libcurl Multi Interface

Overview

This topic describes how to use the libcurl multi interface in your C programs. There are specific MAN pages for each function mentioned. There is also the libcurl-tutorial MAN page for a complete tutorial to programming with libcurl and the libcurl-easy 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 does not. 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, 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. Create them, and setup the appropriate options for each easy handle, as outlined in the libcurl MAN page, using curl_easy_setopt.

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_multi_add_handle. The multi handle is occasionally referred to as a ‘multi stack’ because of the fact that it may hold a large amount of easy handles.

Should you reassess, the easy handle is again removed from the multi stack using curl_multi_remove_handle. After removal from the multi handle, you can again use other easy interface functions like curl_easy_perform on the handle or whatever you deem 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. libcurl will then transfer data if there is anything available to transfer. It will use the callbacks and everything else you have setup in the individual easy handles. It will 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 do not have to busy-loop and call that curl_multi_perform repeatedly. curl_multi_fdset offers an interface using which you can extract fd_sets from libcurl to use in select or poll calls to discover when the transfers in the multi stack might need attention. This also makes it easy for your program to wait for input on your own private file descriptors at the same time or perhaps timeout occasionally, should you want that.

A note about the return codes from the multi functions, and especially the curl_multi_perform: if you receive CURLM_CALL_MULTI_PERFORM, this indicates that you should call curl_multi_perform again, before you select on more actions. You do not have to do it immediately, but the return code indicates that libcurl may have more data available to return or that there may be more data to send off before it is "satisfied".

The function, curl_multi_perform stores the number of still running transfers in one of its input arguments, and by reading that you can determine when all the transfers in the multi handles are done. 'done' does not indicate successful. One or more of the transfers may have failed. Tracking when this number changes, indicates when one or more transfers are done.

To get information about completed transfers, to determine success 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. 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 and then close it with curl_easy_cleanup, or possibly set new options to it and add it again with curl_multi_add_handle to start another transfer.

When all transfers in the multi stack are done, cleanup the multi handle with curl_multi_cleanup. Be careful and note that you must invoke separate curl_easy_cleanup calls on every single easy handle to clean them up properly.

If you want to reuse an easy handle that was added to the multi handle for transfer, 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

NOTE:
Though multi_socket APIs are made available in Cafe libcurl, they are not tested and are not officially supported.

The curl_multi_socket_action 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 then could be used instead of curl_multi_perform.

Blocking

A few areas in the code still use blocking code, even when used from the multi interface. While we want and intend for these to get fixed in the future, you should be aware of the following current restrictions:

See Also

HTTP Client Library (libcurl)

Revision History

2013/09/12 Converted.


CONFIDENTIAL