one we call multi_socket. You will benefit from reading
61 through the description of both versions to fully understand how they work and
62 differentiate. We start out with the select() oriented version.
63
64 When an easy handle is setup and ready for transfer, then instead of using
65 \fIcurl_easy_perform(3)\fP like when using the easy interface for transfers,
66 you should add the easy handle to the multi handle with
67 \fIcurl_multi_add_handle(3)\fP. You can add more easy handles to a multi
68 handle at any point, even if other transfers are already running.
69
70 Should you change your mind, the easy handle is again removed from the multi
71 stack using \fIcurl_multi_remove_handle(3)\fP. Once removed from the multi
72 handle, you can again use other easy interface functions like
73 \fIcurl_easy_perform(3)\fP on the handle or whatever you think is
74 necessary. You can remove handles at any point in time during transfers.
75
76 Adding the easy handle to the multi handle does not start the transfer.
77 Remember that one of the main ideas with this interface is to let your
78 application drive. You drive the transfers by invoking
79 \fIcurl_multi_perform(3)\fP. libcurl will then transfer data if there is
80 anything available to transfer. It'll use the callbacks and everything else
81 you have setup in the individual easy handles. It'll transfer data on all
82 current transfers in the multi stack that are ready to transfer anything. It
83 may be all, it may be none. When there's nothing more to do for now, it
84 returns back to the calling application.
85
86 Your application extracts info from libcurl about when it would like to get
87 invoked to transfer data or do other work. The most convenient way is to use
88 \fIcurl_multi_wait(3)\fP that will help you wait until the application should
89 call libcurl again. The older API to accomplish the same thing is
90 \fIcurl_multi_fdset(3)\fP that extracts fd_sets from libcurl to use in
91 select() or poll() calls in order to get to know when the transfers in the
92 multi stack might need attention. Both these APIs allow for your program to
93 wait for input on your own private file descriptors at the same time
94 \fIcurl_multi_timeout(3)\fP also helps you with providing a suitable timeout
95 period for your select() calls.
96
97 \fIcurl_multi_perform(3)\fP stores the number of still running transfers in
98 one of its input arguments, and by reading that you can figure out when all
99 the transfers in the multi handles are done. 'done' does not mean
100 successful. One or more of the transfers may have failed. Tracking when this
101 number changes, you know when one or more transfers are done.
102
103 To get information about completed transfers, to figure out success or not and
104 similar, \fIcurl_multi_info_read(3)\fP should be called. It can return a
105 message about a current or previous transfer. Repeated invokes of the function
106 get more messages until the message queue is empty. The information you
107 receive there includes an easy handle pointer which you may use to identify
108 which easy handle the information regards.
109
110 When a single transfer is completed, the easy handle is still left added to
111 the multi stack. You need to first remove the easy handle with
112 \fIcurl_multi_remove_handle(3)\fP and then close it with
113 \fIcurl_easy_cleanup(3)\fP, or possibly set new options to it and add it again
114 with \fIcurl_multi_add_handle(3)\fP to start another transfer.
115
116 When all transfers in the multi stack are done, close the multi handle with
117 \fIcurl_multi_cleanup(3)\fP. Be careful and please note that you \fBMUST\fP
118 invoke separate \fIcurl_easy_cleanup(3)\fP calls for every single easy handle
119 to clean them up properly.
120
121 If you want to re-use an easy handle that was added to the multi handle for
122 transfer, you must first remove it from the multi stack and then re-add it
123 again (possibly after having altered some options at your own choice).
124 .SH "MULTI_SOCKET"
125 \fIcurl_multi_socket_action(3)\fP function offers a way for applications to
126 not only avoid being forced to use select(), but it also offers a much more
127 high-performance API that will make a significant difference for applications
128 using large numbers of simultaneous connections.
129
130 \fIcurl_multi_socket_action(3)\fP is then used instead of
131 \fIcurl_multi_perform(3)\fP.
132
133 When using this API, you add easy handles to the multi handle just as with the
134 normal multi interface. Then you also set two callbacks with the
135 \fICURLMOPT_SOCKETFUNCTION(3)\fP and \fICURLMOPT_TIMERFUNCTION(3)\fP options
136 to \fIcurl_multi_setopt(3)\fP. They are two callback functions that libcurl
137 will call with information about what sockets to wait for, and for what
138 activity, and what the current timeout time is - if that expires libcurl
139 should be notified.
140
141 The multi_socket API is designed to inform your application about which
142 sockets libcurl is currently using and for what activities (read and/or write)
143 on those sockets your application is expected to wait for.
144
145 Your application must make sure to receive all sockets informed about in the
146 \fICURLMOPT_SOCKETFUNCTION(3)\fP callback and make sure it reacts on the given
147 activity on them. When a socket has the given activity, you call
148 \fIcurl_multi_socket_action(3)\fP specifying which socket and action there
149 are.
150
151 The \fICURLMOPT_TIMERFUNCTION(3)\fP callback is called to set a timeout. When
152 that timeout expires, your application should call the
153 \fIcurl_multi_socket_action(3)\fP function saying it was due to a timeout.
154
155 This API is typically used with an event-driven underlying functionality (like
156 libevent, libev, kqueue, epoll or similar) with which the application
157 "subscribes" on socket changes. This allows applications and libcurl to much
158 better scale upward and beyond thousands of simultaneous transfers without
159 losing performance.
160
161 When you've added your initial set of handles, you call
162 \fIcurl_multi_socket_action(3)\fP with CURL_SOCKET_TIMEOUT set in the sockfd
163 argument, and you'll get callbacks call that sets you up and you then continue
164 to call \fIcurl_multi_socket_action(3)\fP accordingly when you get activity on
165 the sockets you've been asked to wait on, or if the timeout timer expires.
166
167 You can poll \fIcurl_multi_info_read(3)\fP to see if any transfer has
168 completed, as it then has a message saying so.
169 .SH "BLOCKING"
170 A few areas in the code are still using blocking code, even when used from the
171 multi interface. While we certainly want and intend for these to get fixed in
172 the future, you should be aware of the following current restrictions:
173
174 .nf
175 - Name resolves unless the c-ares or threaded-resolver backends are used
176 - SOCKS proxy handshakes
177 - file:// transfers
178 - TELNET transfers
179 .fi
180 .SH "SEE ALSO"
181 .BR libcurl-errors "(3), " libcurl-easy "(3), " libcurl "(3) "