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96 lines
4.9 KiB
Groff
96 lines
4.9 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 3 "13 Oct 2001" "libcurl 7.10.1" "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 \fIlibcurl-tutorial(3)\fP man page for a complete
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tutorial to programming with libcurl and the \fIlibcurl-easy(3)\fP man page
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for an overview of the libcurl easy interface.
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All functions in the multi interface are prefixed with curl_multi.
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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(3)\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, using \fIcurl_easy_setopt(3)\fP.
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When the easy handle is setup for a transfer, then instead of using
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\fIcurl_easy_perform(3)\fP (as when using the easy interface for transfers),
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you should instead add the easy handle to the multi handle using
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\fIcurl_multi_add_handle(3)\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(3)\fP. Once removed from the multi
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handle, you can again use other easy interface functions like
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\fIcurl_easy_perform(3)\fP on the handle or whatever you think is necessary.
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Adding the easy handle to the multi handle does not start the transfer.
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Remember that one of the main ideas with this interface is to let your
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application drive. You drive the transfers by invoking
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\fIcurl_multi_perform(3)\fP. libcurl will then transfer data if there is
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anything available to transfer. It'll use the callbacks and everything else
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you have setup in the individual easy handles. It'll transfer data on all
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current transfers in the multi stack that are ready to transfer anything. It
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may be 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(3)\fP like crazy. \fIcurl_multi_fdset(3)\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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A little note here about the return codes from the multi functions, and
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especially the \fIcurl_multi_perform(3)\fP: if you receive
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\fICURLM_CALL_MULTI_PERFORM\fP, this basically means that you should call
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\fIcurl_multi_perform(3)\fP again, before you select() on more actions. You
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don't have to do it immediately, but the return code means that libcurl may
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have more data available to return or that there may be more data to send off
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before it is "satisfied".
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\fIcurl_multi_perform(3)\fP stores the number of still running transfers in
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one of its input arguments, and by reading that you can figure out when all
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the transfers in the multi handles are done. 'done' does not mean
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successful. One or more of the transfers may have failed. Tracking when this
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number changes, you know when one or more transfers are done.
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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(3)\fP should be called. It can return a
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message about a current or previous transfer. Repeated invokes of the function
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get more messages until the message queue is empty. The information you
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receive there includes an easy handle pointer which you may use to identify
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which easy handle the information regards.
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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(3)\fP. Be careful and please note that you \fBMUST\fP
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invoke separate \fIcurl_easy_cleanup(3)\fP calls on every single easy handle
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to clean them up properly.
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If you want to re-use an easy handle that was added to the multi handle for
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transfer, you must first remove it from the multi stack and then re-add it
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again (possibly after having altered some options at your own choice).
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