1.. _nacl-and-pnacl: 2 3############## 4NaCl and PNaCl 5############## 6 7This document describes the differences between **Native Client** and 8**Portable Native Client**, and provides recommendations for when to use each. 9 10.. contents:: 11 :local: 12 :backlinks: none 13 :depth: 2 14 15Native Client (NaCl) 16==================== 17 18Native Client enables the execution of native code 19securely inside web applications through the use of advanced 20`Software Fault Isolation (SFI) techniques <https://developers.google.com/native-client/community/talks#research>`_. 21Since its launch in 2011, Native Client has provided 22developers with the ability to harness a client machine's computational power 23to a much fuller extent than traditional web technologies, by running compiled C 24and C++ code at near-native speeds and taking advantage of multiple cores with 25shared memory. 26 27While Native Client provides operating system independence, it requires 28developers to generate architecture-specific executable modules 29(**nexe** modules) for each hardware platform. This is not only inconvenient 30for developers, but architecture-specific machine code is not portable and thus 31not well-suited for the open web. The traditional method of application 32distribution on the web is through a self-contained bundle of HTML, CSS, 33JavaScript, and other resources (images, etc.) that can be hosted on a server 34and run inside a web browser. With this type of distribution, a website 35created today should still work years later, on all platforms. 36Architecture-specific executables are clearly not a good fit for distribution 37on the web. As a consequence, Native Client has been restricted to 38applications and browser extensions that are installed through the 39Chrome Web Store. 40 41Portable Native Client (PNaCl) 42============================== 43 44PNaCl solves the portability problem by splitting the compilation process 45into two parts: 46 47#. compiling the source code to a portable bitcode format, and 48#. translating the bitcode to a host-specific executable. 49 50PNaCl enables developers 51to distribute **portable executables** (**pexe** modules) that the hosting 52environment (e.g., the Chrome browser) can translate to native code before 53executing. This portability aligns Native Client with existing open web 54technologies such as JavaScript: A developer can distribute a **pexe** 55as part of an application (along with HTML, CSS, and JavaScript), 56and the user's machine is simply able to run it. 57 58With PNaCl, a developer generates a single **pexe** from source code, 59rather than multiple platform-specific nexes. The **pexe** provides both 60architecture- and OS-independence. Since the **pexe** uses an abstract, 61architecture-independent format, it does not suffer from the portability 62problem described above. Future versions of hosting environments should 63have no problem executing the **pexe**, even on new architectures. 64Moreover, if an existing architecture is subsequently enhanced, the 65**pexe** doesn't even have to be recompiled---in some cases the 66client-side translation will automatically be able to take advantage of 67the new capabilities. 68 69**In short, PNaCl combines the portability of existing web technologies with 70the performance and security benefits of Native Client.** 71 72With the advent of PNaCl, the distribution restriction of Native Client 73can be lifted. Specifically, a **pexe** module can be part of any web 74application---it does not have to be distributed through the Chrome Web 75Store. 76 77PNaCl is a new technology, and as such it still has a few limitations 78as compared to NaCl. These limitations are described below. 79 80When to use PNaCl 81================= 82 83PNaCl is the preferred toolchain for Native Client, and the only way to deploy 84Native Client modules on the open web. Unless your project is subject to one 85of the narrow limitations described below 86(see :ref:`When to use NaCl<when-to-use-nacl>`), you should use PNaCl. 87 88Beginning with version 31, the Chrome browser supports translation of 89**pexe** modules and their use in web applications, without requiring 90any installation (either of a browser plugin or of the applications 91themselves). Native Client and PNaCl are open-source technologies, and 92our hope is that they will be added to other hosting platforms in the 93future. 94 95If controlled distribution through the Chrome Web Store is an important part 96of your product plan, the benefits of PNaCl are less critical for you. But 97you can still use the PNaCl toolchain and distribute your application 98through the Chrome Web Store, and thereby take advantage of the 99conveniences of PNaCl, such as not having to explicitly compile your application 100for all supported architectures. 101 102.. _when-to-use-nacl: 103 104When to use NaCl 105================ 106 107The limitations below apply to the current release of PNaCl. If any of 108these limitations are critical for your application, you should use 109non-portable NaCl: 110 111* By its nature, PNaCl does not support architecture-specific instructions in 112 an application (i.e., inline assembly). Future editions of PNaCl will 113 attempt to mitigate this problem by introducing portable intrinsics for vector 114 operations. 115* Currently PNaCl only supports static linking with the ``newlib`` 116 C standard library (the Native Client SDK provides a PNaCl port of 117 ``newlib``). Dynamic linking and ``glibc`` are not yet supported. 118 Work is under way to enable dynamic linking in future versions of PNaCl. 119* In the initial release, PNaCl does not support vector types and SIMD. 120* In the initial release, PNaCl does not support some GNU extensions 121 like taking the address of a label for computed ``goto``, or nested 122 functions. 123