Add debugging information for nacl_io library.

native_client_sdk/doc_generated/devguide/devcycle/debugging.html

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 <section id="debugging">
 <span id="devcycle-debugging"></span><h1 id="debugging"><span id="devcycle-debugging"></span>Debugging</h1>
 <p>This document describes tools and techniques you can use to debug, monitor,
 and measure your application&#8217;s performance.</p>
 <div class="contents local" id="table-of-contents" style="display: none">
 <p class="topic-title first">Table Of Contents</p>
 <ul class="small-gap">
 <li><p class="first"><a class="reference internal" href="#diagnostic-information" id="id2">Diagnostic information</a></p>
@@ skipping 14 matching lines @@
 <li><a class="reference internal" href="#debugging-with-visual-studio" id="id10">Debugging with Visual Studio</a></li>
 <li><p class="first"><a class="reference internal" href="#debugging-with-nacl-gdb" id="id11">Debugging with nacl-gdb</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#debugging-pnacl-pexes-pepper-35-or-later" id="id12">Debugging PNaCl pexes (Pepper 35 or later)</a></li>
 <li><a class="reference internal" href="#debugging-pnacl-pexes-with-older-pepper-toolchains" id="id13">Debugging PNaCl pexes (with older Pepper toolchains)</a></li>
 <li><a class="reference internal" href="#running-nacl-gdb" id="id14">Running nacl-gdb</a></li>
 </ul>
 </li>
 </ul>
 </li>
-<li><a class="reference internal" href="#debugging-with-other-tools" id="id15">Debugging with other tools</a></li>
+<li><p class="first"><a class="reference internal" href="#non-standard-debugging" id="id15">Non-Standard Debugging</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#nacl-io" id="id16">nacl_io</a></li>
+<li><a class="reference internal" href="#alternative-debuggers" id="id17">Alternative Debuggers</a></li>
+</ul>
+</li>
 </ul>
 
 </div><h2 id="diagnostic-information">Diagnostic information</h2>
 <h3 id="viewing-process-statistics-with-the-task-manager">Viewing process statistics with the task manager</h3>
 <p>You can use Chrome&#8217;s Task Manager to display information about a Native Client
 application:</p>
 <ol class="arabic simple">
 <li>Open the Task Manager by clicking the menu icon <img alt="menu-icon" src="/native-client/images/menu-icon.png" /> and choosing
 <strong>Tools &gt; Task manager</strong>.</li>
 <li>When the Task Manager window appears, verify that the columns displaying
@@ skipping 38 matching lines @@
 #include &lt;stdio.h&gt;
 void logmsg(const char* pMsg){
   fprintf(stdout,&quot;logmsg: %s\n&quot;,pMsg);
 }
 void errormsg(const char* pMsg){
   fprintf(stderr,&quot;logerr: %s\n&quot;,pMsg);
 }
 </pre>
 <p>By default stdout and stderr will appear in Chrome&#8217;s stdout and stderr stream
 but they can also be redirected as described below.</p>
-<h4 id="redirecting-output-to-log-files">Redirecting output to log files</h4>
+<h4 id="redirecting-output-to-log-files"><span id="redirecting-output-to-log"></span>Redirecting output to log files</h4>
 <p>You can redirect stdout and stderr to output files by setting these environment
 variables:</p>
 <ul class="small-gap">
 <li><code>NACL_EXE_STDOUT=c:\nacl_stdout.log</code></li>
 <li><code>NACL_EXE_STDERR=c:\nacl_stderr.log</code></li>
 </ul>
 <p>There is another variable, <code>NACLLOG</code>, that you can use to redirect Native
 Client&#8217;s internally-generated messages. This variable is set to stderr by
 default; you can redirect these messages to an output file by setting the
 variable as follows:</p>
@@ skipping 345 matching lines @@
 <dt><code>help &lt;command&gt;</code></dt>
 <dd>print documentation for the specified gdb &lt;command&gt;</dd>
 <dt><code>quit</code></dt>
 <dd>quit gdb</dd>
 </dl>
 <p>See the <a class="reference external" href="http://sourceware.org/gdb/current/onlinedocs/gdb/#toc_Top">gdb documentation</a> for a
 comprehensive list of gdb commands. Note that you can abbreviate most commands
 to just their first letter (<code>b</code> for break, <code>c</code> for continue, and so on).</p>
 <p>To interrupt execution of your module, press &lt;Ctrl-c&gt;. When you&#8217;re done
 debugging, close the Chrome window and type <code>q</code> to quit gdb.</p>
-<h2 id="debugging-with-other-tools">Debugging with other tools</h2>
+<h2 id="non-standard-debugging">Non-Standard Debugging</h2>
+<h3 id="nacl-io"><span id="nacl-io-debugging"></span>nacl_io</h3>
+<p>The logging system for the <a class="reference internal" href="/native-client/devguide/coding/nacl_io.html"><em>nacl_io library</em></a> writes
+directly to the <code>stderr</code> stream of the NaCl process. It deliberately bypasses
+the standard library functions implemented in nacl_io to avoid circular calls to
+itself.</p>
+<p>The stderr from the process is by default visible in the terminal that launched
+chrome (at least on mac and linux). Be sure that when you launch chrome it
+doesn&#8217;t attach to an existing instance. One simple way to do this is to pass a
+new directory to chrome as your user data directory (<code>chrome
+--user-data-dir=&lt;newdir&gt;</code>). It&#8217;s also possible to redirect the NaCl&#8217;s stderr to
+other places such as, for example, <a class="reference internal" href="#redirecting-output-to-log"><em>log files</em></a>.</p>
+<h3 id="alternative-debuggers">Alternative Debuggers</h3>
 <p>If you cannot use the <a class="reference internal" href="#visual-studio"><em>Visual Studio add-in</em></a>, or you want
 to use a debugger other than nacl-gdb, you must manually build your module as a
 Pepper plugin (sometimes referred to as a &#8220;<a class="reference external" href="http://www.chromium.org/nativeclient/getting-started/getting-started-background-and-basics#TOC-Trusted-vs-Untrusted">trusted</a>&#8221;
 or &#8220;in-process&#8221; plugin).  Pepper plugins (.DLL files on Windows; .so files on
 Linux; .bundle files on Mac) are loaded directly in either the Chrome renderer
 process or a separate plugin process, rather than in Native Client. Building a
 module as a trusted Pepper plugin allows you to use standard debuggers and
 development tools on your system, but when you&#8217;re finished developing the
 plugin, you need to port it to Native Client (i.e., build the module with one
 of the toolchains in the NaCl SDK so that the module runs in Native Client).
 For details on this advanced development technique, see <a class="reference external" href="http://www.chromium.org/nativeclient/how-tos/debugging-documentation/debugging-a-trusted-plugin">Debugging a Trusted
 Plugin</a>.
 Note that starting with the <code>pepper_22</code> bundle, the NaCl SDK for Windows
 includes pre-built libraries and library source code, making it much easier to
 build a module into a .DLL.</p>
 </section>
 
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