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1{{+bindTo:partials.standard_nacl_article}} 2 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="id3">Native Client (NaCl)</a></li> 10<li><a class="reference internal" href="#portable-native-client-pnacl" id="id4">Portable Native Client (PNaCl)</a></li> 11<li><a class="reference internal" href="#when-to-use-pnacl" id="id5">When to use PNaCl</a></li> 12<li><a class="reference internal" href="#when-to-use-nacl" id="id6">When to use NaCl</a></li> 13</ul> 14 15</div><section id="native-client-nacl"> 16<h2 id="native-client-nacl">Native Client (NaCl)</h2> 17<p>Native Client enables the execution of native code 18securely inside web applications through the use of advanced 19<a class="reference external" href="https://developers.google.com/native-client/community/talks#research">Software Fault Isolation (SFI) techniques</a>. 20Since its launch in 2011, Native Client has provided 21developers with the ability to harness a client machine’s computational power 22to a much fuller extent than traditional web technologies, by running compiled C 23and C++ code at near-native speeds and taking advantage of multiple cores with 24shared memory.</p> 25<p>While Native Client provides operating system independence, it requires 26developers to generate architecture-specific executable modules 27(<strong>nexe</strong> modules) for each hardware platform. This is not only inconvenient 28for developers, but architecture-specific machine code is not portable and thus 29not well-suited for the open web. The traditional method of application 30distribution on the web is through a self-contained bundle of HTML, CSS, 31JavaScript, and other resources (images, etc.) that can be hosted on a server 32and run inside a web browser. With this type of distribution, a website 33created today should still work years later, on all platforms. 34Architecture-specific executables are clearly not a good fit for distribution 35on the web. As a consequence, Native Client has been restricted to 36applications and browser extensions that are installed through the 37Chrome Web Store.</p> 38</section><section id="portable-native-client-pnacl"> 39<h2 id="portable-native-client-pnacl">Portable Native Client (PNaCl)</h2> 40<p>PNaCl solves the portability problem by splitting the compilation process 41into two parts:</p> 42<ol class="arabic simple"> 43<li>compiling the source code to a portable bitcode format, and</li> 44<li>translating the bitcode to a host-specific executable.</li> 45</ol> 46<p>PNaCl enables developers 47to distribute <strong>portable executables</strong> (<strong>pexe</strong> modules) that the hosting 48environment (e.g., the Chrome browser) can translate to native code before 49executing. This portability aligns Native Client with existing open web 50technologies such as JavaScript: A developer can distribute a <strong>pexe</strong> 51as part of an application (along with HTML, CSS, and JavaScript), 52and the user’s machine is simply able to run it.</p> 53<p>With PNaCl, a developer generates a single <strong>pexe</strong> from source code, 54rather than multiple platform-specific nexes. The <strong>pexe</strong> provides both 55architecture- and OS-independence. Since the <strong>pexe</strong> uses an abstract, 56architecture-independent format, it does not suffer from the portability 57problem described above. Future versions of hosting environments should 58have no problem executing the <strong>pexe</strong>, even on new architectures. 59Moreover, if an existing architecture is subsequently enhanced, the 60<strong>pexe</strong> doesn’t even have to be recompiled—in some cases the 61client-side translation will automatically be able to take advantage of 62the new capabilities.</p> 63<p><strong>In short, PNaCl combines the portability of existing web technologies with 64the performance and security benefits of Native Client.</strong></p> 65<p>With the advent of PNaCl, the distribution restriction of Native Client 66can be lifted. Specifically, a <strong>pexe</strong> module can be part of any web 67application—it does not have to be distributed through the Chrome Web 68Store.</p> 69<p>PNaCl is a new technology, and as such it still has a few limitations 70as compared to NaCl. These limitations are described below.</p> 71</section><section id="when-to-use-pnacl"> 72<h2 id="when-to-use-pnacl">When to use PNaCl</h2> 73<p>PNaCl is the preferred toolchain for Native Client, and the only way to deploy 74Native Client modules on the open web. Unless your project is subject to one 75of the narrow limitations described below 76(see <a class="reference internal" href="#when-to-use-nacl"><em>When to use NaCl</em></a>), you should use PNaCl.</p> 77<p>Beginning with version 31, the Chrome browser supports translation of 78<strong>pexe</strong> modules and their use in web applications, without requiring 79any installation (either of a browser plugin or of the applications 80themselves). Native Client and PNaCl are open-source technologies, and 81our hope is that they will be added to other hosting platforms in the 82future.</p> 83<p>If controlled distribution through the Chrome Web Store is an important part 84of your product plan, the benefits of PNaCl are less critical for you. But 85you can still use the PNaCl toolchain and distribute your application 86through the Chrome Web Store, and thereby take advantage of the 87conveniences of PNaCl, such as not having to explicitly compile your application 88for all supported architectures.</p> 89</section><section id="when-to-use-nacl"> 90<span id="id2"></span><h2 id="when-to-use-nacl"><span id="id2"></span>When to use NaCl</h2> 91<p>The limitations below apply to the current release of PNaCl. If any of 92these limitations are critical for your application, you should use 93non-portable NaCl:</p> 94<ul class="small-gap"> 95<li>By its nature, PNaCl does not support architecture-specific instructions in 96an application (i.e., inline assembly). Future editions of PNaCl will 97attempt to mitigate this problem by introducing portable intrinsics for vector 98operations.</li> 99<li>Currently PNaCl only supports static linking with the <code>newlib</code> 100C standard library (the Native Client SDK provides a PNaCl port of 101<code>newlib</code>). Dynamic linking and <code>glibc</code> are not yet supported. 102Work is under way to enable dynamic linking in future versions of PNaCl.</li> 103<li>In the initial release, PNaCl does not support vector types and SIMD.</li> 104<li>In the initial release, PNaCl does not support some GNU extensions 105like taking the address of a label for computed <code>goto</code>, or nested 106functions.</li> 107</ul> 108</section></section> 109 110{{/partials.standard_nacl_article}} 111