Native Client

Technical Overview


Native Client (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.

Google has implemented the open-source Native Client project in the Chrome browser on Windows, Mac, Linux, and Chrome OS. The Native Client Software Development Kit (SDK), itself an open-source project, lets developers create web applications that use NaCl and run in Chrome across multiple platforms.

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.


Why use Native Client?

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—it provides the means to fully harness the client’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.

Here are a few of the key benefits that Native Client offers:

  • Graphics, audio, and much more: 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—all without requiring the user to install a plugin.
  • Portability: Write your applications once and you’ll be able to run them across operating systems (Windows, Linux, Mac, and Chrome OS) and CPU architectures (x86 and ARM).
  • Easy migration path to the web: 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++.
  • Security: Native Client uses a double sandbox model designed to protect the user’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.
  • Performance: 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.

Common use cases

Typical use cases for Native Client include the following:

  • Existing software components: With support for C and C++, Native Client enables you to reuse existing software modules in web applications—you don’t need to rewrite and debug code that’s already proven to work well.
  • Legacy desktop applications: 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.
  • Heavy computation in enterprise applications: 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.
  • Multimedia applications: Codecs for processing sounds, images, and movies can be added to the browser in a Native Client module.
  • Games: 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.
  • Any application that requires acceleration: Native Client fits seamlessly into web applications—it’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.

How Native Client works

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.

At a high level, Native Client consists of:

  • Toolchains: collections of development tools (compilers, linkers, etc.) that transform C/C++ code to Native Client modules.
  • Runtime components: components embedded in the browser or other host platforms that allow execution of Native Client modules securely and efficiently.

The following diagram shows how these components interact:


The left side of the diagram shows how to use Portable Native Client (PNaCl, pronounced “pinnacle”). Developers use the PNaCl toolchain to produce a single, portable (pexe) module. At runtime, a translator built into the browser translates the pexe into native code for the relevant client architecture.

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 (nexe) modules, which are packaged into an application. At runtime, the browser decides which nexe to load based on the architecture of the client machine.


Since Native Client permits the execution of native code on client machines, special security measures have to be implemented:

  • 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.
  • 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.

The above security measures are in addition to the existing sandbox in the Chrome browser—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.


Portable Native Client (PNaCl, prounounced “pinnacle”) employs state-of-the-art compiler technology to compile C/C++ source code to a portable bitcode executable (pexe). PNaCl bitcode is an OS- and architecture-independent format that can be freely distributed on the web and embedded in web applications.

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’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’s browser.

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—they can only be used as part of applications and extensions that are installed from the Chrome Web Store.

For more details on the difference between NaCl and PNaCl, see NaCl and PNaCl.


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.

The Native Client SDK provides two toolchains:

  • a PNaCl toolchain for generating portable NaCl modules (pexe files)
  • a gcc-based toolchain (nacl-gcc) for generating non-portable NaCl modules (nexe files)

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.

Native Client in a web application

A Native Client application consists of a set of files:

  • HTML, CSS, and JavaScript files, as in any modern web application. The JavaScript code is responsible for communicating with the NaCl module.
  • A pexe (portable NaCl) file. This module uses the Pepper API, which provides the bridge to JavaScript and browser resources.
  • A Native Client manifest file that specifies the pexe to load, along with some loading options. This manifest file is embedded into the HTML page through an <embed> tag, as shown in the figure below.

For more details, see Application Structure.

Pepper Plugin API

The Pepper Plugin API (PPAPI), called Pepper 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.

You can use the Pepper APIs to gain access to the full array of browser capabilities, including:

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 Pepper Concepts.


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 versions of the Pepper APIs to help developers make adjustments to API changes and take advantage of new features.

Where to start

The Quick Start document provides links to downloads and documentation that should help you get started with developing and distributing Native Client applications.

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