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+{{+bindTo:partials.standard_nacl_article}}
+
+<section id="dynamic-linking-and-loading-with-glibc">
+<h1 id="dynamic-linking-and-loading-with-glibc">Dynamic Linking and Loading with glibc</h1>
+<div class="contents local topic" id="contents">
+<ul class="small-gap">
+<li><p class="first"><a class="reference internal" href="#introduction" id="id1">Introduction</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#c-standard-libraries-glibc-and-newlib" id="id2">C standard libraries: glibc and newlib</a></li>
+<li><a class="reference internal" href="#sdk-toolchains" id="id3">SDK toolchains</a></li>
+<li><a class="reference internal" href="#specifying-and-delivering-shared-libraries" id="id4">Specifying and delivering shared libraries</a></li>
+</ul>
+</li>
+<li><a class="reference internal" href="#building-a-dynamically-linked-application" id="id5">Building a dynamically linked application</a></li>
+<li><a class="reference internal" href="#generating-a-native-client-manifest-file-for-a-dynamically-linked-application" id="id6">Generating a Native Client manifest file for a dynamically linked application</a></li>
+<li><a class="reference internal" href="#deploying-a-dynamically-linked-application" id="id7">Deploying a dynamically linked application</a></li>
+<li><a class="reference internal" href="#opening-a-shared-library-at-runtime" id="id8">Opening a shared library at runtime</a></li>
+<li><a class="reference internal" href="#troubleshooting" id="id9">Troubleshooting</a></li>
+</ul>
+</div>
+<section id="introduction">
+<h2 id="introduction">Introduction</h2>
+<aside class="caution">
+Portable Native Client currently only supports static linking, and the
+only C library available for it is newlib. This page is only valid for
+Native Client, though PNaCl will eventually support some form of
+dynamic linking.
+</aside>
+<p>This document describes how to create and deploy dynamically linked and loaded
+applications with the glibc library in the Native Client SDK. Before reading
+this document, we recommend reading <a class="reference internal" href="/native-client/devguide/devcycle/building.html"><em>Building Native Client Modules</em></a></p>
+<section id="c-standard-libraries-glibc-and-newlib">
+<span id="c-libraries"></span><h3 id="c-standard-libraries-glibc-and-newlib"><span id="c-libraries"></span>C standard libraries: glibc and newlib</h3>
+<p>The Native Client SDK comes with two C standard libraries &#8212; glibc and
+newlib. These libraries are described in the table below.</p>
+<table border="1" class="docutils">
+<colgroup>
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Library</th>
+<th class="head">Linking</th>
+<th class="head">License</th>
+<th class="head">Description</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td>glibc</td>
+<td>dynamic
+or static</td>
+<td>GNU Lesser
+General
+Public
+License
+(LGPL)</td>
+<td>glibc is the GNU implementation of the
+<a class="reference external" href="http://en.wikipedia.org/wiki/POSIX">POSIX</a> standard runtime library for the C
+programming language. Designed for
+portability and performance, glibc is one
+of the most popular implementations of the
+C library. It is comprised of a set of
+interdependent libraries including libc,
+libpthreads, libdl, and others. For
+documentation, FAQs, and additional
+information about glibc, see <a class="reference external" href="http://www.gnu.org/software/libc/index.html">GLIBC</a></td>
+</tr>
+<tr class="row-odd"><td>newlib</td>
+<td>static</td>
+<td>Berkeley
+Software
+Distribution
+(BSD) type
+free
+software
+licenses</td>
+<td>newlib is a C library intended for use in
+embedded systems. Like glibc, newlib is a
+conglomeration of several library parts.
+It is available for use under BSD-type free
+software licenses, which generally makes it
+more suitable to link statically in
+commercial, closed-source applications. For
+documentation, FAQs, and additional
+information about newlib, see the <a class="reference external" href="http://sourceware.org/newlib/">newlib</a>
+documentation.</td>
+</tr>
+</tbody>
+</table>
+<p>For proprietary (closed-source) applications, your options are to either
+statically link to newlib, or dynamically link to glibc. We recommend
+dynamically linking to glibc, for a couple of reasons:</p>
+<ul class="small-gap">
+<li>The glibc library is widely distributed (it&#8217;s included in Linux
+distributions), and as such it&#8217;s mature, hardened, and feature-rich. Your
+code is more likely to compile out-of-the-box with glibc.</li>
+<li>Dynamic loading can provide a big performance benefit for your application if
+you can structure the application to defer loading of code that&#8217;s not needed
+for initial interaction with the user. It takes some work to put such code in
+shared libraries and to load the libraries at runtime, but the payoff is
+usually worth it. In future releases, Chrome may also support caching of
+common dynamically linked libraries such as libc.so between applications.
+This could significantly reduce download size and provide a further potential
+performance benefit (for example, the hello_world example would only require
+downloading a .nexe file that&#8217;s on the order of 30KB, rather than a .nexe
+file and several libraries, which are on the order of 1.5MB).</li>
+</ul>
+<p>Native Client support for dynamic linking and loading is based on glibc. Thus,
+<strong>if your Native Client application must dynamically link and load code (e.g.,
+due to licensing considerations), we recommend that you use the glibc
+library.</strong></p>
+<aside class="note">
+<p><strong>Notes:</strong></p>
+<ul class="small-gap">
+<li><strong>None of the above constitutes legal advice, or a description of the legal
+obligations you need to fulfill in order to be compliant with the LGPL or
+newlib licenses. The above description is only a technical explanation of
+the differences between newlib and glibc, and the choice you must make
+between the two libraries.</strong></li>
+<li>Static linking with glibc is rarely used. Use this feature with caution.</li>
+<li>The standard C++ runtime in Native Client is provided by libstdc++; this
+library is independent from and layered on top of glibc. Because of
+licensing restrictions, libstdc++ must be statically linked for commercial
+uses, even if the rest of an application is dynamically linked.</li>
+</ul>
+
+</aside>
+</section><section id="sdk-toolchains">
+<h3 id="sdk-toolchains">SDK toolchains</h3>
+<p>The Native Client SDK contains multiple toolchains, which are differentiated by
+<a class="reference internal" href="/native-client/devguide/devcycle/building.html#target-architectures"><em>target architecture</em></a> and C library:</p>
+<table border="1" class="docutils">
+<colgroup>
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Target architecture</th>
+<th class="head">C library</th>
+<th class="head">Toolchain directory</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td>x86</td>
+<td>newlib</td>
+<td>toolchain/&lt;platform&gt;_x86_newlib</td>
+</tr>
+<tr class="row-odd"><td>x86</td>
+<td>glibc</td>
+<td>toolchain/&lt;platform&gt;_x86_glibc</td>
+</tr>
+<tr class="row-even"><td>ARM</td>
+<td>newlib</td>
+<td>toolchain/&lt;platform&gt;_arm_newlib</td>
+</tr>
+<tr class="row-odd"><td>PNaCl</td>
+<td>newlib</td>
+<td>toolchain/&lt;platform&gt;_pnacl</td>
+</tr>
+</tbody>
+</table>
+<p>In the directories listed above, &lt;platform&gt; is the platform of your development
+machine (i.e., win, mac, or linux). For example, in the Windows SDK, the x86
+toolchain that uses glibc is in <code>toolchain/win_x86_glibc</code>.</p>
+<aside class="note">
+<strong>Note:</strong> The ARM and PNaCl toolchains are currently restricted to newlib.
+</aside>
+<p>To use the glibc library and dynamic linking in your application, you <strong>must</strong>
+use a glibc toolchain. (Currently the only glibc toolchain is
+<code>&lt;platform&gt;_x86_glibc</code>.) Note that you must build all code in your application
+with one toolchain. Code from multiple toolchains cannot be mixed.</p>
+</section><section id="specifying-and-delivering-shared-libraries">
+<h3 id="specifying-and-delivering-shared-libraries">Specifying and delivering shared libraries</h3>
+<p>One significant difference between newlib and glibc applications is that glibc
+applications must explicitly list and deploy the shared libraries that they
+use.</p>
+<p>In a desktop environment, when the user launches a dynamically linked
+application, the operating system&#8217;s program loader determines the set of
+libraries the application requires by reading explicit inter-module
+dependencies from executable file headers, and loads the required libraries
+into the address space of the application process. Typically the required
+libraries will have been installed on the system as a part of the application&#8217;s
+installation process. Often the desktop application developer doesn&#8217;t know or
+think about the libraries that are required by an application, as those details
+are taken care of by the user&#8217;s operating system.</p>
+<p>In the Native Client sandbox, dynamic linking can&#8217;t rely in the same way on the
+operating system or the local file system. Instead, the application developer
+must identify the set of libraries that are required by an application, list
+those libraries in a Native Client <a class="reference internal" href="/native-client/devguide/coding/application-structure.html#manifest-file"><em>manifest file</em></a>, and
+deploy the libraries along with the application. Instructions for how to build
+a dynamically linked Native Client application, generate a Native Client
+manifest (.nmf) file, and deploy an application are provided below.</p>
+</section></section><section id="building-a-dynamically-linked-application">
+<h2 id="building-a-dynamically-linked-application">Building a dynamically linked application</h2>
+<p>Applications built with the glibc toolchain will by dynamically linked by
+default. Application that load shared libraries at runtime using <code>dlopen()</code>
+must link with the libdl library (<code>-ldl</code>).</p>
+<p>Like other gcc-based toolchains building a dynamic library for NaCl is normally
+done by linking with the <code>-shared</code> flag and compiling with the <code>-fPIC</code> flag.
+The SDK build system will do this automatically when the <code>SO_RULE</code> Makefile
+rule is used.</p>
+<p>The Native Client SDK includes an example that demonstrates how to build a
+shared library, and how to use the <code>dlopen()</code> interface to load that library
+at runtime (after the application is already running). Many applications load
+and link shared libraries at launch rather than at runtime, and hence do not
+use the <code>dlopen()</code> interface. The SDK example is nevertheless instructive, as
+it demonstrates how to build Native Client modules (.nexe files) and shared
+libraries (.so files) with the x86 glibc toolchain, and how to generate a
+Native Client manifest file for glibc applications.</p>
+<p>The SDK example, located in <code>examples/tutorial/dlopen</code>, includes three C++
+files:</p>
+<dl class="docutils">
+<dt>eightball.cc</dt>
+<dd>This file implements the function <code>Magic8Ball()</code>, which is used to provide
+whimsical answers to user questions. This file is compiled into a shared
+library called <code>libeightball.so</code>. This library gets included in the
+.nmf file and is therefore directly loadable with <code>dlopen()</code>.</dd>
+<dt>reverse.cc</dt>
+<dd>This file implements the function <code>Reverse()</code>, which returns reversed
+copies of strings that are passed to it. This file is compiled into a shared
+library called <code>libreverse.so</code>. This library is <strong>not</strong> included in the
+.nmf file and is loaded via an http mount using the <a class="reference internal" href="/native-client/devguide/coding/nacl_io.html#nacl-io"><em>nacl_io library</em></a>.</dd>
+<dt>dlopen.cc</dt>
+<dd>This file implements the Native Client module, which loads the two shared
+libraries and handles communcation with with JavaScript. The file is compiled
+into a Native Client executable (.nexe).</dd>
+</dl>
+<p>Run <code>make</code> in the dlopen directory to see the commands the Makefile executes
+to build x86 32-bit and 64-bit .nexe and .so files, and to generate a .nmf
+file. These commands are described below.</p>
+<aside class="note">
+<strong>Note:</strong> The Makefiles for most of the examples in the SDK build the
+examples using multiple toolchains (x86 newlib, x86 glibc, ARM, and PNaCl).
+With a few exceptions (listed in the <a class="reference internal" href="/native-client/sdk/release-notes.html#sdk-release-notes"><em>Release Notes</em></a>), running &#8220;make&#8221; in each example&#8217;s directory builds
+multiple versions of the example using the SDK toolchains. The dlopen example
+is one of those exceptions – it is only built with the x86 glibc toolchain,
+as that is currently the only toolchain that supports glibc and thus dynamic
+linking and loading. Take a look at the example Makefiles and the generated
+.nmf files for details on how to build dynamically linked applications.
+</aside>
+</section><section id="generating-a-native-client-manifest-file-for-a-dynamically-linked-application">
+<span id="dynamic-loading-manifest"></span><h2 id="generating-a-native-client-manifest-file-for-a-dynamically-linked-application"><span id="dynamic-loading-manifest"></span>Generating a Native Client manifest file for a dynamically linked application</h2>
+<p>The Native Client manifest file specifies the name of the executable to run
+and must also specify any shared libraries that the application directly
+depends on. For indirect dependencies (such as libraries opened via
+<code>dlopen()</code>) it is also convenient to list libraries in the manifest file.
+However it is possile to load arbitrary shared libraries at runtime that
+are not mentioned in the manifest by using the <a class="reference external" href="nacl_io">nacl_io library</a>
+to mount a filesystem that contains the shared libraries which will then
+allow <code>dlopen()</code> to access them.</p>
+<p>In this example we demonstrate both loading directly from via the manifest
+file (<code>libeightball.so</code>) and loading indirectly via a http mount
+(<code>libreverse.so</code>).</p>
+<p>Take a look at the manifest file in the dlopen example to see how
+a glibc-style manifest file is structured. (Run <code>make</code> in the dlopen directory to
+generate the manifest file if you haven&#8217;t done so already.) Here is an excerpt
+from <code>dlopen.nmf</code>:</p>
+<pre class="prettyprint">
+{
+  &quot;files&quot;: {
+    &quot;libeightball.so&quot;: {
+      &quot;x86-64&quot;: {
+        &quot;url&quot;: &quot;lib64/libeightball.so&quot;
+      },
+      &quot;x86-32&quot;: {
+        &quot;url&quot;: &quot;lib32/libeightball.so&quot;
+      }
+    },
+    &quot;libstdc++.so.6&quot;: {
+      &quot;x86-64&quot;: {
+        &quot;url&quot;: &quot;lib64/libstdc++.so.6&quot;
+      },
+      &quot;x86-32&quot;: {
+        &quot;url&quot;: &quot;lib32/libstdc++.so.6&quot;
+      }
+    },
+    &quot;libppapi_cpp.so&quot;: {
+      &quot;x86-64&quot;: {
+        &quot;url&quot;: &quot;lib64/libppapi_cpp.so&quot;
+      },
+      &quot;x86-32&quot;: {
+        &quot;url&quot;: &quot;lib32/libppapi_cpp.so&quot;
+      }
+    },
+... etc.
+</pre>
+<p>In most cases, you can use the <code>create_nmf.py</code> script in the SDK to generate
+a manifest file for your application. The script is located in the tools
+directory (e.g. <code>pepper_28/tools</code>).</p>
+<p>The Makefile in the dlopen example generates the manifest automatically using
+the <code>NMF_RULE</code> provided by the SDK build system. Running <code>make V=1</code> will
+show the full command line which is used to generate the nmf:</p>
+<pre class="prettyprint">
+create_nmf.py -o dlopen.nmf glibc/Release/dlopen_x86_32.nexe \
+   glibc/Release/dlopen_x86_64.nexe glibc/Release/libeightball_x86_32.so \
+   glibc/Release/libeightball_x86_64.so  -s ./glibc/Release \
+   -n libeightball_x86_32.so,libeightball.so \
+   -n libeightball_x86_64.so,libeightball.so
+</pre>
+<p>Run python <code>create_nmf.py --help</code> to see a full description of the command-line
+flags. A few of the important flags are described below.</p>
+<dl class="docutils">
+<dt><code>-s</code> <em>directory</em></dt>
+<dd>use <em>directory</em> to stage libraries (libraries are added to <code>lib32</code> and
+<code>lib64</code> subfolders)</dd>
+<dt><code>-L</code> <em>directory</em></dt>
+<dd>add <em>directory</em> to the library search path. The default search path
+already includes the toolchain and SDK libraries directories.</dd>
+</dl>
+<aside class="note">
+<strong>Note:</strong> The <code>create_nmf</code> script can only automatically detect explicit
+shared library dependencies (for example, dependencies specified with the -l
+flag for the compiler/linker). If you want to include libraries that you
+intend to dlopen() at runtime you must explcitly list them in your call to
+<code>create_nmf</code>.
+</aside>
+<p>As an alternative to using <code>create_nmf</code>, it is possible to manually calculate
+the list of shared library dependencies using tools such as <code>objdump_</code>.</p>
+</section><section id="deploying-a-dynamically-linked-application">
+<h2 id="deploying-a-dynamically-linked-application">Deploying a dynamically linked application</h2>
+<p>As described above, an application&#8217;s manifest file must explicitly list all the
+executable code modules that the application directly depends on, including
+modules from the application itself (.nexe and .so files), modules from the
+Native Client SDK (e.g., libppapi_cpp.so), and perhaps also modules from
+<a class="reference external" href="naclports_">naclport</a> or from <a class="reference internal" href="/native-client/community/middleware.html"><em>middleware systems</em></a> that
+the application uses. You must provide all of those modules as part of the
+application deployment process.</p>
+<p>As explained in <a class="reference internal" href="/native-client/devguide/distributing.html"><em>Distributing Your Application</em></a>, there are two basic ways to deploy an application:</p>
+<ul class="small-gap">
+<li><strong>hosted application:</strong> all modules are hosted together on a web server of
+your choice</li>
+<li><strong>packaged application:</strong> all modules are packaged into one file, hosted in
+the Chrome Web Store, and downloaded to the user&#8217;s machine</li>
+</ul>
+<p>You must deploy all the modules listed in your application&#8217;s manifest file for
+either the hosted application or the packaged application case. For hosted
+applications, you must upload the modules to your web server. For packaged
+applications, you must include the modules in the application&#8217;s Chrome Web
+Store .crx file. Modules should use URLs/names that are consistent with those
+in the Native Client manifest file, and be named relative to the location of
+the manifest file. Remember that some of the libraries named in the manifest
+file may be located in directories you specified with the -L option to
+<code>create_nmf.py</code>. You are free to rename/rearrange files and directories
+referenced by the Native Client manifest file, so long as the modules are
+available in the locations indicated by the manifest file. If you move or
+rename modules, it may be easier to re-run <code>create_nmf.py</code> to generate a new
+manifest file rather than edit the original manifest file. For hosted
+applications, you can check for name mismatches during testing by watching the
+request log of the web server hosting your test deployment.</p>
+</section><section id="opening-a-shared-library-at-runtime">
+<h2 id="opening-a-shared-library-at-runtime">Opening a shared library at runtime</h2>
+<p>Native Client supports a version of the POSIX standard <code>dlopen()</code> interface
+for opening libraries explicitly, after an application is already running.
+Calling <code>dlopen()</code> may cause a library download to occur, and automatically
+loads all libraries that are required by the named library.</p>
+<aside class="note">
+<strong>Caution:</strong> Since <code>dlopen()</code> can potentially block, you must initially
+call <code>dlopen()</code> off your application&#8217;s main thread. Initial calls to
+<code>dlopen()</code> from the main thread will always fail in the current
+implementation of Native Client.
+</aside>
+<p>The best practice for opening libraries with <code>dlopen()</code> is to use a worker
+thread to pre-load libraries asynchronously during initialization of your
+application, so that the libraries are available when they&#8217;re needed. You can
+call <code>dlopen()</code> a second time when you need to use a library &#8211; per the
+specification, subsequent calls to <code>dlopen()</code> return a handle to the
+previously loaded library. Note that you should only call <code>dlclose()</code> to
+close a library when you no longer need the library; otherwise, subsequent
+calls to <code>dlopen()</code> could cause the library to be fetched again.</p>
+<p>The dlopen example in the SDK demonstrates how to open a shared libraries
+at runtime. To reiterate, the example includes three C++ files:</p>
+<ul class="small-gap">
+<li><code>eightball.cc</code>: this is the shared library that implements the function
+<code>Magic8Ball()</code> (this file is compiled into libeightball.so)</li>
+<li><code>reverse.cc</code>: this is the shared library that implements the function
+<code>Reverse()</code> (this file is compiled into libreverse.so)</li>
+<li><code>dlopen.cc</code>: this is the Native Client module that loads the shared libraries
+and makes calls to <code>Magic8Ball()</code> and <code>Reverse()</code> in response to requests
+from JavaScript.</li>
+</ul>
+<p>When the Native Client module starts, it kicks off a worker thread that calls
+<code>dlopen()</code> to load the two shared libraries. Once the module has a handle to
+the library, it fetches the addresses of the <code>Magic8Ball()</code> and <code>Reverse()</code>
+functions using <code>dlsym()</code>. When a user types in a query and clicks the &#8216;ASK!&#8217;
+button, the module calls <code>Magic8Ball()</code> to generate an answer, and returns
+the result to the user. Likewise when the user clicks the &#8216;Reverse&#8217; button
+it calls the <code>Reverse()</code> function to reverse the string.</p>
+</section><section id="troubleshooting">
+<h2 id="troubleshooting">Troubleshooting</h2>
+<p>If your .nexe isn&#8217;t loading, the best place to look for information that can
+help you troubleshoot the JavaScript console and standard output from Chrome.
+See <a class="reference internal" href="/native-client/devguide/devcycle/debugging.html#devcycle-debugging"><em>Debugging</em></a> for more information.</p>
+<p>Here are a few common error messages and explanations of what they mean:</p>
+<dl class="docutils">
+<dt><strong>/main.nexe: error while loading shared libraries: /main.nexe: failed to allocate code and data space for executable</strong></dt>
+<dd>The .nexe may not have been compiled correctly (e.g., the .nexe may be
+statically linked). Try cleaning and recompiling with the glibc toolchain.</dd>
+<dt><strong>/main.nexe: error while loading shared libraries: libpthread.so.xxxx: cannot open shared object file: Permission denied</strong></dt>
+<dd>(xxxx is a version number, for example, 5055067a.) This error can result from
+having the wrong path in the .nmf file. Double-check that the path in the
+.nmf file is correct.</dd>
+<dt><strong>/main.nexe: error while loading shared libraries: /main.nexe: cannot open shared object file: No such file or directory</strong></dt>
+<dd>If there are no obvious problems with your main.nexe entry in the .nmf file,
+check where main.nexe is being requested from. Use Chrome&#8217;s Developer Tools:
+Click the menu icon <img alt="menu-icon" src="/native-client/images/menu-icon.png" />, select Tools &gt; Developer Tools, click the
+Network tab, and look at the path in the Name column.</dd>
+<dt><strong>NaCl module load failed: ELF executable text/rodata segment has wrong starting address</strong></dt>
+<dd>This error happens when using a newlib-style .nmf file instead of a
+glibc-style .nmf file. Make sure you build your application with the glic
+toolchain, and use the create_nmf.py script to generate your .nmf file.</dd>
+<dt><strong>NativeClient: NaCl module load failed: Nexe crashed during startup</strong></dt>
+<dd>This error message indicates that a module crashed while being loaded. You
+can determine which module crashed by looking at the Network tab in Chrome&#8217;s
+Developer Tools (see above). The module that crashed will be the last one
+that was loaded.</dd>
+<dt><strong>/lib/main.nexe: error while loading shared libraries: /lib/main.nexe: only ET_DYN and ET_EXEC can be loaded</strong></dt>
+<dd>This error message indicates that there is an error with the .so files listed
+in the .nmf file &#8211; either the files are the wrong type or kind, or an
+expected library is missing.</dd>
+<dt><strong>undefined reference to &#8216;dlopen&#8217; collect2: ld returned 1 exit status</strong></dt>
+<dd>This is a linker ordering problem that usually results from improper ordering
+of command line flags when linking. Reconfigure your command line string to
+list libraries after the -o flag.</dd>
+</dl>
+</section></section>
+
+{{/partials.standard_nacl_article}}