NAME
libcurl-multi - how to use the multi interface
DESCRIPTION
This is an overview on how to use the libcurl multi inter
face in your C programs. There are specific man pages for
each function mentioned in here. There's also the libcurl-
the-guide document for a complete tutorial to programming
with libcurl and the libcurl(3) man page for an overview of
the libcurl easy interface.
All functions in the multi interface are prefixed with
curl_multi.
PLEASE NOTICE
The multi interface is a rather new member of the libcurl
family. It has not yet been very widely used. It may still
be a few more bugs lurking in there than we are used to.
That said, it might also just work in every aspect you try
it. Please report all bugs and oddities you see.
OBJECTIVES
The multi interface introduces several new abilities that
the easy interface refuses to offer. They are mainly:
1. Enable a "pull" interface. The application that uses
libcurl decides where and when to ask libcurl to get/send
data.
2. Enable multiple simultaneous transfers in the same thread
without making it complicated for the application.
3. Enable the application to select() on its own file
descriptors and curl's file descriptors simultaneous easily.
ONE MULTI HANDLE MANY EASY HANDLES
To use the multi interface, you must first create a 'multi
handle' with curl_multi_init. 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 libcurl(3) man page.
When the easy handle is setup for a transfer, then instead
of using curl_easy_perform (as when using the easy interface
for transfers), you should instead add the easy handle to
the multi handle using curl_easy_add_handl. 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.
Should you change your mind, the easy handle is again
removed from the multi stack using curl_multi_remove_handle.
Once removed from the multi handle, you can again use other
easy interface functions like curl_easy_perform or whatever
you think is necessary.
Adding the easy handles to the multi handle does not start
any transfer. Remember that one of the main ideas with this
interface is to let your application drive. You drive the
transfers by invoking curl_multi_perform. libcurl will then
transfer data if there is 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 curl_multi_perform
like a mad man! curl_multi_fdset 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 per
haps timeout every now and then, should you want that.
curl_multi_perform stores the number of still running trans
fers in one of its input arguments, and by reading that you
can figure out when all the transfers in the multi handles
are done. 'done' does not mean successful. One or more of
the transfers may have failed.
To get information about completed transfers, to figure out
success or not and similar, curl_multi_info_read should be
called. It can return a message about a current or previous
transfer. Repeated invokes of the function get more messages
until the message queue is empty.
When all transfers in the multi stack are done, cleanup the
multi handle with curl_multi_cleanup. Be careful and please
note that you MUST invoke separate curl_easy_cleanup calls
on every single easy handle to clean them up properly.
Man(1) output converted with
man2html