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86 lines
4.2 KiB
Groff
86 lines
4.2 KiB
Groff
.\" You can view this file with:
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.\" nroff -man [file]
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.\" $Id$
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.\"
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.TH libcurl-multi 5 "20 March 2001" "libcurl 7.9.5" "libcurl multi interface"
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.SH NAME
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libcurl-multi \- how to use the multi interface
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.SH DESCRIPTION
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This is an overview on how to use the libcurl multi interface in your C
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programs. There are specific man pages for each function mentioned in
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here. There's also the libcurl-the-guide document for a complete tutorial to
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programming with libcurl and the \fIlibcurl(3)\fP man page for an overview of
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the libcurl easy interface.
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All functions in the multi interface are prefixed with curl_multi.
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.SH "PLEASE NOTICE"
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The multi interface is a rather new member of the libcurl family. It has not
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yet been very widely used. It may still be a few more bugs lurking in there
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than we are used to. That said, it might also just work in every aspect you
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try it. Please report all bugs and oddities you see.
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.SH "OBJECTIVES"
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The multi interface introduces several new abilities that the easy interface
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refuses to offer. They are mainly:
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1. Enable a "pull" interface. The application that uses libcurl decides where
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and when to ask libcurl to get/send data.
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2. Enable multiple simultaneous transfers in the same thread without making it
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complicated for the application.
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3. Enable the application to select() on its own file descriptors and curl's
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file descriptors simultaneous easily.
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.SH "ONE MULTI HANDLE MANY EASY HANDLES"
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To use the multi interface, you must first create a 'multi handle' with
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\fIcurl_multi_init\fP. This handle is then used as input to all further
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curl_multi_* functions.
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Each single transfer is built up with an easy handle. You must create them,
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and setup the appropriate options for each easy handle, as outlined in the
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\fIlibcurl(3)\fP man page.
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When the easy handle is setup for a transfer, then instead of using
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\fIcurl_easy_perform\fP (as when using the easy interface for transfers), you
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should instead add the easy handle to the multi handle using
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\fIcurl_easy_add_handl\fP. The multi handle is sometimes referred to as a
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\'multi stack\' because of the fact that it may hold a large amount of easy
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handles.
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Should you change your mind, the easy handle is again removed from the multi
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stack using \fIcurl_multi_remove_handle\fP. Once removed from the multi
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handle, you can again use other easy interface functions like
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curl_easy_perform or whatever you think is necessary.
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Adding the easy handles to the multi handle does not start any
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transfer. Remember that one of the main ideas with this interface is to let
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your application drive. You drive the transfers by invoking
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\fIcurl_multi_perform\fP. libcurl will then transfer data if there is anything
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available to transfer. It'll use the callbacks and everything else you have
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setup in the individual easy handles. It'll transfer data on all current
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transfers in the multi stack that are ready to transfer anything. It may be
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all, it may be none.
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Your application can acquire knowledge from libcurl when it would like to get
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invoked to transfer data, so that you don't have to busy-loop and call that
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\fIcurl_multi_perform\fP like a mad man! \fIcurl_multi_fdset\fP offers an
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interface using which you can extract fd_sets from libcurl to use in select()
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or poll() calls in order to get to know when the transfers in the multi stack
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might need attention. This also makes it very easy for your program to wait
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for input on your own private file descriptors at the same time or perhaps
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timeout every now and then, should you want that.
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\fIcurl_multi_perform\fP stores the number of still running transfers in one
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of its input arguments, and by reading that you can figure out when all the
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transfers in the multi handles are done. 'done' does not mean successful. One
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or more of the transfers may have failed.
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To get information about completed transfers, to figure out success or not and
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similar, \fIcurl_multi_info_read\fP should be called. It can return a message
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about a current or previous transfer. Repeated invokes of the function get
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more messages until the message queue is empty.
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When all transfers in the multi stack are done, cleanup the multi handle with
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\fIcurl_multi_cleanup\fP. Be careful and please note that you \fBMUST\fP
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invoke separate \fIcurl_easy_cleanup\fP calls on every single easy handle to
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clean them up properly.
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