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Ref6_bufferevent.html
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Ref6_bufferevent.html
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<body class="article">
<div id="header">
</div>
<div id="content">
<div class="quoteblock">
<div class="content">
<div class="paragraph"><p>These documents are Copyright (c) 2009-2012 by Nick Mathewson, and are made
available under the Creative Commons Attribution-Noncommercial-Share Alike
license, version 3.0. Future versions may be made available under a less
restrictive license.</p></div>
<div class="paragraph"><p>Additionally, the source code examples in these documents are also licensed
under the so-called "3-Clause" or "Modified" BSD license. See
<a href="license_bsd.html">the license_bsd file</a> distributed with these documents
for the full terms.</p></div>
<div class="paragraph"><p>For the latest version of this document, see
<a href="http://www.wangafu.net/~nickm/libevent-book/TOC.html">http://www.wangafu.net/~nickm/libevent-book/TOC.html</a></p></div>
<div class="paragraph"><p>To get the source for the latest version of this document, install git
and run "git clone git://github.com/nmathewson/libevent-book.git"</p></div>
</div>
<div class="attribution">
</div></div>
<div class="sect1">
<h2 id="_bufferevents_concepts_and_basics">Bufferevents: concepts and basics</h2>
<div class="sectionbody">
<div class="paragraph"><p>Most of the time, an application wants to perform some amount of data
buffering in addition to just responding to events. When we want to
write data, for example, the usual pattern runs something like:</p></div>
<div class="ulist"><ul>
<li>
<p>
Decide that we want to write some data to a connection; put that
data in a buffer.
</p>
</li>
<li>
<p>
Wait for the connection to become writable
</p>
</li>
<li>
<p>
Write as much of the data as we can
</p>
</li>
<li>
<p>
Remember how much we wrote, and if we still have more data to write,
wait for the connection to become writable again.
</p>
</li>
</ul></div>
<div class="paragraph"><p>This buffered IO pattern is common enough that Libevent provides a
generic mechanism for it. A "bufferevent" consists of an
underlying transport (like a socket), a read buffer, and a write
buffer. Instead of regular events, which give callbacks when the
underlying transport is ready to be read or written, a bufferevent
invokes its user-supplied callbacks when it has read or written enough
data.</p></div>
<div class="paragraph"><p>There are multiple types of bufferevent that all share a common
interface. As of this writing, the following types exist:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
socket-based bufferevents
</dt>
<dd>
<p>
A bufferevent that sends and receives data from an underlying
stream socket, using the event_* interface as its backend.
</p>
</dd>
<dt class="hdlist1">
asynchronous-IO bufferevents
</dt>
<dd>
<p>
A bufferevent that uses the Windows IOCP interface to send and
receive data to an underlying stream socket. (Windows only;
experimental.)
</p>
</dd>
<dt class="hdlist1">
filtering bufferevents
</dt>
<dd>
<p>
A bufferevent that processes incoming and outgoing data before
passing it to an underlying bufferevent object—for example, to
compress or translate data.
</p>
</dd>
<dt class="hdlist1">
paired bufferevents
</dt>
<dd>
<p>
Two bufferevents that transmit data to one another.
</p>
</dd>
</dl></div>
<div class="paragraph"><div class="title">NOTE</div><p>As of Libevent 2.0.2-alpha, the bufferevents interfaces here are still
not fully orthogonal across all bufferevent types. In other words,
not every interface described below will work on all bufferevent types.
The Libevent developers intend to correct this in future versions.</p></div>
<div class="paragraph"><div class="title">NOTE ALSO</div><p>Bufferevents currently only work for stream-oriented protocols like TCP.
There may in the future be support for datagram-oriented protocols like UDP.</p></div>
<div class="paragraph"><p>All of the functions and types in this section are declared in
event2/bufferevent.h. Functions specifically related to evbuffers are
declared in event2/buffer.h; see the next chapter for information on
those.</p></div>
<div class="sect2">
<h3 id="_bufferevents_and_evbuffers">Bufferevents and evbuffers</h3>
<div class="paragraph"><p>Every bufferevent has an input buffer and an output buffer. These are
of type "struct evbuffer". When you have data to write on a
bufferevent, you add it to the output buffer; when a bufferevent has
data for you to read, you drain it from the input buffer.</p></div>
<div class="paragraph"><p>The evbuffer interface supports many operations; we discuss them in a
later section.</p></div>
</div>
<div class="sect2">
<h3 id="_callbacks_and_watermarks">Callbacks and watermarks</h3>
<div class="paragraph"><p>Every bufferevent has two data-related callbacks: a read callback
and a write callback. By default, the read callback is called
whenever any data is read from the underlying transport, and the write
callback is called whenever enough data from the output buffer is emptied to
the underlying transport. You can override the behavior of these functions
by adjusting the read and write "watermarks" of the bufferevent.</p></div>
<div class="paragraph"><p>Every bufferevent has four watermarks:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
Read low-water mark
</dt>
<dd>
<p>
Whenever a read occurs that leaves the bufferevent’s input buffer
at this level or higher, the bufferevent’s read callback is invoked.
Defaults to 0, so that every read results in the read callback
being invoked.
</p>
</dd>
<dt class="hdlist1">
Read high-water mark
</dt>
<dd>
<p>
If the bufferevent’s input buffer ever gets to this level, the
bufferevent stops reading until enough data is drained from the
input buffer to take us below it again. Defaults to unlimited, so
that we never stop reading because of the size of the input buffer.
</p>
</dd>
<dt class="hdlist1">
Write low-water mark
</dt>
<dd>
<p>
Whenever a write occurs that takes us to this level or below, we
invoke the write callback. Defaults to 0, so that a write callback
is not invoked unless the output buffer is emptied.
</p>
</dd>
<dt class="hdlist1">
Write high-water mark
</dt>
<dd>
<p>
Not used by a bufferevent directly, this watermark can have special
meaning when a bufferevent is used as the underlying transport of
another bufferevent. See notes on filtering bufferevents below.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>A bufferevent also has an "error" or "event" callback that gets
invoked to tell the application about non-data-oriented events, like
when a connection is closed or an error occurs. The following event
flags are defined:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
BEV_EVENT_READING
</dt>
<dd>
<p>
An event occured during a read operation on the bufferevent. See
the other flags for which event it was.
</p>
</dd>
<dt class="hdlist1">
BEV_EVENT_WRITING
</dt>
<dd>
<p>
An event occured during a write operation on the bufferevent. See
the other flags for which event it was.
</p>
</dd>
<dt class="hdlist1">
BEV_EVENT_ERROR
</dt>
<dd>
<p>
An error occurred during a bufferevent operation. For more
information on what the error was, call EVUTIL_SOCKET_ERROR().
</p>
</dd>
<dt class="hdlist1">
BEV_EVENT_TIMEOUT
</dt>
<dd>
<p>
A timeout expired on the bufferevent.
</p>
</dd>
<dt class="hdlist1">
BEV_EVENT_EOF
</dt>
<dd>
<p>
We got an end-of-file indication on the bufferevent.
</p>
</dd>
<dt class="hdlist1">
BEV_EVENT_CONNECTED
</dt>
<dd>
<p>
We finished a requested connection on the bufferevent.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>(The above event names are new in Libevent 2.0.2-alpha.)</p></div>
</div>
<div class="sect2">
<h3 id="_deferred_callbacks">Deferred callbacks</h3>
<div class="paragraph"><p>By default, a bufferevent callbacks are executed <em>immediately</em> when
the corresponding condition happens. (This is true of evbuffer
callbacks too; we’ll get to those later.) This immediate invocation
can make trouble when dependencies get complex. For example, suppose
that there is a callback that moves data into evbuffer A when it grows
empty, and another callback that processes data out of evbuffer A when
it grows full. Since these calls are all happening on the stack, you
might risk a stack overflow if the dependency grows nasty enough.</p></div>
<div class="paragraph"><p>To solve this, you can tell a bufferevent (or an evbuffer) that its
callbacks should be <em>deferred</em>. When the conditions are met for a
deferred callback, rather than invoking it immediately, it is queued
as part of the event_loop() call, and invoked after the regular events'
callbacks.</p></div>
<div class="paragraph"><p>(Deferred callbacks were introduced in Libevent 2.0.1-alpha.)</p></div>
</div>
<div class="sect2">
<h3 id="_option_flags_for_bufferevents">Option flags for bufferevents</h3>
<div class="paragraph"><p>You can use one or more flags when creating a bufferevent to alter its
behavior. Recognized flags are:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
BEV_OPT_CLOSE_ON_FREE
</dt>
<dd>
<p>
When the bufferevent is freed, close the underlying transport.
This will close an underlying socket, free an underlying
bufferevent, etc.
</p>
</dd>
<dt class="hdlist1">
BEV_OPT_THREADSAFE
</dt>
<dd>