libcurl-multi - how to use the multi interface
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.
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.
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 eas- ily.
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, using curl_easy_setopt(3). When the easy handle is setup for a transfer, then instead of using curl_easy_perform (as when using the easy inter- face for transfers), you should instead add the easy han- dle 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. removed from the multi stack using curl_multi_remove_han- dle. Once removed from the multi handle, you can again use other easy interface functions like curl_easy_perform 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. 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 crazy. 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 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: if you receive CURLM_CALL_MULTI_PERFORM, this basicly means that you should call curl_multi_perform again, before you select() on more actions. You don't have to do it immedi- ately, 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 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 trans- fers are done. 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. The infor- mation you receive there includes an easy handle pointer which you may use to identify which easy handle the infor- mation regards. please note that you MUST invoke separate curl_easy_cleanup 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 (possbily after having altered some options at your own choice).
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