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177 lines
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HTML
177 lines
6.6 KiB
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<HTML>
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<BODY>
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<PRE>
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<!-- Manpage converted by man2html 3.0.1 -->
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libcurl-multi - how to use the multi interface
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</PRE>
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<H2>DESCRIPTION</H2><PRE>
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This is an overview on how to use the libcurl multi inter-
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face in your C programs. There are specific man pages for
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each function mentioned in here. There's also the libcurl-
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the-guide document for a complete tutorial to programming
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with libcurl and the <I>libcurl(3)</I> man page for an overview
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of the libcurl easy interface.
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All functions in the multi interface are prefixed with
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curl_multi.
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</PRE>
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<H2>PLEASE NOTICE</H2><PRE>
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The multi interface is a rather new member of the libcurl
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family. It has not yet been very widely used. It may still
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be a few more bugs lurking in there than we are used to.
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That said, it might also just work in every aspect you try
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it. Please report all bugs and oddities you see.
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</PRE>
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<H2>OBJECTIVES</H2><PRE>
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The multi interface introduces several new abilities that
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the easy interface refuses to offer. They are mainly:
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1. Enable a "pull" interface. The application that uses
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libcurl decides where and when to ask libcurl to get/send
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data.
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2. Enable multiple simultaneous transfers in the same
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thread without making it complicated for the application.
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3. Enable the application to select() on its own file
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descriptors and curl's file descriptors simultaneous eas-
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ily.
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</PRE>
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<H2>ONE MULTI HANDLE MANY EASY HANDLES</H2><PRE>
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To use the multi interface, you must first create a 'multi
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handle' with <I>curl</I><B>_</B><I>multi</I><B>_</B><I>init</I>. This handle is then used as
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input to all further curl_multi_* functions.
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Each single transfer is built up with an easy handle. You
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must create them, and setup the appropriate options for
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each easy handle, as outlined in the <I>libcurl(3)</I> man page,
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using <I>curl</I><B>_</B><I>easy</I><B>_</B><I>setopt(3)</I>.
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When the easy handle is setup for a transfer, then instead
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of using <I>curl</I><B>_</B><I>easy</I><B>_</B><I>perform</I> (as when using the easy inter-
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face for transfers), you should instead add the easy han-
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dle to the multi handle using <I>curl</I><B>_</B><I>easy</I><B>_</B><I>add</I><B>_</B><I>handl</I>. The
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multi handle is sometimes referred to as a 'multi stack'
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because of the fact that it may hold a large amount of
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easy handles.
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removed from the multi stack using <I>curl</I><B>_</B><I>multi</I><B>_</B><I>remove</I><B>_</B><I>han-</I>
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<I>dle</I>. Once removed from the multi handle, you can again use
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other easy interface functions like <I>curl</I><B>_</B><I>easy</I><B>_</B><I>perform</I> on
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the handle or whatever you think is necessary.
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Adding the easy handle to the multi handle does not start
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the transfer. Remember that one of the main ideas with
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this interface is to let your application drive. You drive
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the transfers by invoking <I>curl</I><B>_</B><I>multi</I><B>_</B><I>perform</I>. libcurl will
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then transfer data if there is anything available to
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transfer. It'll use the callbacks and everything else you
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have setup in the individual easy handles. It'll transfer
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data on all current transfers in the multi stack that are
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ready to transfer anything. It may be all, it may be none.
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Your application can acquire knowledge from libcurl when
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it would like to get invoked to transfer data, so that you
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don't have to busy-loop and call that <I>curl</I><B>_</B><I>multi</I><B>_</B><I>perform</I>
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like crazy. <I>curl</I><B>_</B><I>multi</I><B>_</B><I>fdset</I> offers an interface using
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which you can extract fd_sets from libcurl to use in
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select() or poll() calls in order to get to know when the
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transfers in the multi stack might need attention. This
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also makes it very easy for your program to wait for input
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on your own private file descriptors at the same time or
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perhaps timeout every now and then, should you want that.
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A little note here about the return codes from the multi
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functions, and especially the <I>curl</I><B>_</B><I>multi</I><B>_</B><I>perform</I>: if you
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receive <I>CURLM</I><B>_</B><I>CALL</I><B>_</B><I>MULTI</I><B>_</B><I>PERFORM</I>, this basicly means that
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you should call <I>curl</I><B>_</B><I>multi</I><B>_</B><I>perform</I> again, before you
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select() on more actions. You don't have to do it immedi-
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ately, but the return code means that libcurl may have
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more data available to return or that there may be more
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data to send off before it is "satisfied".
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<I>curl</I><B>_</B><I>multi</I><B>_</B><I>perform</I> stores the number of still running
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transfers in one of its input arguments, and by reading
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that you can figure out when all the transfers in the
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multi handles are done. 'done' does not mean successful.
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One or more of the transfers may have failed. Tracking
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when this number changes, you know when one or more trans-
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fers are done.
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To get information about completed transfers, to figure
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out success or not and similar, <I>curl</I><B>_</B><I>multi</I><B>_</B><I>info</I><B>_</B><I>read</I>
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should be called. It can return a message about a current
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or previous transfer. Repeated invokes of the function get
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more messages until the message queue is empty. The infor-
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mation you receive there includes an easy handle pointer
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which you may use to identify which easy handle the infor-
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mation regards.
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please note that you <B>MUST</B> invoke separate
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<I>curl</I><B>_</B><I>easy</I><B>_</B><I>cleanup</I> calls on every single easy handle to
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clean them up properly.
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If you want to re-use an easy handle that was added to the
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multi handle for transfer, you must first remove it from
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the multi stack and then re-add it again (possbily after
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having altered some options at your own choice).
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</PRE>
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<HR>
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<ADDRESS>
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|
|
Man(1) output converted with
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|
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<a href="http://www.oac.uci.edu/indiv/ehood/man2html.html">man2html</a>
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</ADDRESS>
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</BODY>
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</HTML>
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