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Web native plug-ins for Emscripten

Web native plug-ins for Emscripten.
Read time 3 minutesLast updated 12 days ago
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 IL2CPP to convert the .NET game code in your C# scripts 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 Editor uses varies by Editor version:

Unity Version

Emscripten version

Supported native plug-in file formats

Unity 2023.2 and laterEmscripten 3.1.38-unity.a, .bc
Unity 2022.2 and laterEmscripten 3.1.8-unity.a, .bc
Unity 2021.2 and laterEmscripten 2.0.19.6-unity.a, .bc
Unity 2019.2 to 2021.11.38.11-unity.bc
Unity 2018.4 up until Unity 2019.21.37.40-unity.bc
Unity 2018.2 up until Unity 2018.41.37.33-unity.bc
Notes:
  • The following list provides expansion for the acronyms of file formats listed in the above 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 (see Native plug-ins) 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, you should 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, Unity recommends 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

If compiling WebAssembly Object Files to use with WebAssembly 2023 or C/C++ multithreading, include the following Emscripten build flags. Without these flags, the code might not link properly with code built with Unity.

Target

Required Emscripten build flags

WebAssembly 2023
-msimd128
-fwasm-exceptions
-mbulk-memory
-mnontrapping-fptoint
-msse4.2
-sWASM_BIGINT
-sSUPPORT_LONGJMP=wasm
C/C++ native multithreading
-pthread

Additional resources: