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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Handler Tracking</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.handler_tracking"></a><a class="link" href="handler_tracking.html" title="Handler Tracking">Handler
Tracking</a>
</h4></div></div></div>
<p>
To aid in debugging asynchronous programs, Boost.Asio provides support
for handler tracking. When enabled by defining <code class="computeroutput"><span class="identifier">BOOST_ASIO_ENABLE_HANDLER_TRACKING</span></code>,
Boost.Asio writes debugging output to the standard error stream. The output
records asynchronous operations and the relationships between their handlers.
</p>
<p>
This feature is useful when debugging and you need to know how your asynchronous
operations are chained together, or what the pending asynchronous operations
are. As an illustration, here is the output when you run the HTTP Server
example, handle a single request, then shut down via Ctrl+C:
</p>
<pre class="programlisting">@asio|1512254357.979980|0*1|signal_set@0x7ffeaaaa20d8.async_wait
@asio|1512254357.980127|0*2|socket@0x7ffeaaaa20f8.async_accept
@asio|1512254357.980150|.2|non_blocking_accept,ec=asio.system:11
@asio|1512254357.980162|0|resolver@0x7ffeaaaa1fd8.cancel
@asio|1512254368.457147|.2|non_blocking_accept,ec=system:0
@asio|1512254368.457193|>2|ec=system:0
@asio|1512254368.457219|2*3|socket@0x55cf39f0a238.async_receive
@asio|1512254368.457244|.3|non_blocking_recv,ec=system:0,bytes_transferred=141
@asio|1512254368.457275|2*4|socket@0x7ffeaaaa20f8.async_accept
@asio|1512254368.457293|.4|non_blocking_accept,ec=asio.system:11
@asio|1512254368.457301|<2|
@asio|1512254368.457310|>3|ec=system:0,bytes_transferred=141
@asio|1512254368.457441|3*5|socket@0x55cf39f0a238.async_send
@asio|1512254368.457502|.5|non_blocking_send,ec=system:0,bytes_transferred=156
@asio|1512254368.457511|<3|
@asio|1512254368.457519|>5|ec=system:0,bytes_transferred=156
@asio|1512254368.457544|5|socket@0x55cf39f0a238.close
@asio|1512254368.457559|<5|
@asio|1512254371.385106|>1|ec=system:0,signal_number=2
@asio|1512254371.385130|1|socket@0x7ffeaaaa20f8.close
@asio|1512254371.385163|<1|
@asio|1512254371.385175|>4|ec=asio.system:125
@asio|1512254371.385182|<4|
@asio|1512254371.385202|0|signal_set@0x7ffeaaaa20d8.cancel
</pre>
<p>
Each line is of the form:
</p>
<pre class="programlisting"><tag>|<timestamp>|<action>|<description>
</pre>
<p>
The <code class="computeroutput"><tag></code> is always <code class="computeroutput">@asio</code>, and is used
to identify and extract the handler tracking messages from the program
output.
</p>
<p>
The <code class="computeroutput"><timestamp></code> is seconds and microseconds from 1 Jan
1970 UTC.
</p>
<p>
The <code class="computeroutput"><action></code> takes one of the following forms:
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">>n</span></dt>
<dd><p>
The program entered the handler number <code class="computeroutput">n</code>. The <code class="computeroutput"><description></code>
shows the arguments to the handler.
</p></dd>
<dt><span class="term"><n</span></dt>
<dd><p>
The program left handler number <code class="computeroutput">n</code>.
</p></dd>
<dt><span class="term">!n</span></dt>
<dd><p>
The program left handler number n due to an exception.
</p></dd>
<dt><span class="term">~n</span></dt>
<dd><p>
The handler number <code class="computeroutput">n</code> was destroyed without having been
invoked. This is usually the case for any unfinished asynchronous
operations when the <code class="computeroutput">io_context</code> is destroyed.
</p></dd>
<dt><span class="term">n*m</span></dt>
<dd><p>
The handler number <code class="computeroutput">n</code> created a new asynchronous operation
with completion handler number <code class="computeroutput">m</code>. The <code class="computeroutput"><description></code>
shows what asynchronous operation was started.
</p></dd>
<dt><span class="term">n</span></dt>
<dd><p>
The handler number <code class="computeroutput">n</code> performed some other operation.
The <code class="computeroutput"><description></code> shows what function was called.
Currently only <code class="computeroutput">close()</code> and <code class="computeroutput">cancel()</code> operations
are logged, as these may affect the state of pending asynchronous
operations.
</p></dd>
<dt><span class="term">.n</span></dt>
<dd><p>
The implementation performed a system call as part of the asynchronous
operation for which handler number <code class="computeroutput">n</code> is the completion
handler. The <code class="computeroutput"><description></code> shows what function
was called and its results. These tracking events are only emitted
when using a reactor-based implementation.
</p></dd>
</dl>
</div>
<p>
Where the <code class="computeroutput"><description></code> shows a synchronous or asynchronous
operation, the format is <code class="computeroutput"><object-type>@<pointer>.<operation></code>.
For handler entry, it shows a comma-separated list of arguments and their
values.
</p>
<p>
As shown above, Each handler is assigned a numeric identifier. Where the
handler tracking output shows a handler number of 0, it means that the
action was performed outside of any handler.
</p>
<h6>
<a name="boost_asio.overview.core.handler_tracking.h0"></a>
<span class="phrase"><a name="boost_asio.overview.core.handler_tracking.visual_representations"></a></span><a class="link" href="handler_tracking.html#boost_asio.overview.core.handler_tracking.visual_representations">Visual
Representations</a>
</h6>
<p>
The handler tracking output may be post-processed using the included <code class="literal">handlerviz.pl</code>
tool to create a visual representation of the handlers (requires the GraphViz
tool <code class="literal">dot</code>).
</p>
<h6>
<a name="boost_asio.overview.core.handler_tracking.h1"></a>
<span class="phrase"><a name="boost_asio.overview.core.handler_tracking.custom_tracking"></a></span><a class="link" href="handler_tracking.html#boost_asio.overview.core.handler_tracking.custom_tracking">Custom
Tracking</a>
</h6>
<p>
Handling tracking may be customised by defining the <code class="computeroutput"><span class="identifier">BOOST_ASIO_CUSTOM_HANDLER_TRACKING</span></code>
macro to the name of a header file (enclosed in <code class="computeroutput"><span class="string">""</span></code>
or <code class="computeroutput"><span class="special"><></span></code>). This header
file must implement the following preprocessor macros:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Macro
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_INHERIT_TRACKED_HANDLER</span></code>
</p>
</td>
<td>
<p>
Specifies a base class for classes that implement asynchronous
operations. When used, the macro immediately follows the class
name, so it must have the form <code class="computeroutput"><span class="special">:</span>
<span class="keyword">public</span> <span class="identifier">my_class</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_ALSO_INHERIT_TRACKED_HANDLER</span></code>
</p>
</td>
<td>
<p>
Specifies a base class for classes that implement asynchronous
operations. When used, the macro follows other base classes,
so it must have the form <code class="computeroutput"><span class="special">,</span>
<span class="keyword">public</span> <span class="identifier">my_class</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_TRACKING_INIT</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is used to initialise the tracking mechanism.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_CREATION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called on creation of an asynchronous operation.
<code class="computeroutput"><span class="identifier">args</span></code> is a parenthesised
function argument list containing the owning execution context,
the tracked handler, the name of the object type, a pointer to
the object, the object's native handle, and the operation name.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_COMPLETION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called on completion of an asynchronous
operation. <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the tracked
handler.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_INVOCATION_BEGIN</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called immediately before a completion
handler is invoked. <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the arguments
to the completion handler.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_INVOCATION_END</span></code>
</p>
</td>
<td>
<p>
An expression that is called immediately after a completion handler
is invoked.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_OPERATION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called when some synchronous object operation
is called (such as <code class="computeroutput"><span class="identifier">close</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">cancel</span><span class="special">()</span></code>). <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the owning
execution context, the name of the object type, a pointer to
the object, the object's native handle, and the operation name.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_REGISTRATION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called when an object is registered with
the reactor. <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the owning
execution context, the object's native handle, and a unique registration
key.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_DEREGISTRATION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called when an object is deregistered from
the reactor. <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the owning
execution context, the object's native handle, and a unique registration
key.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_READ_EVENT</span></code>
</p>
</td>
<td>
<p>
A bitmask constant used to identify reactor read readiness events.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_WRITE_EVENT</span></code>
</p>
</td>
<td>
<p>
A bitmask constant used to identify reactor write readiness events.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_ERROR_EVENT</span></code>
</p>
</td>
<td>
<p>
A bitmask constant used to identify reactor error readiness events.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_EVENTS</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called when an object registered with the
reactor becomes ready. <code class="computeroutput"><span class="identifier">args</span></code>
is a parenthesised function argument list containing the owning
execution context, the unique registration key, and a bitmask
of the ready events.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ASIO_HANDLER_REACTOR_OPERATION</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
An expression that is called when the implementation performs
a system call as part of a reactor-based asynchronous operation.
<code class="computeroutput"><span class="identifier">args</span></code> is a parenthesised
function argument list containing the tracked handler, the operation
name, the error code produced by the operation, and (optionally)
the number of bytes transferred.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_asio.overview.core.handler_tracking.h2"></a>
<span class="phrase"><a name="boost_asio.overview.core.handler_tracking.see_also"></a></span><a class="link" href="handler_tracking.html#boost_asio.overview.core.handler_tracking.see_also">See Also</a>
</h6>
<p>
<a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.handler_tracking">Custom
handler tracking example</a>.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2018 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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