1 .. _nacl-and-pnacl: 2 3 ############## 4 NaCl and PNaCl 5 ############## 6 7 This 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 15 Native Client (NaCl) 16 ==================== 17 18 Native Client enables the execution of native code securely inside web 19 applications through the use of advanced `Software Fault Isolation (SFI) 20 techniques </native-client/community/talks#research>`_. Since its launch in 21 2011, Native Client has provided developers with the ability to harness a 22 client machine's computational power to a much fuller extent than traditional 23 web technologies, by running compiled C and C++ code at near-native speeds and 24 taking advantage of multiple cores with shared memory. 25 26 While Native Client provides operating system independence, it requires 27 developers to generate architecture-specific executable modules 28 (**nexe** modules) for each hardware platform. This is not only inconvenient 29 for developers, but architecture-specific machine code is not portable and thus 30 not well-suited for the open web. The traditional method of application 31 distribution on the web is through a self-contained bundle of HTML, CSS, 32 JavaScript, and other resources (images, etc.) that can be hosted on a server 33 and run inside a web browser. With this type of distribution, a website 34 created today should still work years later, on all platforms. 35 Architecture-specific executables are clearly not a good fit for distribution 36 on the web. As a consequence, Native Client has been restricted to 37 applications and browser extensions that are installed through the 38 Chrome Web Store. 39 40 Portable Native Client (PNaCl) 41 ============================== 42 43 PNaCl solves the portability problem by splitting the compilation process 44 into two parts: 45 46 #. compiling the source code to a portable bitcode format, and 47 #. translating the bitcode to a host-specific executable. 48 49 PNaCl enables developers 50 to distribute **portable executables** (**pexe** modules) that the hosting 51 environment (e.g., the Chrome browser) can translate to native code before 52 executing. This portability aligns Native Client with existing open web 53 technologies such as JavaScript: A developer can distribute a **pexe** 54 as part of an application (along with HTML, CSS, and JavaScript), 55 and the user's machine is simply able to run it. 56 57 With PNaCl, a developer generates a single **pexe** from source code, 58 rather than multiple platform-specific nexes. The **pexe** provides both 59 architecture- and OS-independence. Since the **pexe** uses an abstract, 60 architecture-independent format, it does not suffer from the portability 61 problem described above. Future versions of hosting environments should 62 have no problem executing the **pexe**, even on new architectures. 63 Moreover, if an existing architecture is subsequently enhanced, the 64 **pexe** doesn't even have to be recompiled---in some cases the 65 client-side translation will automatically be able to take advantage of 66 the new capabilities. 67 68 **In short, PNaCl combines the portability of existing web technologies with 69 the performance and security benefits of Native Client.** 70 71 With the advent of PNaCl, the distribution restriction of Native Client 72 can be lifted. Specifically, a **pexe** module can be part of any web 73 application---it does not have to be distributed through the Chrome Web 74 Store. 75 76 PNaCl is a new technology, and as such it still has a few limitations 77 as compared to NaCl. These limitations are described below. 78 79 When to use PNaCl 80 ================= 81 82 PNaCl is the preferred toolchain for Native Client, and the only way to deploy 83 Native Client modules on the open web. Unless your project is subject to one 84 of the narrow limitations described below 85 (see :ref:`When to use NaCl<when-to-use-nacl>`), you should use PNaCl. 86 87 Beginning with version 31, the Chrome browser supports translation of 88 **pexe** modules and their use in web applications, without requiring 89 any installation (either of a browser plugin or of the applications 90 themselves). Native Client and PNaCl are open-source technologies, and 91 our hope is that they will be added to other hosting platforms in the 92 future. 93 94 If controlled distribution through the Chrome Web Store is an important part 95 of your product plan, the benefits of PNaCl are less critical for you. But 96 you can still use the PNaCl toolchain and distribute your application 97 through the Chrome Web Store, and thereby take advantage of the 98 conveniences of PNaCl, such as not having to explicitly compile your application 99 for all supported architectures. 100 101 .. _when-to-use-nacl: 102 103 When to use NaCl 104 ================ 105 106 The limitations below apply to the current release of PNaCl. If any of 107 these limitations are critical for your application, you should use 108 non-portable NaCl: 109 110 * By its nature, PNaCl does not support architecture-specific 111 instructions in an application (i.e., inline assembly), but tries to 112 offer high-performance portable equivalents. One such example is 113 PNaCl's :ref:`Portable SIMD Vectors <portable_simd_vectors>`. 114 * Currently PNaCl only supports static linking with the ``newlib`` 115 C standard library (the Native Client SDK provides a PNaCl port of 116 ``newlib``). Dynamic linking and ``glibc`` are not yet supported. 117 Work is under way to enable dynamic linking in future versions of PNaCl. 118 * In the initial release, PNaCl does not support some GNU extensions 119 like taking the address of a label for computed ``goto``, or nested 120 functions. 121
