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+
+<section id="technical-overview">
+<span id="overview"></span><h1 id="technical-overview"><span id="overview"></span>Technical Overview</h1>
+<div class="contents local topic" id="contents">
+<ul class="small-gap">
+<li><a class="reference internal" href="#introduction" id="id2">Introduction</a></li>
+<li><a class="reference internal" href="#why-use-native-client" id="id3">Why use Native Client?</a></li>
+<li><a class="reference internal" href="#common-use-cases" id="id4">Common use cases</a></li>
+<li><p class="first"><a class="reference internal" href="#how-native-client-works" id="id5">How Native Client works</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#security" id="id6">Security</a></li>
+<li><a class="reference internal" href="#portability" id="id7">Portability</a></li>
+<li><a class="reference internal" href="#toolchains" id="id8">Toolchains</a></li>
+</ul>
+</li>
+<li><p class="first"><a class="reference internal" href="#native-client-in-a-web-application" id="id9">Native Client in a web application</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#pepper-plugin-api" id="id10">Pepper Plugin API</a></li>
+</ul>
+</li>
+<li><a class="reference internal" href="#versioning" id="id11">Versioning</a></li>
+<li><a class="reference internal" href="#where-to-start" id="id12">Where to start</a></li>
+</ul>
+</div>
+<section id="introduction">
+<h2 id="introduction">Introduction</h2>
+<p><strong>Native Client</strong> (NaCl) is an open-source technology for running native
+compiled code in the browser, with the goal of maintaining the portability
+and safety that users expect from web applications. Native Client expands web
+programming beyond JavaScript, enabling developers to enhance their web
+applications using their preferred language. This document describes some of
+the key benefits and common use cases of Native Client.</p>
+<p>Google has implemented the open-source <a class="reference external" href="http://www.chromium.org/nativeclient">Native Client project</a> in the Chrome browser on Windows, Mac,
+Linux, and Chrome OS. The <a class="reference internal" href="/native-client/sdk/download.html"><em>Native Client Software Development Kit (SDK)</em></a>, itself an open-source project, lets developers create web
+applications that use NaCl and run in Chrome across multiple platforms.</p>
+<p>A web application that uses Native Client generally consists of a combination of
+JavaScript, HTML, CSS, and a NaCl module that is written in a language supported
+by the SDK. The NaCl SDK currently supports C and C++; as compilers for
+additional languages are developed, the SDK will be updated to support those
+languages as well.</p>
+<img alt="/native-client/images/web-app-with-nacl.png" src="/native-client/images/web-app-with-nacl.png" />
+</section><section id="why-use-native-client">
+<h2 id="why-use-native-client">Why use Native Client?</h2>
+<p>Native Client open-source technology is designed to run compiled code
+securely inside a browser at near-native speeds. Native Client puts web
+applications on the same playing field as traditional (locally-run)
+software&#8212;it provides the means to fully harness the client&#8217;s computational
+resources for applications such as 3D games, multimedia editors, CAD modeling,
+client-side data analytics, and interactive simulations.
+Native Client also aims to give C and C++ (and eventually other languages) the
+same level of portability and safety that JavaScript provides on the web today.</p>
+<p>Here are a few of the key benefits that Native Client offers:</p>
+<ul class="small-gap">
+<li><strong>Graphics, audio, and much more:</strong> Run native code modules that render 2D
+and 3D graphics, play audio, respond to mouse and keyboard events, run on
+multiple threads, and access memory directly&#8212;all without requiring
+the user to install a plugin.</li>
+<li><strong>Portability:</strong> Write your applications once and you&#8217;ll be able to run them
+across operating systems (Windows, Linux, Mac, and Chrome OS) and CPU
+architectures (x86 and ARM).</li>
+<li><strong>Easy migration path to the web:</strong> Many developers and companies have years
+of work invested in existing desktop applications. Native Client makes the
+transition from the desktop to a web application significantly easier because
+it supports C and C++.</li>
+<li><strong>Security:</strong> Native Client uses a double sandbox model designed to protect
+the user&#8217;s system from malicious or buggy applications. This model offers the
+safety of traditional web applications without sacrificing performance and
+without requiring users to install a plugin.</li>
+<li><strong>Performance:</strong> Native Client allows web applications to run at speeds
+comparable to desktop applications (within 5-15% of native speed).
+Native Client also allows applications to harness all available CPU cores via
+a threading API; this enables demanding applications such as console-quality
+games to run inside the browser.</li>
+</ul>
+</section><section id="common-use-cases">
+<h2 id="common-use-cases">Common use cases</h2>
+<p>Typical use cases for Native Client include the following:</p>
+<ul class="small-gap">
+<li><strong>Existing software components:</strong> With support for C and C++, Native
+Client enables you to reuse existing software modules in
+web applications&#8212;you don&#8217;t need to rewrite and debug code
+that&#8217;s already proven to work well.</li>
+<li><strong>Legacy desktop applications:</strong> Native Client provides a smooth migration
+path from desktop applications to the web. You can port and recompile existing
+code for the computation engine of your application directly to Native Client,
+and need repurpose only the user interface and event handling portions to the
+new browser platform. Native Client allows you to embed existing functionality
+directly into the browser. At the same time, your application can take
+advantage of things the browser does well: handling user interaction and
+processing events, based on the latest developments in HTML5.</li>
+<li><strong>Heavy computation in enterprise applications:</strong> Native Client can handle the
+number crunching required by large-scale enterprise applications. To ensure
+protection of user data, Native Client enables you to build complex
+cryptographic algorithms directly into the browser so that unencrypted data
+never goes out over the network.</li>
+<li><strong>Multimedia applications:</strong> Codecs for processing sounds, images, and movies
+can be added to the browser in a Native Client module.</li>
+<li><strong>Games:</strong> Native Client lets web applications run at close to native
+speed, reuse existing multithreaded/multicore C/C++ code bases, and
+access low-latency audio, networking APIs, and OpenGL ES with programmable
+shaders. Native Client is a natural fit for running a physics engine or
+artificial intelligence module that powers a sophisticated web game.
+Native Client also enables applications to run unchanged across
+many platforms.</li>
+<li><strong>Any application that requires acceleration</strong>: Native Client fits seamlessly
+into web applications&#8212;it&#8217;s up to you to decide to what extent to use it.
+Use of Native Client covers the full spectrum from complete applications to
+small optimized routines that accelerate vital parts of web apps.</li>
+</ul>
+</section><section id="how-native-client-works">
+<span id="link-how-nacl-works"></span><h2 id="how-native-client-works"><span id="link-how-nacl-works"></span>How Native Client works</h2>
+<p>Native Client is an umbrella name for a set of interrelated software components
+that work together to provide a way to develop C/C++ applications and run them
+securely on the web.</p>
+<p>At a high level, Native Client consists of:</p>
+<ul class="small-gap">
+<li><strong>Toolchains</strong>: collections of development tools (compilers, linkers, etc.)
+that transform C/C++ code to Native Client modules.</li>
+<li><strong>Runtime components</strong>: components embedded in the browser or other
+host platforms that allow execution of  Native Client modules
+securely and efficiently.</li>
+</ul>
+<p>The following diagram shows how these components interact:</p>
+<img alt="/native-client/images/nacl-pnacl-component-diagram.png" src="/native-client/images/nacl-pnacl-component-diagram.png" />
+<p>The left side of the diagram shows how to use Portable Native Client
+(PNaCl, pronounced &#8220;pinnacle&#8221;). Developers use the PNaCl toolchain
+to produce a single, portable (<strong>pexe</strong>) module. At runtime, a translator
+built into the browser translates the pexe into native code for the
+relevant client architecture.</p>
+<p>The right side of the diagram shows how to use traditional (non-portable)
+Native Client. Developers use a nacl-gcc based toolchain to produce multiple
+architecture-dependent (<strong>nexe</strong>) modules, which are packaged into an
+application. At runtime, the browser decides which nexe to load based
+on the architecture of the client machine.</p>
+<section id="security">
+<h3 id="security">Security</h3>
+<p>Since Native Client permits the execution of native code on client machines,
+special security measures have to be implemented:</p>
+<ul class="small-gap">
+<li>The NaCl sandbox ensures that code accesses system resources only through
+safe, whitelisted APIs, and operates within its limits without attempting to
+interfere with other code running either within the browser or outside it.</li>
+<li>The NaCl validator statically analyzes code prior to running it
+to make sure it only uses code and data patterns that are permitted and safe.</li>
+</ul>
+<p>The above security measures are in addition to the existing sandbox in the
+Chrome browser&#8212;the Native Client module always executes in a process with
+restricted permissions. The only interaction between this process and the
+outside world is through sanctioned browser interfaces. Because of the
+combination of the NaCl sandbox and the Chrome sandbox, we say that
+Native Client employs a double sandbox design.</p>
+</section><section id="portability">
+<h3 id="portability">Portability</h3>
+<p>Portable Native Client (PNaCl, prounounced &#8220;pinnacle&#8221;) employs state-of-the-art
+compiler technology to compile C/C++ source code to a portable bitcode
+executable (<strong>pexe</strong>). PNaCl bitcode is an OS- and architecture-independent
+format that can be freely distributed on the web and <a class="reference internal" href="#link-nacl-in-web-apps"><em>embedded in web
+applications</em></a>.</p>
+<p>The PNaCl translator is a component embedded in the Chrome browser; its task is
+to run pexe modules. Internally, the translator compiles a pexe to a nexe
+(a native executable for the client platform&#8217;s architecture), and then executes
+the nexe within the Native Client sandbox as described above. It also uses
+intelligent caching to avoid re-compiling the pexe if it was previously compiled
+on the client&#8217;s browser.</p>
+<p>Native Client also supports the execution of nexe modules directly in the
+browser. However, since nexes contain architecture-specific machine code,
+they are not allowed to be distributed on the open web&#8212;they can only be
+used as part of applications and extensions that are installed from the
+Chrome Web Store.</p>
+<p>For more details on the difference between NaCl and PNaCl, see
+<a class="reference internal" href="/native-client/nacl-and-pnacl.html"><em>NaCl and PNaCl</em></a>.</p>
+</section><section id="toolchains">
+<span id="id1"></span><h3 id="toolchains"><span id="id1"></span>Toolchains</h3>
+<p>A toolchain is a set of tools used to create an application from a set of
+source files. In the case of Native Client, a toolchain consists of a compiler,
+linker, assembler and other tools that are used to convert an
+application written in C/C++ into a module that is loadable by the browser.</p>
+<p>The Native Client SDK provides two toolchains:</p>
+<ul class="small-gap">
+<li>a <strong>PNaCl toolchain</strong> for generating portable NaCl modules (pexe files)</li>
+<li>a <strong>gcc-based toolchain (nacl-gcc)</strong> for generating non-portable NaCl modules
+(nexe files)</li>
+</ul>
+<p>The PNaCl toolchain is recommended for most applications. The nacl-gcc
+toolchain should only be used for applications that will not be distributed
+on the open web.</p>
+</section></section><section id="native-client-in-a-web-application">
+<span id="link-nacl-in-web-apps"></span><h2 id="native-client-in-a-web-application"><span id="link-nacl-in-web-apps"></span>Native Client in a web application</h2>
+<p id="application-files">A Native Client application consists of a set of files:</p>
+<ul class="small-gap">
+<li><strong>HTML</strong>, <strong>CSS</strong>, and <strong>JavaScript</strong> files, as in any modern web
+application. The JavaScript code is responsible for communicating with the
+NaCl module.</li>
+<li>A <strong>pexe</strong> (portable NaCl) file. This module uses the <a class="reference internal" href="#link-pepper"><em>Pepper</em></a> API, which provides the bridge to JavaScript and
+browser resources.</li>
+<li>A Native Client <strong>manifest</strong> file that specifies the pexe to load, along with
+some loading options. This manifest file is embedded into the HTML page
+through an <code>&lt;embed&gt;</code> tag, as shown in the figure below.</li>
+</ul>
+<img alt="/native-client/images/nacl-in-a-web-app.png" src="/native-client/images/nacl-in-a-web-app.png" />
+<p>For more details, see <a class="reference internal" href="/native-client/devguide/coding/application-structure.html"><em>Application Structure</em></a>.</p>
+<section id="pepper-plugin-api">
+<span id="link-pepper"></span><h3 id="pepper-plugin-api"><span id="link-pepper"></span>Pepper Plugin API</h3>
+<p>The Pepper Plugin API (PPAPI), called <strong>Pepper</strong> for convenience, is an
+open-source, cross-platform C/C++ API for web browser plugins. From the point
+of view of Native Client, Pepper allows a C/C++ module to communicate with
+the hosting browser and get access to system-level functions in a safe and
+portable way. One of the security constraints in Native Client is that modules
+cannot make any OS-level calls directly. Pepper provides analogous APIs that
+modules can target instead.</p>
+<p>You can use the Pepper APIs to gain access to the full array of browser
+capabilities, including:</p>
+<ul class="small-gap">
+<li><a class="reference internal" href="/native-client/devguide/coding/message-system.html"><em>Talking to the JavaScript code in your application</em></a> from the C++ code in your NaCl module.</li>
+<li><a class="reference internal" href="/native-client/devguide/coding/file-io.html"><em>Doing file I/O</em></a>.</li>
+<li><a class="reference internal" href="/native-client/devguide/coding/audio.html"><em>Playing audio</em></a>.</li>
+<li><a class="reference internal" href="/native-client/devguide/coding/3D-graphics.html"><em>Rendering 3D graphics</em></a>.</li>
+</ul>
+<p>Pepper includes both a C API and a C++ API. The C++ API is a set of bindings
+written on top of the C API. For additional information about Pepper, see
+<a class="reference external" href="http://code.google.com/p/ppapi/wiki/Concepts">Pepper Concepts</a>.</p>
+</section></section><section id="versioning">
+<h2 id="versioning">Versioning</h2>
+<p>Chrome is released on a six week cycle, and developer versions of Chrome are
+pushed to the public beta channel three weeks before each release. As with any
+software, each release of Chrome may include changes to Native Client and the
+Pepper interfaces that may require modification to existing applications.
+However, modules compiled for one version of Pepper/Chrome should work with
+subsequent versions of Pepper/Chrome. The SDK includes multiple <a class="reference external" href="https://developers.google.com/native-client/version">versions</a> of the Pepper APIs to
+help developers make adjustments to API changes and take advantage of new
+features.</p>
+</section><section id="where-to-start">
+<h2 id="where-to-start">Where to start</h2>
+<p>The <a class="reference internal" href="/native-client/quick-start.html"><em>Quick Start</em></a> document provides links to downloads and
+documentation that should help you get started with developing and distributing
+Native Client applications.</p>
+</section></section>
+
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