.. _about-emscripten:

================ About Emscripten

Emscripten is a complete :ref:Open Source <emscripten-license> compiler toolchain to WebAssembly. Using Emscripten you can:

• Compile C and C++ code, or any other language that uses LLVM, into WebAssembly, and run it on the Web, Node.js, or other Wasm runtimes.
• Compile the C/C++ runtimes of other languages into WebAssembly, and then run code in those other languages in an indirect way (for example, this has been done for Python <https://github.com/pyodide/pyodide>_ and Lua <https://daurnimator.github.io/lua.vm.js/lua.vm.js.html>_).

Practically any portable C or C++ codebase can be compiled into WebAssembly using Emscripten, ranging from high-performance games that need to render graphics, play sounds, and load and process files, through to application frameworks like Qt. Emscripten has already been used to convert a very long list of real-world codebases to WebAssembly, including commercial codebases like the Unreal Engine 4 <https://blog.mozilla.org/blog/2014/03/12/mozilla-and-epic-preview-unreal-engine-4-running-in-firefox/>_ and the Unity <https://blogs.unity3d.com/2018/08/15/webassembly-is-here/>_ engine. For examples and demos, see the community-maintained list on the wiki <https://github.com/emscripten-core/emscripten/wiki/Porting-Examples-and-Demos>_.

Emscripten generates small and fast code! Its default output format is WebAssembly <http://webassembly.org/>_ , a highly optimizable executable format, that runs almost as fast as native code, while being portable and safe. Emscripten does a lot of optimization work for you automatically, by careful integration with LLVM, Binaryen <https://github.com/WebAssembly/binaryen>, Closure Compiler <https://developers.google.com/closure/compiler>, and other tools.

.. _about-emscripten-toolchain:

Emscripten Toolchain

A high level view of the Emscripten toolchain is given below. The main tool is the :ref:emccdoc. This is a drop-in replacement for a standard compiler like gcc or clang.

Emcc uses :term:Clang and LLVM to compile to WebAssembly. Emcc also emits JavaScript that provides API support to the compiled code. That JavaScript can be executed by :term:Node.js, or from within HTML in a browser.

The :ref:Emscripten SDK <sdk-download-and-install> is used to install the entire toolchain, including emcc and LLVM and so forth. The :ref:emsdk can be used on Linux, Windows or MacOS.

.. _about-emscripten-porting-code:

Porting code to use Emscripten

Emscripten support for portable C/C++ code is fairly comprehensive. Support for the C standard library, C++ standard library, C++ exceptions, etc. is very good, as is SDL2 <https://www.libsdl.org/>_ and other APIs. :ref:OpenGL-support support is excellent for OpenGL ES 2.0-type code, and acceptable for other types.

There are differences between the native and :ref:emscripten-runtime-environment, which mean some changes usually need to be made to the native code. That said, many applications will only need to change the way they define their main loop, and also modify their :ref:file handling <file-system-overview> to adapt to the limitations of the browser/JavaScript.

There are also limitations that can make some code easier to port — read :ref:code-portability-guidelines to determine where you may need to spend more effort.