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<title>Logging and Crash Handling</title>
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<h1>Logging and Crash Handling</h1>
<p> This example illustrates techniques for tracking the state of a NaCl
module via PostMessage and status of the module's lastError attribute.
Messages from the modules are in the form of
<ul>
<li>
"LOG: <data>" which adds the message to the log.
</li>
<li>
"TRC: <data>" which provides a JSON string defining an exception.
</li>
</ul>
<h2> Exception API </h2>
<p> As of Chrome 28, NativeClient exception handling is possible without
requiring special command-line flags. This feature is not always available
so developers should avoid requring it under normal operation. However it
can be a very useful tool for diagnosing crashes, especially in the field.
NativeClient provides a library called "error_handling" for registering
the exception handler, as well as unwinding the exception context.
<br><b>NOTE: The library requires '-fno-omit-frame-pointer' to facilitate
unwinding the stack.</b></p>
<h2> Trace Walkthrough </h2>
<p> First we request the exception handler interface, and use it to register
both a handler and an exception stack. We use a separate stack since we
do not know the state of stack for the thread handling the exception.
Next, we create a worker thread which will take the exception. It is
recommended that modules do as much work as possible off the main thread.
Failure to do so can block the browser, making the page unresponsive and/or
preventing communication with JavaScript. In addition blocking calls,
which can greatly simplify code, are only allowed off the main thread.</p>
<p> After two seconds, JavaScript sends a message to the module which will
cause it take an exception on the worker thread. The exception handler
unwinds the stack while creating a stringified JSON object containing the
stack frame information. Once unwound, or the buffer is exhausted, the
JSON object is sent to JavaScript for processing.
<p> The message handler in JavaScript takes the JSON object and uses the
arch key to load the appropriate MAP file using an XMLHttpRequest. It
then processes the MAP file and prints out a stack trace using the exception
data in the JSON object.</p>
<h2> Exception Handling in the Field </h2>
<p>
For real world applications, it's important to get the crash information
back the developer. In this case, the JSON object could be sent via
XMLHttpRequest. The JSON object can the be processed by the developers
QA team to manage bugs in the field. The handler.py script provided in
the example sources shows how the JSON object can be used with the tools
to provide a better stack trace. Simply cut and paste the JSON object
to a text file and run the handler.py script on it.
</p>
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<h2>Status: <code id="statusField">NO-STATUS</code></h2>
<table>
<tr>
<td><h2>Log</h2></td>
<td><h2>JSON</h2></td>
</tr>
<tr>
<td>
<textarea id="log" rows="10" cols="60" readonly="readonly"></textarea>
</td>
<td>
<textarea id="json" rows="10" cols="60" readonly="readonly"></textarea>
</td>
</tr>
</table>
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<h2>Stack Trace</h2>
<textarea id="trace" rows="10" cols="130" readonly="readonly"></textarea>
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