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   3 <section id="nacl-and-pnacl">
   4 <span id="id1"></span><h1 id="nacl-and-pnacl"><span id="id1"></span>NaCl and PNaCl</h1>
   5 <p>This document describes the differences between <strong>Native Client</strong> and
   6 <strong>Portable Native Client</strong>, and provides recommendations for when to use each.</p>
   7 <div class="contents local" id="contents" style="display: none">
   8 <ul class="small-gap">
   9 <li><a class="reference internal" href="#native-client-nacl" id="id6">Native Client (NaCl)</a></li>
  10 <li><a class="reference internal" href="#portable-native-client-pnacl" id="id7">Portable Native Client (PNaCl)</a></li>
  11 <li><a class="reference internal" href="#when-to-use-pnacl" id="id8">When to use PNaCl</a></li>
  12 <li><a class="reference internal" href="#when-to-use-nacl" id="id9">When to use NaCl</a></li>
  13 </ul>
  14
  15 </div><h2 id="native-client-nacl"><span id="id2"></span>Native Client (NaCl)</h2>
  16 <p>Native Client enables the execution of native code securely inside web
  17 applications through the use of advanced <a class="reference external" href="http://research.google.com/pubs/pub35649.html">Software Fault Isolation (SFI)
  18 techniques</a>. Native Client
  19 allows you to harness a client machine&#8217;s computational power to a fuller extent
  20 than traditional web technologies. It does this by running compiled C and C++
  21 code at near-native speeds, and exposing a CPU&#8217;s full capabilities, including
  22 SIMD vectors and multiple-core processing with shared memory.</p>
  23 <p>While Native Client provides operating system independence, it requires you to
  24 generate architecture-specific executables (<strong>nexe</strong>) for each hardware
  25 platform. This is neither portable nor convenient, making it ill-suited for the
  26 open web.</p>
  27 <p>The traditional method of application distribution on the web is through self-
  28 contained bundles of HTML, CSS, JavaScript, and other resources (images, etc.)
  29 that can be hosted on a server and run inside a web browser. With this type of
  30 distribution, a website created today should still work years later, on all
  31 platforms. Architecture-specific executables are clearly not a good fit for
  32 distribution on the web. Consequently, Native Client has been until recently
  33 restricted to applications and browser extensions that are installed through the
  34 Chrome Web Store.</p>
  35 <h2 id="portable-native-client-pnacl"><span id="id3"></span>Portable Native Client (PNaCl)</h2>
  36 <p>PNaCl solves the portability problem by splitting the compilation process
  37 into two parts:</p>
  38 <ol class="arabic simple">
  39 <li>compiling the source code to a bitcode executable (pexe), and</li>
  40 <li>translating the bitcode to a host-specific executable as soon as the module
  41 loads in the browser but before any code execution.</li>
  42 </ol>
  43 <p>This portability aligns Native Client with existing open web technologies such
  44 as JavaScript. You can distribute a pexe as part of an application (along with
  45 HTML, CSS, and JavaScript), and the user&#8217;s machine is simply able to run it.</p>
  46 <p>With PNaCl, you&#8217;ll generate a single pexe, rather than multiple platform-
  47 specific nexes. Since the pexe uses an abstract, architecture- and OS-
  48 independent format, it does not suffer from the portability problem described
  49 above. Although, PNaCl can be more efficient on some operating systems than on
  50 others. PNaCl boasts the same level of security as NaCl. Future versions of
  51 hosting environments should have no problem executing the pexe, even on new
  52 architectures. Moreover, if an existing architecture is enhanced, the pexe
  53 doesn&#8217;t need to be recompiled. In some cases the client-side translation will
  54 automatically take advantage of new capabilities. A pexe can be part of any web
  55 application. It does not have to be distributed through the Chrome Web Store. In
  56 short, PNaCl combines the portability of existing web technologies with the
  57 performance and security benefits of Native Client.</p>
  58 <p>PNaCl is a new technology, and as such it still has a few limitations
  59 as compared to NaCl. These limitations are described below.</p>
  60 <h2 id="when-to-use-pnacl"><span id="id4"></span>When to use PNaCl</h2>
  61 <p>PNaCl is the preferred toolchain for Native Client, and the only way to deploy
  62 Native Client modules without the Google Web Store. Unless your project is
  63 subject to one of the narrow limitations described under &#8220;<a class="reference internal" href="#when-to-use-nacl"><em>When to use
  64 NaCl</em></a>&#8221;, you should use PNaCl.</p>
  65 <p>Chrome supports translation of pexe modules and their use in web applications
  66 without requiring installation either of a browser plug-in or of the
  67 applications themselves. Native Client and PNaCl are open-source technologies,
  68 and our hope is that they will be added to other hosting platforms in the
  69 future.</p>
  70 <p>If controlled distribution through the Chrome Web Store is an important part of
  71 your product plan, the benefits of PNaCl are less critical for you. But you can
  72 still use the PNaCl toolchain and distribute your application through the Chrome
  73 Web Store, and thereby take advantage of the conveniences of PNaCl, such as not
  74 having to explicitly compile your application for all supported architectures.</p>
  75 <h2 id="when-to-use-nacl"><span id="id5"></span>When to use NaCl</h2>
  76 <p>Use NaCl if any of the following apply to your application:</p>
  77 <ul class="small-gap">
  78 <li>Your application requires architecture-specific instructions such as, for
  79 example, inline assembly. PNaCl tries to offer high-performance portable
  80 equivalents. One such example is PNaCl&#8217;s <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#portable-simd-vectors"><em>Portable SIMD Vectors</em></a>.</li>
  81 <li>Your application uses dynamic linking. PNaCl only supports static linking
  82 with a PNaCl port of the <code>newlib</code> C standard library. Dynamic linking and
  83 <code>glibc</code> are not yet supported in PNaCl. Work is under way to enable dynamic
  84 linking in future versions of PNaCl.</li>
  85 <li>Your application uses certain GNU extensions not supported by PNaCl&#8217;s LLVM
  86 toolchain, like taking the address of a label for computed <code>goto</code>, or nested
  87 functions.</li>
  88 </ul>
  89 </section>
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