Web native plug-ins for Emscripten

Unity uses the Emscripten compiler toolchain to cross-compile the Unity runtime code written in C and C++ into WebAssembly (also called Wasm). Emscripten generates small code, is load-time and memory efficient, and aims to deliver close to native speed execution.

Unity uses IL2CPPA Unity-developed scripting back-end which you can use as an alternative to Mono when building projects for some platforms. More info
See in Glossary to convert the .NET game code in your C# scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary to Wasm. IL2CPP takes .NET bytecode and converts it to corresponding C++ source files, which is then compiled using Emscripten to convert your scripts to Wasm.

Targeted versions

Unity bundles the Emscripten compiler with the Web platform package, so that you don’t have to manually download it. The version of Emscripten that the Unity Editor uses varies by Editor version:

Notes:

• The following list provides expansion for the acronyms of file formats listed in the table:

  • GNU archive file format (.a)
  • WebAssembly Object Files (.o)
  • LLVM Bitcode file format (.bc)

• With Emscripten versions before Emscripten 2.0, Unity recommends building Bitcode plug-ins (files of type .bc). From Emscripten 2.0 onwards, Unity recommends to build Wasm Object File plug-ins that are Wasm object files of type .o, bundled into GNU archive files of type .a.

You can determine the path to the exact version of Emscripten toolchain from the emscripten-version.txt located at:

C:\Program Files\Unity\Hub\Editor\<Editor version>\Editor\Data\PlaybackEngines\WebGLSupport\BuildTools\Emscripten\emscripten\emscripten-version.txt

Compile native plug-ins with Emscripten

If you’re creating native C/C++ plug-ins for your Unity application, you can either bundle the C/C++ code with your project (refer to Native plug-insA platform-specific native code library that is created outside of Unity for use in Unity. Allows you can access features like OS calls and third-party code libraries that would otherwise not be available to Unity. More info
See in Glossary) or build the native code to a plug-in archive ahead-of-time with the Emscripten compiler toolchain.

If you choose to build plug-in code in advance, use the Emscripten compiler toolchain. To ensure LLVM binary format compatibility, the version of Emscripten that’s used to compile the plug-ins must match the version of Emscripten that Unity uses.

Typically, plug-ins are compiled to a static library archive format. For versions before Unity 2021.2 (Emscripten 2.0), LLVM Bitcode file format (.bc) is the preferred plug-in format.

From Unity 2021.2, the preferred plug-in format is GNU archive file format (.a) that contain WebAssembly Object Files (.o). Although, earlier LLVM Bitcode .bc plug-in files are still supported, using them might have a negative impact to compilation times.

Note: When migrating a native plug-in from one Unity version to a version that uses a different version of Emscripten, the recommended best practice is to recompile the Unity plug-in from source, because the LLVM compiler project doesn’t guarantee binary compatibility of build artifact files across compiler versions.

Emscripten build flags

When you compile your C/C++ code, include specific build flags to ensure compatibility with certain Player Settings (menu: Edit > Project Settings > Player).

Without these flags, the plug-in might not link correctly with the code generated by Unity.

Note: Setting Maximum Memory Size to more than 4 GB enables WebAssembly 64-bit support, which also enables Target WebAssembly 2023.

Support different Player Settings in a native plug-in

To support different Player SettingsSettings that let you set various player-specific options for the final game built by Unity. More info
See in Glossary in a native plug-in, compile multiple versions of the plug-in using the Emscripten build flags and add them to your project.

You can then use the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary to configure which plug-in version Unity includes in the build, depending on the active Player Settings. For each native plug-in, the Inspector provides the following Web platform settings:

Unity automatically selects the correct plug-in variant at build time based on the active Player Settings. If multiple plug-ins match the same configuration, or no plug-in matches, Unity reports an error.

Additional resources

• Import and configure plug-ins
• Native plug-ins