.\" **************************************************************************.\" *                                  _   _ ____  _.\" *  Project                     ___| | | |  _ \| |.\" *                             / __| | | | |_) | |.\" *                            | (__| |_| |  _ <| |___.\" *                             \___|\___/|_| \_\_____|.\" *.\" * Copyright (C) 1998 - 2006, Daniel Stenberg, <daniel@haxx.se>, et al..\" *.\" * This software is licensed as described in the file COPYING, which.\" * you should have received as part of this distribution. The terms.\" * are also available at http://curl.haxx.se/docs/copyright.html..\" *.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell.\" * copies of the Software, and permit persons to whom the Software is.\" * furnished to do so, under the terms of the COPYING file..\" *.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY.\" * KIND, either express or implied..\" *.\" * $Id: libcurl-multi.3,v 1.13 2006-06-26 06:43:45 bagder Exp $.\" **************************************************************************.\".TH libcurl-multi 3 "13 Oct 2001" "libcurl 7.10.1" "libcurl multi interface".SH NAMElibcurl-multi \- how to use the multi interface.SH DESCRIPTIONThis is an overview on how to use the libcurl multi interface in your Cprograms. There are specific man pages for each function mentioned inhere. There's also the \fIlibcurl-tutorial(3)\fP man page for a completetutorial to programming with libcurl and the \fIlibcurl-easy(3)\fP man pagefor an overview of the libcurl easy interface.All functions in the multi interface are prefixed with curl_multi..SH "OBJECTIVES"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 whereand when to ask libcurl to get/send data.2. Enable multiple simultaneous transfers in the same thread without making itcomplicated for the application.3. Enable the application to wait for action on its own file descriptors andcurl's file descriptors simultaneous easily..SH "ONE MULTI HANDLE MANY EASY HANDLES"To use the multi interface, you must first create a 'multi handle' with\fIcurl_multi_init(3)\fP. This handle is then used as input to all furthercurl_multi_* functions.Each single transfer is built up with an easy handle. You must create them,and setup the appropriate options for each easy handle, as outlined in the\fIlibcurl(3)\fP man page, using \fIcurl_easy_setopt(3)\fP.When the easy handle is setup for a transfer, then instead of using\fIcurl_easy_perform(3)\fP (as when using the easy interface for transfers),you should instead add the easy handle to the multi handle using\fIcurl_multi_add_handle(3)\fP. The multi handle is sometimes referred to as a\'multi stack\' because of the fact that it may hold a large amount of easyhandles.Should you change your mind, the easy handle is again removed from the multistack using \fIcurl_multi_remove_handle(3)\fP. Once removed from the multihandle, you can again use other easy interface functions like\fIcurl_easy_perform(3)\fP on the handle or whatever you think is necessary.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 yourapplication drive. You drive the transfers by invoking\fIcurl_multi_perform(3)\fP. libcurl will then transfer data if there isanything available to transfer. It'll use the callbacks and everything elseyou have setup in the individual easy handles. It'll transfer data on allcurrent transfers in the multi stack that are ready to transfer anything. Itmay be all, it may be none.Your application can acquire knowledge from libcurl when it would like to getinvoked to transfer data, so that you don't have to busy-loop and call that\fIcurl_multi_perform(3)\fP like crazy. \fIcurl_multi_fdset(3)\fP offers aninterface using which you can extract fd_sets from libcurl to use in select()or poll() calls in order to get to know when the transfers in the multi stackmight need attention. This also makes it very easy for your program to waitfor input on your own private file descriptors at the same time or perhapstimeout every now and then, should you want that.A little note here about the return codes from the multi functions, andespecially the \fIcurl_multi_perform(3)\fP: if you receive\fICURLM_CALL_MULTI_PERFORM\fP, this basically means that you should call\fIcurl_multi_perform(3)\fP again, before you select() on more actions. Youdon't have to do it immediately, but the return code means that libcurl mayhave more data available to return or that there may be more data to send offbefore it is "satisfied".\fIcurl_multi_perform(3)\fP stores the number of still running transfers inone of its input arguments, and by reading that you can figure out when allthe transfers in the multi handles are done. 'done' does not meansuccessful. One or more of the transfers may have failed. Tracking when thisnumber changes, you know when one or more transfers are done.To get information about completed transfers, to figure out success or not andsimilar, \fIcurl_multi_info_read(3)\fP should be called. It can return amessage about a current or previous transfer. Repeated invokes of the functionget more messages until the message queue is empty. The information youreceive there includes an easy handle pointer which you may use to identifywhich easy handle the information regards.When all transfers in the multi stack are done, cleanup the multi handle with\fIcurl_multi_cleanup(3)\fP. Be careful and please note that you \fBMUST\fPinvoke separate \fIcurl_easy_cleanup(3)\fP calls on every single easy handleto clean them up properly.If you want to re-use an easy handle that was added to the multi handle fortransfer, you must first remove it from the multi stack and then re-add itagain (possibly after having altered some options at your own choice).