libcurl-multi.md (8140B)
1 --- 2 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3 SPDX-License-Identifier: curl 4 Title: libcurl-multi 5 Section: 3 6 Source: libcurl 7 See-also: 8 - libcurl (3) 9 - libcurl-easy (3) 10 - libcurl-errors (3) 11 Protocol: 12 - All 13 Added-in: 7.9.6 14 --- 15 16 # NAME 17 18 libcurl-multi - how to use the multi interface 19 20 # DESCRIPTION 21 22 This is an overview on how to use the libcurl multi interface in your C 23 programs. There are specific man pages for each function mentioned in 24 here. There is also the libcurl-tutorial(3) man page for a complete 25 tutorial to programming with libcurl and the libcurl-easy(3) man page 26 for an overview of the libcurl easy interface. 27 28 All functions in the multi interface are prefixed with curl_multi. 29 30 # OBJECTIVES 31 32 The multi interface offers several abilities that the easy interface does not. 33 They are mainly: 34 35 1. Enable a "pull" interface. The application that uses libcurl decides where 36 and when to ask libcurl to get/send data. 37 38 2. Enable multiple simultaneous transfers in the same thread without making it 39 complicated for the application. 40 41 3. Enable the application to wait for action on its own file descriptors and 42 curl's file descriptors simultaneously. 43 44 4. Enable event-based handling and scaling transfers up to and beyond 45 thousands of parallel connections. 46 47 # ONE MULTI HANDLE MANY EASY HANDLES 48 49 To use the multi interface, you must first create a 'multi handle' with 50 curl_multi_init(3). This handle is then used as input to all further 51 curl_multi_* functions. 52 53 With a multi handle and the multi interface you can do several simultaneous 54 transfers in parallel. Each single transfer is built up around an easy 55 handle. You create all the easy handles you need, and setup the appropriate 56 options for each easy handle using curl_easy_setopt(3). 57 58 There are two flavors of the multi interface, the select() oriented one and 59 the event based one we call multi_socket. You benefit from reading through the 60 description of both versions to fully understand how they work and 61 differentiate. We start out with the select() oriented version. 62 63 When an easy handle is setup and ready for transfer, then instead of using 64 curl_easy_perform(3) like when using the easy interface for transfers, 65 you should add the easy handle to the multi handle with 66 curl_multi_add_handle(3). You can add more easy handles to a multi 67 handle at any point, even if other transfers are already running. 68 69 Should you change your mind, the easy handle is again removed from the multi 70 stack using curl_multi_remove_handle(3). Once removed from the multi 71 handle, you can again use other easy interface functions like 72 curl_easy_perform(3) on the handle or whatever you think is 73 necessary. You can remove handles at any point during transfers. 74 75 Adding the easy handle to the multi handle does not start the transfer. 76 Remember that one of the main ideas with this interface is to let your 77 application drive. You drive the transfers by invoking 78 curl_multi_perform(3). libcurl then transfers data if there is anything 79 available to transfer. It uses the callbacks and everything else you have 80 setup in the individual easy handles. It transfers data on all current 81 transfers in the multi stack that are ready to transfer anything. It may be 82 all, it may be none. When there is nothing more to do for now, it returns back 83 to the calling application. 84 85 Your application extracts info from libcurl about when it would like to get 86 invoked to transfer data or do other work. The most convenient way is to use 87 curl_multi_poll(3) that helps you wait until the application should call 88 libcurl again. The older API to accomplish the same thing is 89 curl_multi_fdset(3) that extracts *fd_sets* from libcurl to use in 90 select() or poll() calls in order to get to know when the transfers in the 91 multi stack might need attention. Both these APIs allow for your program to 92 wait for input on your own private file descriptors at the same time. 93 curl_multi_timeout(3) also helps you with providing a suitable timeout 94 period for your select() calls. 95 96 curl_multi_perform(3) stores the number of still running transfers in 97 one of its input arguments, and by reading that you can figure out when all 98 the transfers in the multi handles are done. 'done' does not mean 99 successful. One or more of the transfers may have failed. 100 101 To get information about completed transfers, to figure out success or not and 102 similar, curl_multi_info_read(3) should be called. It can return a 103 message about a current or previous transfer. Repeated invokes of the function 104 get more messages until the message queue is empty. The information you 105 receive there includes an easy handle pointer which you may use to identify 106 which easy handle the information regards. 107 108 When a single transfer is completed, the easy handle is still left added to 109 the multi stack. You need to first remove the easy handle with 110 curl_multi_remove_handle(3) and then close it with 111 curl_easy_cleanup(3), or possibly set new options to it and add it again 112 with curl_multi_add_handle(3) to start another transfer. 113 114 When all transfers in the multi stack are done, close the multi handle with 115 curl_multi_cleanup(3). Be careful and please note that you **MUST** 116 invoke separate curl_easy_cleanup(3) calls for every single easy handle 117 to clean them up properly. 118 119 If you want to reuse an easy handle that was added to the multi handle for 120 transfer, you must first remove it from the multi stack and then re-add it 121 again (possibly after having altered some options at your own choice). 122 123 # MULTI_SOCKET 124 125 curl_multi_socket_action(3) 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 makes a significant difference for applications 128 using large numbers of simultaneous connections. 129 130 curl_multi_socket_action(3) is then used instead of 131 curl_multi_perform(3). 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 CURLMOPT_SOCKETFUNCTION(3) and CURLMOPT_TIMERFUNCTION(3) options 136 to curl_multi_setopt(3). They are two callback functions that libcurl 137 calls with information about what sockets to wait for, and for what activity, 138 and what the current timeout time is - if that expires libcurl should be 139 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 CURLMOPT_SOCKETFUNCTION(3) callback and make sure it reacts on the given 147 activity on them. When a socket has the given activity, you call 148 curl_multi_socket_action(3) specifying which socket and action there 149 are. 150 151 The CURLMOPT_TIMERFUNCTION(3) callback is called to set a timeout. When 152 that timeout expires, your application should call the 153 curl_multi_socket_action(3) 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 have added your initial set of handles, you call 162 curl_multi_socket_action(3) with CURL_SOCKET_TIMEOUT set in the 163 *sockfd* argument, and you get callbacks invoked that set you up and you 164 then continue to call curl_multi_socket_action(3) accordingly when you 165 get activity on the sockets you have been asked to wait on, or if the timeout 166 timer expires. 167 168 You can poll curl_multi_info_read(3) to see if any transfer has 169 completed, as it then has a message saying so. 170 171 # BLOCKING 172 173 A few areas in the code are still using blocking code, even when used from the 174 multi interface. While we certainly want and intend for these to get fixed in 175 the future, you should be aware of the following current restrictions: 176 177 ~~~c 178 - Name resolves unless the c-ares or threaded-resolver backends are used 179 - file:// transfers 180 - TELNET transfers 181 ~~~