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-.TH libcurl-multi 3 "3 Feb 2007" "libcurl 7.16.0" "libcurl multi interface"
+.TH libcurl-multi 3 "19 Sep 2014" "libcurl" "libcurl multi interface"
.SH NAME
libcurl-multi \- how to use the multi interface
.SH DESCRIPTION
3. Enable the application to wait for action on its own file descriptors and
curl's file descriptors simultaneous easily.
+
+4. Enable event-based handling and scaling transfers up to and beyond
+thousands of parallel connections.
.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 further
curl_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.
+With a multi handle and the multi interface you can do several simultaneous
+transfers in parallel. Each single transfer is built up around an easy
+handle. You create all the easy handles you need, and setup the appropriate
+options for each easy handle using \fIcurl_easy_setopt(3)\fP.
+
+There are two flavours of the multi interface, the select() oriented one and
+the event based one we call multi_socket. You will benefit from reading
+through the description of both versions to fully understand how they work and
+differentiate. We start out with the select() oriented version.
-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 easy
-handles.
+When an easy handle is setup and ready for transfer, then instead of using
+\fIcurl_easy_perform(3)\fP like when using the easy interface for transfers,
+you should add the easy handle to the multi handle with
+\fIcurl_multi_add_handle(3)\fP. You can add more easy handles to a multi
+handle at any point, even if other transfers are already running.
Should you change your mind, the easy handle is again removed from the multi
stack using \fIcurl_multi_remove_handle(3)\fP. Once removed from the multi
handle, you can again use other easy interface functions like
-\fIcurl_easy_perform(3)\fP on the handle or whatever you think is necessary.
+\fIcurl_easy_perform(3)\fP on the handle or whatever you think is
+necessary. You can remove handles at any point in time during transfers.
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 your
anything available to transfer. It'll use the callbacks and everything else
you have setup in the individual easy handles. It'll transfer data on all
current transfers in the multi stack that are ready to transfer anything. It
-may be all, it may be none.
-
-Your application can acquire knowledge from libcurl when it would like to get
-invoked 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 an
-interface 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 stack
-might need attention. This also makes it very easy for your program to wait
-for input on your own private file descriptors at the same time or perhaps
-timeout every now and then, should you want that.
-
-A little note here about the return codes from the multi functions, and
-especially 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. You
-don't have to do it immediately, but the return code means that libcurl may
-have more data available to return or that there may be more data to send off
-before it is "satisfied".
+may be all, it may be none. When there's nothing more to do for now, it
+returns back to the calling application.
+
+Your application extracts info from libcurl about when it would like to get
+invoked to transfer data or do other work. The most convenient way is to use
+\fIcurl_multi_wait(3)\fP that will help you wait until the application should
+call libcurl again. The older API to accomplish the same thing is
+\fIcurl_multi_fdset(3)\fP that extracts fd_sets from libcurl to use in
+select() or poll() calls in order to get to know when the transfers in the
+multi stack might need attention. Both these APIs allow for your program to
+wait for input on your own private file descriptors at the same time
+\fIcurl_multi_timeout(3)\fP also helps you with providing a suitable timeout
+period for your select() calls.
\fIcurl_multi_perform(3)\fP stores the number of still running transfers in
one of its input arguments, and by reading that you can figure out when all
\fIcurl_easy_cleanup(3)\fP, or possibly set new options to it and add it again
with \fIcurl_multi_add_handle(3)\fP to start another transfer.
-When all transfers in the multi stack are done, cleanup the multi handle with
+When all transfers in the multi stack are done, close the multi handle with
\fIcurl_multi_cleanup(3)\fP. Be careful and please note that you \fBMUST\fP
-invoke separate \fIcurl_easy_cleanup(3)\fP calls on every single easy handle
+invoke separate \fIcurl_easy_cleanup(3)\fP calls for every single easy handle
to clean them up properly.
If you want to re-use an easy handle that was added to the multi handle for
transfer, you must first remove it from the multi stack and then re-add it
again (possibly after having altered some options at your own choice).
.SH "MULTI_SOCKET"
-Since 7.16.0, the \fIcurl_multi_socket_action(3)\fP function offers a way for
-applications to not only avoid being forced to use select(), but it also
-offers a much more high-performance API that will make a significant
-difference for applications using large numbers of simultaneous connections.
-
-\fIcurl_multi_socket_action(3)\fP is then used
-instead of \fIcurl_multi_perform(3)\fP.
+\fIcurl_multi_socket_action(3)\fP function offers a way for applications to
+not only avoid being forced to use select(), but it also offers a much more
+high-performance API that will make a significant difference for applications
+using large numbers of simultaneous connections.
+
+\fIcurl_multi_socket_action(3)\fP is then used instead of
+\fIcurl_multi_perform(3)\fP.
+
+When using this API, you add easy handles to the multi handle just as with the
+normal multi interface. Then you also set two callbacks with the
+CURLMOPT_SOCKETFUNCTION and CURLMOPT_TIMERFUNCTION options to
+\fIcurl_multi_setopt(3)\fP. They are two callback functions that libcurl will
+call with information about what sockets to wait for, and for what activity,
+and what the current timeout time is - if that expires libcurl should be
+notified.
+
+The multi_socket API is designed to inform your application about which
+sockets libcurl is currently using and for what activities (read and/or write)
+on those sockets your application is expected to wait for.
+
+Your application must make sure to receive all sockets informed about in the
+CURLMOPT_SOCKETFUNCTION callback and make sure it reacts on the given activity
+on them. When a socket has the given activity, you call
+\fIcurl_multi_socket_action(3)\fP specifying which socket and action there
+are.
+
+The CURLMOPT_TIMERFUNCTION callback is called to set a timeout. When that
+timeout expires, your application should call the
+\fIcurl_multi_socket_action(3)\fP function saying it was due to a timeout.
+
+This API is typically used with an event-driven underlying functionality (like
+libevent, libev, kqueue, epoll or similar) with which the application
+"subscribes" on socket changes. This allows applications and libcurl to much
+better scale upward and beyond thousands of simultaneous transfers without
+losing performance.
+
+When you've added your initial set of handles, you call
+\fIcurl_multi_socket_action(3)\fP with CURL_SOCKET_TIMEOUT set in the sockfd
+argument, and you'll get callbacks call that sets you up and you then continue
+to call \fIcurl_multi_socket_action(3)\fP accordingly when you get activity on
+the sockets you've been asked to wait on, or if the timeout timer expires.
+
+You can poll \fIcurl_multi_info_read(3)\fP to see if any transfer has
+completed, as it then has a message saying so.
.SH "BLOCKING"
A few areas in the code are still using blocking code, even when used from the
multi interface. While we certainly want and intend for these to get fixed in
the future, you should be aware of the following current restrictions:
.nf
- - Name resolves on non-windows unless c-ares is used
- - GnuTLS SSL connections
+ - Name resolves unless the c-ares or threaded-resolver backends are used
- NSS SSL connections
- - Active FTP connections
- HTTP proxy CONNECT operations
- SOCKS proxy handshakes
- file:// transfers
- TELNET transfers
.fi
+.SH "SEE ALSO"
+.BR libcurl-errors "(3), " libcurl-easy "(3), " libcurl "(3) "
projects / platform / upstream / curl.git / blobdiff