xref: /aosp_15_r20/external/curl/docs/libcurl/libcurl-multi.md (revision 6236dae45794135f37c4eb022389c904c8b0090d)

---
c: Copyright (C) Daniel Stenberg, <[email protected]>, et al.
SPDX-License-Identifier: curl
Title: libcurl-multi
Section: 3
Source: libcurl
See-also:
  - libcurl (3)
  - libcurl-easy (3)
  - libcurl-errors (3)
Protocol:
  - All
Added-in: 7.9.6
---

# 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 is 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 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 simultaneously.

4. Enable event-based handling and scaling transfers up to and beyond
thousands of parallel connections.

# 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.

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 curl_easy_setopt(3).

There are two flavors of the multi interface, the select() oriented one and
the event based one we call multi_socket. You 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 an easy handle is setup and ready for transfer, then instead of using
curl_easy_perform(3) like when using the easy interface for transfers,
you should add the easy handle to the multi handle with
curl_multi_add_handle(3). 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 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. You can remove handles at any point 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
application drive. You drive the transfers by invoking
curl_multi_perform(3). libcurl then transfers data if there is anything
available to transfer. It uses the callbacks and everything else you have
setup in the individual easy handles. It transfers data on all current
transfers in the multi stack that are ready to transfer anything. It may be
all, it may be none. When there is 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
curl_multi_poll(3) that helps you wait until the application should call
libcurl again. The older API to accomplish the same thing is
curl_multi_fdset(3) 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.
curl_multi_timeout(3) also helps you with providing a suitable timeout
period for your select() calls.

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.

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, close the multi handle with
curl_multi_cleanup(3). Be careful and please note that you **MUST**
invoke separate curl_easy_cleanup(3) calls for 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, 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

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 makes 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).

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(3) and CURLMOPT_TIMERFUNCTION(3) options
to curl_multi_setopt(3). They are two callback functions that libcurl
calls 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(3) callback and make sure it reacts on the given
activity on them. When a socket has the given activity, you call
curl_multi_socket_action(3) specifying which socket and action there
are.

The CURLMOPT_TIMERFUNCTION(3) callback is called to set a timeout. When
that timeout expires, your application should call the
curl_multi_socket_action(3) 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 have added your initial set of handles, you call
curl_multi_socket_action(3) with CURL_SOCKET_TIMEOUT set in the
*sockfd* argument, and you get callbacks invoked that set you up and you
then continue to call curl_multi_socket_action(3) accordingly when you
get activity on the sockets you have been asked to wait on, or if the timeout
timer expires.

You can poll curl_multi_info_read(3) to see if any transfer has
completed, as it then has a message saying so.

# 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:

~~~c
 - Name resolves unless the c-ares or threaded-resolver backends are used
 - file:// transfers
 - TELNET transfers
~~~