1 <!DOCTYPE html> 2 <html> 3 <!-- 4 Copyright (c) 2012 The Chromium Authors. All rights reserved. 5 Use of this source code is governed by a BSD-style license that can be 6 found in the LICENSE file. 7 --> 8 <head> 9 <meta http-equiv="Pragma" content="no-cache"> 10 <meta http-equiv="Expires" content="-1"> 11 <title>Logging and Crash Handling</title> 12 <script type="text/javascript" src="common.js"></script> 13 <script type="text/javascript" src="example.js"></script> 14 </head> 15 <body data-width="0" data-height="0" data-custom-load="true" {{attrs}}> 16 <h1>Logging and Crash Handling</h1> 17 <p> This example illustrates techniques for tracking the state of a NaCl 18 module via PostMessage and status of the module's lastError attribute. 19 Messages from the modules are in the form of 20 <ul> 21 <li> 22 "LOG: <data>" which adds the message to the log. 23 </li> 24 <li> 25 "TRC: <data>" which provides a JSON string defining an exception. 26 </li> 27 </ul> 28 <h2> Exception API </h2> 29 <p> As of Chrome 28, NativeClient exception handling is possible without 30 requiring special command-line flags. This feature is not always available 31 so developers should avoid requring it under normal operation. However it 32 can be a very useful tool for diagnosing crashes, especially in the field. 33 NativeClient provides a library called "error_handling" for registering 34 the exception handler, as well as unwinding the exception context. 35 <br><b>NOTE: The library requires '-fno-omit-frame-pointer' to facilitate 36 unwinding the stack.</b></p> 37 <h2> Trace Walkthrough </h2> 38 <p> First we request the exception handler interface, and use it to register 39 both a handler and an exception stack. We use a separate stack since we 40 do not know the state of stack for the thread handling the exception. 41 Next, we create a worker thread which will take the exception. It is 42 recommended that modules do as much work as possible off the main thread. 43 Failure to do so can block the browser, making the page unresponsive and/or 44 preventing communication with JavaScript. In addition blocking calls, 45 which can greatly simplify code, are only allowed off the main thread.</p> 46 <p> After two seconds, JavaScript sends a message to the module which will 47 cause it take an exception on the worker thread. The exception handler 48 unwinds the stack while creating a stringified JSON object containing the 49 stack frame information. Once unwound, or the buffer is exhausted, the 50 JSON object is sent to JavaScript for processing. 51 <p> The message handler in JavaScript takes the JSON object and uses the 52 arch key to load the appropriate MAP file using an XMLHttpRequest. It 53 then processes the MAP file and prints out a stack trace using the exception 54 data in the JSON object.</p> 55 <h2> Exception Handling in the Field </h2> 56 <p> 57 For real world applications, it's important to get the crash information 58 back the developer. In this case, the JSON object could be sent via 59 XMLHttpRequest. The JSON object can the be processed by the developers 60 QA team to manage bugs in the field. The handler.py script provided in 61 the example sources shows how the JSON object can be used with the tools 62 to provide a better stack trace. Simply cut and paste the JSON object 63 to a text file and run the handler.py script on it. 64 </p> 65 66 <div id="listener"></div> 67 <hr> 68 <h2>Status: <code id="statusField">NO-STATUS</code></h2> 69 <table> 70 <tr> 71 <td><h2>Log</h2></td> 72 <td><h2>JSON</h2></td> 73 </tr> 74 <tr> 75 <td> 76 <textarea id="log" rows="10" cols="60" readonly="readonly"></textarea> 77 </td> 78 <td> 79 <textarea id="json" rows="10" cols="60" readonly="readonly"></textarea> 80 </td> 81 </tr> 82 </table> 83 <br> 84 <h2>Stack Trace</h2> 85 <textarea id="trace" rows="10" cols="130" readonly="readonly"></textarea> 86 </body> 87 </html> 88
