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Build for WebAssembly

Prerequisites

If you haven't already done so, please read this document to understand the build requirements for your operating system.

The correct version of Emscripten SDK (emsdk) needs to be installed.

  • Run make -C src/mono/wasm provision-wasm to install emsdk into src/mono/wasm/emsdk.
  • Alternatively follow the installation guide. Do not install latest but rather specific version e.g. ./emsdk install 2.0.23. See emscripten-version.txt

Once installed the EMSDK_PATH environment variable needs to be set:

On Linux and macOS:

export EMSDK_PATH=<FULL_PATH_TO_SDK_INSTALL>/emsdk

Building everything

At this time no other build dependencies are necessary to start building for WebAssembly.

This document explains how to work on the runtime or libraries. If you haven't already done so, please read this document to understand configurations.

For Linux and macOS:

./build.sh -os Browser -configuration Release

Artifacts will be placed in artifacts/bin/microsoft.netcore.app.runtime.browser-wasm/Release/. When rebuilding with build.sh after a code change, you need to ensure that the mono.wasmruntime and libs.pretest subsets are included even for a Mono-only change or this directory will not be updated (details below).

Note: do not mix runtime and library configurations

At this time, it is not possible to specify different configurations for the runtime and libraries. That is mixing a Release -runtimeConfiguration with a Debug -libraryConfiguration (or -configuration), or vice versa will not work.

Please only use the -configuration option with Debug or Release, and do not specify a -runtimeConfiguration and -libraryConfiguration.

This is tracked in dotnet#42553

Building Mono's System.Private.CoreLib or runtime

If you are working on core parts of Mono you will probably need to build the Mono runtime and System.Private.CoreLib which can be built with the following:

./build.sh mono -os Browser -c Debug|Release

To build just System.Private.CoreLib without the Mono runtime you can use the Mono.CoreLib subset:

./build.sh mono.corelib -os Browser -c Debug|Release

To build just the Mono runtime without System.Private.CoreLib use the Mono.Runtime subset:

./build.sh mono.runtime -os Browser -c Debug|Release

Building both Mono/System.Private.CoreLib and the managed libraries:

./build.sh mono+libs -os Browser -c Debug|Release

Building the WebAssembly runtime files

The WebAssembly implementation files are built after the libraries source build and made available in the artifacts folder. If you are working on the code base and need to compile just these modules then building the Mono.WasmRuntime subset will allow one to do that:

./build.sh mono.wasmruntime -os Browser -c Debug|Release

Updating in-tree runtime pack

If you don't run the full Libs subset then you can use the Libs.PreTest subset to copy updated runtime/corelib binaries to the runtime pack which is used for running tests:

./build.sh libs.pretest -os Browser -c Debug|Release

Building libraries native components only

The libraries build contains some native code. This includes shims over libc, openssl, gssapi, and zlib. The build system uses CMake to generate Makefiles using clang. The build also uses git for generating some version information.

./build.sh libs.native -os Browser -c Debug|Release

Building individual libraries

Individual projects and libraries can be build by specifying the build configuration.

Examples

  • Build all projects for a given library (e.g.: System.Net.Http) including the tests
./build.sh -os Browser -c Release --projects <full-repository-path>/src/libraries/System.Net.Http/System.Net.Http.sln
  • Build only the source project of a given library (e.g.: System.Net.Http)
 ./build.sh -os Browser -c Release --projects <full-repository-path>/src/libraries/System.Net.Http/src/System.Net.Http.csproj

More information and examples can be found in the libraries document.

Notes

A Debug build sets the following environment variables by default:

  • debugging and logging which will log garbage collection information to the console.
MONO_LOG_LEVEL=debug
MONO_LOG_MASK=gc

Example:

L: GC_MAJOR_SWEEP: major size: 752K in use: 39K
L: GC_MAJOR: (user request) time 3.00ms, stw 3.00ms los size: 0K in use: 0K
  • Redirects the System.Diagnostics.Debug output to stderr which will show up on the console.
    // Setting this env var allows Diagnostic.Debug to write to stderr.  In a browser environment this
    // output will be sent to the console.  Right now this is the only way to emit debug logging from
    // corlib assemblies.
    monoeg_g_setenv ("COMPlus_DebugWriteToStdErr", "1", 0);

Updating Emscripten version in Docker image

First update emscripten version in the webassembly Dockerfile.

ENV EMSCRIPTEN_VERSION=1.39.16

Submit a PR request with the updated version, wait for all checks to pass and for the request to be merged. A master.json file will be updated with the a new docker image.

{
  "platforms": [
    {
      "dockerfile": "src/ubuntu/18.04/webassembly/Dockerfile",
      "simpleTags": [
        "ubuntu-18.04-webassembly-20210707133424-12f133e"
      ],
      "digest": "sha256:1f2d920a70bd8d55bbb329e87c3bd732ef930d64ff288dab4af0aa700c25cfaf",
      "osType": "Linux",
      "osVersion": "Ubuntu 18.04",
      "architecture": "amd64",
      "created": "2020-05-29T22:16:52.5716294Z",
      "commitUrl": "https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/6a6da637580ec557fd3708f86291f3ead2422697/src/ubuntu/18.04/webassembly/Dockerfile"
    }
  ]
},

Copy the docker image tag and replace it in platform-matrix.yml

container:
    image: ubuntu-18.04-webassembly-20210707133424-12f133e
    registry: mcr

Open a PR request with the new image.