NAME
libcurl-multi - how to use the multi interface
DESCRIPTION
This is an overview on how to use the libcurl multi interface in your C
programs. There are specific man pages for each function mentioned in
here. There’s also the libcurl-tutorial(3) man page for a complete
tutorial to programming with libcurl and the libcurl-easy(3) man page
for an overview of the libcurl easy interface.
All functions in the multi interface are prefixed with curl_multi.
OBJECTIVES
The multi interface offers several abilities that the easy interface
doesn’t. 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 wait for action 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(3). 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, using curl_easy_setopt(3).
When the easy handle is setup for a transfer, then instead of using
curl_easy_perform(3) (as when using the easy interface for transfers),
you should instead add the easy handle to the multi handle using
curl_multi_add_handle(3). 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(3). Once removed from the
multi handle, you can again use other easy interface functions like
curl_easy_perform(3) on the handle or whatever you think is necessary.
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
application drive. You drive the transfers by invoking
curl_multi_perform(3). 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(3) like crazy. curl_multi_fdset(3)
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 curl_multi_perform(3): if you receive
CURLM_CALL_MULTI_PERFORM, this basically means that you should call
curl_multi_perform(3) 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".
curl_multi_perform(3) stores the number of still running transfers 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. Tracking when
this number changes, you know when one or more transfers are done.
To get information about completed transfers, to figure out success or
not and similar, curl_multi_info_read(3) 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. The
information you receive there includes an easy handle pointer which you
may use to identify which easy handle the information regards.
When a single transfer is completed, the easy handle is still left
added to the multi stack. You need to first remove the easy handle with
curl_multi_remove_handle(3) and then close it with
curl_easy_cleanup(3), or possibly set new options to it and add it
again with curl_multi_add_handle(3) to start another transfer.
When all transfers in the multi stack are done, cleanup the multi
handle with curl_multi_cleanup(3). Be careful and please note that you
MUST invoke separate curl_easy_cleanup(3) calls on 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).
MULTI_SOCKET
Since 7.16.0, the curl_multi_socket_action(3) 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.
curl_multi_socket_action(3) is then used instead of
curl_multi_perform(3).
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:
- Name resolves on non-windows unless c-ares is used
- GnuTLS SSL connections
- NSS SSL connections
- Active FTP connections
- HTTP proxy CONNECT operations
- SOCKS proxy handshakes
- file:// transfers
- TELNET transfers