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libcurl docs: improvements all over

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Daniel Stenberg
2014-09-19 15:07:58 +02:00
parent 7b85b332cb
commit 3ef73d9a88
5 changed files with 116 additions and 60 deletions
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docs/libcurl/libcurl-multi.3
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@@ -5,7 +5,7 @@
.\" * | (__| |_| | _ <| |___
.\" * \___|\___/|_| \_\_____|
.\" *
.\" * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
.\" * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
.\" *
.\" * This software is licensed as described in the file COPYING, which
.\" * you should have received as part of this distribution. The terms
@@ -20,7 +20,7 @@
.\" *
.\" **************************************************************************
.\"
.TH libcurl-multi 3 "3 Feb 2007" "libcurl 7.16.0" "libcurl multi interface"
.TH libcurl-multi 3 "19 Sep 2014" "libcurl" "libcurl multi interface"
.SH NAME
libcurl-multi \- how to use the multi interface
.SH DESCRIPTION
@@ -43,18 +43,28 @@ complicated for the application.
3. Enable the application to wait for action on its own file descriptors and
curl's file descriptors simultaneous easily.
4. Enable event-based handling and scaling transfers up to and beyond
thousands of parallel connections.
.SH "ONE MULTI HANDLE MANY EASY HANDLES"
To use the multi interface, you must first create a 'multi handle' with
\fIcurl_multi_init(3)\fP. 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
\fIlibcurl(3)\fP man page, using \fIcurl_easy_setopt(3)\fP.
With a multi handle and the multi interface you can do any amount of
simultaneous transfers in parallel. Each single transfer is built up around an
easy handle. You must create the easy handles you need, and setup the
appropriate options for each easy handle, as outlined in the \fIlibcurl(3)\fP
man page, using \fIcurl_easy_setopt(3)\fP.
When the easy handle is setup for a transfer, then instead of using
\fIcurl_easy_perform(3)\fP (as when using the easy interface for transfers),
you should instead add the easy handle to the multi handle using
There are two flavours of the multi interface, the select() oriented one and
the event based one we called multi_socket. You will benefit from reading
through the description of both versions to full understand how they work and
differentiate. We start out with the select() oriented version.
When an easy handle is setup for a transfer, then instead of using
\fIcurl_easy_perform(3)\fP like when using the easy interface for transfers,
you should add the easy handle to the multi handle with
\fIcurl_multi_add_handle(3)\fP. 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.
@@ -71,7 +81,8 @@ application drive. You drive the transfers by invoking
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.
may be all, it may be none. When there's nothing more to do for now, it
returns back to the calling application.
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
@@ -80,7 +91,9 @@ 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.
timeout every now and then, should you want that. \fIcurl_multi_timeout(3)\fP
also helps you with providing a suitable timeout period for your select()
call.
\fIcurl_multi_perform(3)\fP stores the number of still running transfers in
one of its input arguments, and by reading that you can figure out when all
@@ -121,21 +134,39 @@ using large numbers of simultaneous connections.
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 and CURLMOPT_TIMERFUNCTION options to
\fIcurl_multi_setopt(3)\fP.
\fIcurl_multi_setopt(3)\fP. They are two callback functions that libcurl will
call with information about what sockets to wait for, and for what activity,
and what the curret timeout time is - if that expires libcurl should be
notified.
The API is then 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.
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 then make sure to receive all sockets informed about in
the CURLMOPT_SOCKETFUNCTION callback and make sure it reacts on the given
activity on them. When a socket has the given activity, you call
Your application must make sure to receive all sockets informed about in the
CURLMOPT_SOCKETFUNCTION callback and make sure it reacts on the given activity
on them. When a socket has the given activity, you call
\fIcurl_multi_socket_action(3)\fP specifying which socket and action there
are.
The CURLMOPT_TIMERFUNCTION callback is called to set a timeout. When that
timeout expires, your application should call the
\fIcurl_multi_socket_action(3)\fP 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) which 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've added your initial set of handles, you call
\fIcurl_multi_socket_action(3)\fP with CURL_SOCKET_TIMEOUT set in the sockfd
argument, and you'll get callbacks call that sets you up and you then continue
to call \fIcurl_multi_socket_action(3)\fP accordingly when you get activity on
the sockets you've been asked to wait on, or if the timeout timer expires.
You can poll \fIcurl_multi_info_read(3)\fP to see if any transfer has
completed, as it then has a message saying so.
.SH "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