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<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|1298160085.070638|0*1|signal_set@0x7fff50528f40.async_wait
@asio|1298160085.070888|0*2|socket@0x7fff50528f60.async_accept
@asio|1298160085.070913|0|resolver@0x7fff50528e28.cancel
@asio|1298160118.075438|>2|ec=asio.system:0
@asio|1298160118.075472|2*3|socket@0xb39048.async_receive
@asio|1298160118.075507|2*4|socket@0x7fff50528f60.async_accept
@asio|1298160118.075527|<2|
@asio|1298160118.075540|>3|ec=asio.system:0,bytes_transferred=122
@asio|1298160118.075731|3*5|socket@0xb39048.async_send
@asio|1298160118.075778|<3|
@asio|1298160118.075793|>5|ec=asio.system:0,bytes_transferred=156
@asio|1298160118.075831|5|socket@0xb39048.close
@asio|1298160118.075855|<5|
@asio|1298160122.827317|>1|ec=asio.system:0,signal_number=2
@asio|1298160122.827333|1|socket@0x7fff50528f60.close
@asio|1298160122.827359|<1|
@asio|1298160122.827370|>4|ec=asio.system:125
@asio|1298160122.827378|<4|
@asio|1298160122.827394|0|signal_set@0x7fff50528f40.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>
<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_service</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 n 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>
</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><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>
</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 © 2003-2012 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>
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