emnapi

Sponsors


Node-API implementation for Emscripten, wasi-sdk and clang with wasm support.

This project aims to

- Help users port their or existing Node-API native addons to wasm with code change as less as possible.
- Make runtime behavior matches native Node.js as much as possible.

This project also powers the WebAssembly feature for napi-rs, and enables many Node.js native addons to run on StackBlitz's WebContainer.

Node-API changes will be synchronized into this repo.

See documentation for more details:
- https://toyobayashi.github.io/emnapi-docs/guide/
- https://emnapi-docs.vercel.app/guide/

中文文档：
- https://toyobayashi.github.io/emnapi-docs/zh/guide/
- https://emnapi-docs.vercel.app/zh/guide/

Full API List

How to build Node-API official examples

If you want to deep dive into WebAssembly, highly recommend you to visit learn-wasm.dev.

Prerequests

You will need to install:

- Node.js >= v16.15.0
- npm >= v8
- Emscripten >= v3.1.9 / wasi-sdk / LLVM clang with wasm support
- (Optional) CMake >= v3.13
- (Optional) node-gyp >= v10.2.0
- (Optional) ninja
- (Optional) make
- (Optional) node-addon-api >= 6.1.0

There are several choices to get make for Windows user

- Install mingw-w64, then use mingw32-make
- Download MSVC prebuilt binary of GNU make, add to %Path% then rename it to mingw32-make
- Install Visual Studio 2022 C++ desktop workload, use nmake in Visual Studio Developer Command Prompt
- Install Visual C++ Build Tools, use nmake in Visual Studio Developer Command Prompt

Verify your environment:

``bash
node -v
npm -v
emcc -v

clang -v

clang -print-targets # ensure wasm32 target exists

cmake --version

if you use node-gyp

node-gyp --version

if you use ninja

ninja --version

if you use make

make -v

if you use nmake in Visual Studio Developer Command Prompt

nmake /?
`

Build from source

You need to set EMSDK and WASI_SDK_PATH environment variables.

`bash
git clone https://github.com/toyobayashi/emnapi.git
cd ./emnapi
npm install -g node-gyp
npm install
npm run build # output ./packages/*/dist
node ./script/release.js # output ./out

test


npm run rebuild:test
npm test
`

See CONTRIBUTING for more details.

Quick Start

$3

`bash
npm install -D emnapi
npm install @emnapi/runtime

for non-emscripten

npm install @emnapi/core

if you use node-addon-api

npm install node-addon-api
`

Each package should match the same version.

$3

Create hello.c.

`c
#include

#define NODE_API_CALL(env, the_call) \
do { \
if ((the_call) != napi_ok) { \
const napi_extended_error_info *error_info; \
napi_get_last_error_info((env), &error_info); \
bool is_pending; \
const char* err_message = error_info->error_message; \
napi_is_exception_pending((env), &is_pending); \
if (!is_pending) { \
const char* error_message = err_message != NULL ? \
err_message : \
"empty error message"; \
napi_throw_error((env), NULL, error_message); \
} \
return NULL; \
} \
} while (0)

static napi_value js_hello(napi_env env, napi_callback_info info) {
napi_value world;
const char* str = "world";
NODE_API_CALL(env, napi_create_string_utf8(env, str, NAPI_AUTO_LENGTH, &world));
return world;
}

NAPI_MODULE_INIT() {
napi_value hello;
NODE_API_CALL(env, napi_create_function(env, "hello", NAPI_AUTO_LENGTH,
js_hello, NULL, &hello));
NODE_API_CALL(env, napi_set_named_property(env, exports, "hello", hello));
return exports;
}
`

The C code is equivalant to the following JavaScript:

`js
module.exports = (function (exports) {
const hello = function hello () {
// native code in js_hello
const world = 'world'
return world
}

exports.hello = hello
return exports
})(module.exports)
`

#### Building

emscripten

`bash
emcc -O3 \
-DBUILDING_NODE_EXTENSION \
"-DNAPI_EXTERN=__attribute__((__import_module__(\"env\")))" \
-I./node_modules/emnapi/include/node \
-L./node_modules/emnapi/lib/wasm32-emscripten \
--js-library=./node_modules/emnapi/dist/library_napi.js \
-sEXPORTED_FUNCTIONS="['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']" \
-sEXPORTED_RUNTIME_METHODS=['emnapiInit'] \
-o hello.js \
hello.c \
-lemnapi
`

wasi-sdk

`bash
clang -O3 \
-DBUILDING_NODE_EXTENSION \
-I./node_modules/emnapi/include/node \
-L./node_modules/emnapi/lib/wasm32-wasi \
--target=wasm32-wasi \
--sysroot=$WASI_SDK_PATH/share/wasi-sysroot \
-mexec-model=reactor \
-Wl,--initial-memory=16777216 \
-Wl,--export-dynamic \
-Wl,--export=malloc \
-Wl,--export=free \
-Wl,--export=napi_register_wasm_v1 \
-Wl,--export-if-defined=node_api_module_get_api_version_v1 \
-Wl,--import-undefined \
-Wl,--export-table \
-o hello.wasm \
hello.c \
-lemnapi
`

clang wasm32

Choose libdlmalloc.a or libemmalloc.a for malloc and free.

`bash
clang -O3 \
-DBUILDING_NODE_EXTENSION \
-I./node_modules/emnapi/include/node \
-L./node_modules/emnapi/lib/wasm32 \
--target=wasm32 \
-nostdlib \
-Wl,--no-entry \
-Wl,--initial-memory=16777216 \
-Wl,--export-dynamic \
-Wl,--export=malloc \
-Wl,--export=free \
-Wl,--export=napi_register_wasm_v1 \
-Wl,--export-if-defined=node_api_module_get_api_version_v1 \
-Wl,--import-undefined \
-Wl,--export-table \
-o hello.wasm \
hello.c \
-lemnapi \
-ldlmalloc # -lemmalloc
`

#### Initialization

To initialize emnapi, you need to import the emnapi runtime to create a Context by createContext or getDefaultContext first.
Each context owns isolated Node-API object such as napi_env, napi_value, napi_ref. If you have multiple emnapi modules, you should reuse the same Context across them.

`ts
declare namespace emnapi {
// module '@emnapi/runtime'
export class Context { / ... / }
/* Create a new context /
export function createContext (): Context
/* Create or get /
export function getDefaultContext (): Context
// ...
}
`

emscripten

then call Module.emnapiInit after emscripten runtime initialized.
Module.emnapiInit only do initialization once, it will always return the same binding exports after successfully initialized.

`ts
declare namespace Module {
interface EmnapiInitOptions {
context: emnapi.Context

/* node_api_get_module_file_name /
filename?: string

/**
* Support following async_hooks related things
* on Node.js runtime only
*
* napi_async_init,
* napi_async_destroy,
* napi_make_callback,
* async resource parameter of
* napi_create_async_work and napi_create_threadsafe_function
*/
nodeBinding?: typeof import('@emnapi/node-binding')

/* See Multithread part /
asyncWorkPoolSize?: number
}
export function emnapiInit (options: EmnapiInitOptions): any
}
`

`html



`

If you are using Visual Studio Code and have Live Server extension installed, you can right click the HTML file in Visual Studio Code source tree and click Open With Live Server, then you can see the hello world alert!

Running on Node.js:

`js
const emnapi = require('@emnapi/runtime')
const Module = require('./hello.js')

Module.onRuntimeInitialized = function () {
let binding
try {
binding = Module.emnapiInit({ context: emnapi.getDefaultContext() })
} catch (err) {
console.error(err)
return
}
const msg = hello ${binding.hello()}
console.log(msg)
}

// if -sMODULARIZE=1
Module({ / Emscripten module init options / }).then((Module) => {
const binding = Module.emnapiInit({ context: emnapi.getDefaultContext() })
})
`

wasi-sdk or clang wasm32

For non-emscripten, you need to use @emnapi/core. The initialization is similar to emscripten.

`html



`

Using WASI on Node.js

`js
const { instantiateNapiModule } = require('@emnapi/core')
const { getDefaultContext } = require('@emnapi/runtime')
const { WASI } = require('wasi')
const fs = require('fs')

instantiateNapiModule(fs.promises.readFile('./hello.wasm'), {
wasi: new WASI({ / ... / }),
context: getDefaultContext(),
overwriteImports (importObject) {
// importObject.env = {
// ...importObject.env,
// ...importObject.napi,
// ...importObject.emnapi,
// // ...
// }
}
}).then(({ instance, module, napiModule }) => {
const binding = napiModule.exports
// ...
})
`

Using WASI on browser, you can use WASI polyfill in wasm-util,
and memfs-browser

`js
import { instantiateNapiModule } from '@emnapi/core'
import { getDefaultContext } from '@emnapi/runtime'
import { WASI } from '@tybys/wasm-util'
import { Volume, createFsFromVolume } from 'memfs-browser'

const fs = createFsFromVolume(Volume.fromJSON({ / ... / }))
instantiateNapiModule(fetch('./hello.wasm'), {
wasi: new WASI({ fs, / ... / })
context: getDefaultContext(),
overwriteImports (importObject) {
// importObject.env = {
// ...importObject.env,
// ...importObject.napi,
// ...importObject.emnapi,
// // ...
// }
}
}).then(({ instance, module, napiModule }) => {
const binding = napiModule.exports
// ...
})
`

$3

Require node-addon-api >= 6.1.0

`bash
npm install node-addon-api
`

Note: C++ wrapper can only be used to target Node.js v14.6.0+ and modern browsers those support FinalizationRegistry and WeakRef (v8 engine v8.4+)!

Create hello.cpp.

`cpp
#include

Napi::String Method(const Napi::CallbackInfo& info) {
Napi::Env env = info.Env();
return Napi::String::New(env, "world");
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
exports.Set(Napi::String::New(env, "hello"),
Napi::Function::New(env, Method)).Check();
return exports;
}

NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)
`

Compile hello.cpp using em++. C++ exception is disabled by Emscripten default, and not supported by wasi-sdk, so predefine -DNAPI_DISABLE_CPP_EXCEPTIONS and -DNODE_ADDON_API_ENABLE_MAYBE here. If you would like to enable C++ exception, use -sDISABLE_EXCEPTION_CATCHING=0 instead and remove .Check() call. See official documentation here.

#### Building

emscripten

`bash
em++ -O3 \
-DBUILDING_NODE_EXTENSION \
"-DNAPI_EXTERN=__attribute__((__import_module__(\"env\")))" \
-DNAPI_DISABLE_CPP_EXCEPTIONS \
-DNODE_ADDON_API_ENABLE_MAYBE \
-I./node_modules/emnapi/include/node \
-I./node_modules/node-addon-api \
-L./node_modules/emnapi/lib/wasm32-emscripten \
--js-library=./node_modules/emnapi/dist/library_napi.js \
-sEXPORTED_FUNCTIONS="['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']" \
-sEXPORTED_RUNTIME_METHODS=['emnapiInit'] \
-o hello.js \
hello.cpp \
-lemnapi
`

wasi-sdk

`bash
clang++ -O3 \
-DBUILDING_NODE_EXTENSION \
-DNAPI_DISABLE_CPP_EXCEPTIONS \
-DNODE_ADDON_API_ENABLE_MAYBE \
-I./node_modules/emnapi/include/node \
-I./node_modules/node-addon-api \
-L./node_modules/emnapi/lib/wasm32-wasi \
--target=wasm32-wasi \
--sysroot=$WASI_SDK_PATH/share/wasi-sysroot \
-fno-exceptions \
-mexec-model=reactor \
-Wl,--initial-memory=16777216 \
-Wl,--export-dynamic \
-Wl,--export=malloc \
-Wl,--export=free \
-Wl,--export=napi_register_wasm_v1 \
-Wl,--export-if-defined=node_api_module_get_api_version_v1 \
-Wl,--import-undefined \
-Wl,--export-table \
-o hello.wasm \
hello.cpp \
-lemnapi
`

clang wasm32

node-addon-api is using the C++ standard libraries, so you must use WASI if you are using node-addon-api.

You can still use wasm32-unknown-unknown target if you use Node-API C API only in C++.

`bash
clang++ -O3 \
-DBUILDING_NODE_EXTENSION \
-I./node_modules/emnapi/include/node \
-L./node_modules/emnapi/lib/wasm32 \
--target=wasm32 \
-fno-exceptions \
-nostdlib \
-Wl,--no-entry \
-Wl,--initial-memory=16777216 \
-Wl,--export-dynamic \
-Wl,--export=malloc \
-Wl,--export=free \
-Wl,--export=napi_register_wasm_v1 \
-Wl,--export-if-defined=node_api_module_get_api_version_v1 \
-Wl,--import-undefined \
-Wl,--export-table \
-o node_api_c_api_only.wasm \
node_api_c_api_only.cpp \
-lemnapi \
-ldlmalloc # -lemmalloc
`

operator new and operator delete.

`cpp
#include

extern "C" void* malloc(size_t size);
extern "C" void free(void* p);

void* operator new(size_t size) {
return malloc(size);
}

void operator delete(void* p) noexcept {
free(p);
}

void operator delete(void* p, size_t) noexcept {
free(p);
}
`

$3

Create CMakeLists.txt.

`cmake
cmake_minimum_required(VERSION 3.13)

project(emnapiexample)

add_subdirectory("${CMAKE_CURRENT_SOURCE_DIR}/node_modules/emnapi")

add_executable(hello hello.c)

target_link_libraries(hello emnapi)
if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
target_link_options(hello PRIVATE
"-sEXPORTED_FUNCTIONS=['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']"
"-sEXPORTED_RUNTIME_METHODS=['emnapiInit']"
)
elseif(CMAKE_SYSTEM_NAME STREQUAL "WASI")
set_target_properties(hello PROPERTIES SUFFIX ".wasm")
target_link_options(hello PRIVATE
"-mexec-model=reactor"
"-Wl,--export=napi_register_wasm_v1"
"-Wl,--export-if-defined=node_api_module_get_api_version_v1"
"-Wl,--initial-memory=16777216,--export-dynamic,--export=malloc,--export=free,--import-undefined,--export-table"
)
elseif((CMAKE_C_COMPILER_TARGET STREQUAL "wasm32") OR (CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-unknown-unknown"))
set_target_properties(hello PROPERTIES SUFFIX ".wasm")
target_link_options(hello PRIVATE
"-nostdlib"
"-Wl,--export=napi_register_wasm_v1"
"-Wl,--export-if-defined=node_api_module_get_api_version_v1"
"-Wl,--no-entry"
"-Wl,--initial-memory=16777216,--export-dynamic,--export=malloc,--export=free,--import-undefined,--export-table"
)
target_link_libraries(hello dlmalloc)
# target_link_libraries(hello emmalloc)
endif()
`

If you use node-addon-api, you can use -DEMNAPI_FIND_NODE_ADDON_API=ON or manually add node-addon-api directory to the include dir via include_directories() or target_include_directories().

`bash
mkdir build

emscripten

emcmake cmake -DCMAKE_BUILD_TYPE=Release \
-DEMNAPI_FIND_NODE_ADDON_API=ON \
-G Ninja -H. -Bbuild

wasi-sdk

cmake -DCMAKE_TOOLCHAIN_FILE=$WASI_SDK_PATH/share/cmake/wasi-sdk.cmake \
-DWASI_SDK_PREFIX=$WASI_SDK_PATH \
-DEMNAPI_FIND_NODE_ADDON_API=ON \
-DCMAKE_BUILD_TYPE=Release \
-G Ninja -H. -Bbuild

wasm32

cmake -DCMAKE_TOOLCHAIN_FILE=node_modules/emnapi/cmake/wasm32.cmake \
-DLLVM_PREFIX=$WASI_SDK_PATH \
-DCMAKE_BUILD_TYPE=Release \
-G Ninja -H. -Bbuild

cmake --build build
`

Output code can run in recent version modern browsers and Node.js latest LTS. IE is not supported.

$3

Require node-gyp >= 10.2.0

See emnapi-node-gyp-test for examples.

- Variables

Arch: node-gyp configure --arch=

`ts
// node-gyp configure -- -Dvariable_name=value

declare var OS: 'emscripten' | 'wasi' | 'unknown' | 'wasm' | ''

/**
* Enable async work and threadsafe-functions
* @default 0
*/
declare var wasm_threads: 0 | 1

/* @default 1048576 /
declare var stack_size: number

/* @default 16777216 /
declare var initial_memory: number

/* @default 2147483648 /
declare var max_memory: number

/* @default path.join(path.dirname(commonGypiPath,'./dist/library_napi.js')) /
declare var emnapi_js_library: string

/* @default 0 /
declare var emnapi_manual_linking: 0 | 1
`

- Create binding.gyp


`py
{
"targets": [
{
"target_name": "hello",
"sources": [
"hello.c"
],
"conditions": [
["OS == 'emscripten'", {
"product_extension": "js", # required

"cflags": [],
"cflags_c": [],
"cflags_cc": [],
"ldflags": []
}],
["OS == 'wasi'", {
# ...
}],
["OS in ' wasm unknown'", {
# ...
}]
]
}
]
}
`

- Add the following environment variables.

`bash


Linux or macOS

export GYP_CROSSCOMPILE=1

emscripten

export AR_target="$EMSDK/upstream/emscripten/emar"
export CC_target="$EMSDK/upstream/emscripten/emcc"
export CXX_target="$EMSDK/upstream/emscripten/em++"

wasi-sdk

export AR_target="$WASI_SDK_PATH/bin/ar"
export CC_target="$WASI_SDK_PATH/bin/clang"
export CXX_target="$WASI_SDK_PATH/bin/clang++"
`

`bat
@REM Windows

set GYP_CROSSCOMPILE=1

@REM emscripten
call set AR_target=%%EMSDK:\=/%%/upstream/emscripten/emar.bat
call set CC_target=%%EMSDK:\=/%%/upstream/emscripten/emcc.bat
call set CXX_target=%%EMSDK:\=/%%/upstream/emscripten/em++.bat

@REM wasi-sdk
call set AR_target=%%WASI_SDK_PATH:\=/%%/bin/ar.exe
call set CC_target=%%WASI_SDK_PATH:\=/%%/bin/clang.exe
call set CXX_target=%%WASI_SDK_PATH:\=/%%/bin/clang++.exe
`

- Build

`bash


Linux or macOS

emscripten

emmake node-gyp rebuild \
--arch=wasm32 \
--nodedir=./node_modules/emnapi \
-- -f make-emscripten # -Dwasm_threads=1

wasi

node-gyp rebuild \
--arch=wasm32 \
--nodedir=./node_modules/emnapi \
-- -f make-wasi # -Dwasm_threads=1

bare wasm32

node-gyp rebuild \
--arch=wasm32 \
--nodedir=./node_modules/emnapi \
-- -f make-wasm # -Dwasm_threads=1
`

`bat
@REM Use make generator on Windows
@REM Run the bat file in POSIX-like environment (e.g. Cygwin)

@REM emscripten
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-emscripten
call emmake.bat make -C %~dp0build

@REM wasi
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-wasi
make -C %~dp0build

@REM bare wasm32
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-wasm
make -C %~dp0build
`

$3

See napi-rs

$3

Related API:

- napi_*_async_work
- napi_*_threadsafe_function

They are available in emnapi, but you need to know more details before you start to use them.
Now emnapi has 3 implementations of async work and 2 implementations of TSFN:

- Async work
- A. Libuv threadpool and pthread based implementation in C
- B. Single thread mock in JavaScript
- C. Web worker based implementation in C (stack allocation) and JavaScript
- TSFN
- D. Libuv and pthread based implementation in C
- E. Web worker based implementation in JavaScript

| | Library to Link | wasm32-emscripten | wasm32 | wasm32-wasi | wasm32-wasi-threads |
|---|------------------------|---------------------|----------|---------------|-----------------------|
| A | libemnapi-mt.a | ✅ | ❌ | ❌ | ✅ |
| B | libemnapi-basic(-mt).a | ✅ | ✅ | ✅ | ✅ |
| C | libemnapi-basic-mt.a | ❌ | ✅ | ❌ | ✅ |
| D | libemnapi-mt.a | ✅ | ❌ | ❌ | ✅ |
| E | libemnapi-basic(-mt).a | ✅ | ✅ | ✅ | ✅ |

There are some limitations on browser about wasi-libc's pthread implementation, for example
pthread_mutex_lock may call __builtin_wasm_memory_atomic_wait32(memory.atomic.wait32)
which is disallowed in browser JS main thread. While Emscripten's pthread implementation
has considered usage in browser. This issue can be solved by upgrading wasi-sdk to v26+
and emnapi v1.5.0+ then pass --export=emnapi_thread_crashed to the linker. If you need to
run your addon with multithreaded features, we recommend you use A & D or C & E.

Note: For browsers, all the multithreaded features relying on Web Workers (Emscripten pthread also relying on Web Workers)
require cross-origin isolation to enable SharedArrayBuffer. You can make a page cross-origin isolated
by serving the page with these headers:

`
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin
`

If you would like to avoid SharedArrayBuffer and cross-origin isolation, please use B & E (link against libemnapi-basic.a), see the following table for more details.

#### About Prebuilt Libraries

Prebuilt libraries can be found in the lib directory in emnapi npm package.

| Library | Description | wasm32-emscripten | wasm32 | wasm32-wasi | wasm32-wasi-threads |
|----------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------|----------|---------------|-----------------------------------------|
| libemnapi.a | no atomics feature.

no libuv port.

napi__async_work and napi__threadsafe_function always return napi_generic_failure. | ✅ | ✅ | ✅ | ✅ |
| libemnapi-mt.a | atomics feature enabled.

napi__async_work and napi__threadsafe_function are based on pthread and libuv port. | ✅ | ❌ | ❌ | ✅ |
| libemnapi-basic.a | no atomics feature.

no libuv port.

napi__async_work and napi__threadsafe_function are imported from JavaScript land. | ✅ | ✅ | ✅ | ✅ |
| libemnapi-basic-mt.a | atomics feature enabled.

no libuv port.

napi__async_work and napi__threadsafe_function are imported from JavaScript land.

include emnapi_async_worker_create and emnapi_async_worker_init for WebWorker based async work implementation. | ❌ | ✅ | ✅ | ✅ |
| libdlmalloc.a | no atomics feature, no thread safe garanteed. | ❌ | ✅ | ❌ | ❌ |
| libdlmalloc-mt.a | atomics feature enabled, thread safe. | ❌ | ✅ | ❌ | ❌ |
| libemmalloc.a | no atomics feature, no thread safe garanteed. | ❌ | ✅ | ❌ | ❌ |
| libemmalloc-mt.a | atomics feature enabled, thread safe. | ❌ | ✅ | ❌ | ❌ |

#### Usage

`cmake
add_subdirectory("${CMAKE_CURRENT_SOURCE_DIR}/node_modules/emnapi")

add_executable(hello hello.c)

if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
target_link_libraries(hello emnapi-mt)
target_compile_options(hello PRIVATE "-pthread")
target_link_options(hello PRIVATE
"-sALLOW_MEMORY_GROWTH=1"
"-sEXPORTED_FUNCTIONS=['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']"
"-sEXPORTED_RUNTIME_METHODS=['emnapiInit']"
"-pthread"
"-sPTHREAD_POOL_SIZE=4"
# try to specify stack size if you experience pthread errors
"-sSTACK_SIZE=2MB"
"-sDEFAULT_PTHREAD_STACK_SIZE=2MB"
)
elseif(CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-wasi-threads")
target_link_libraries(hello emnapi-mt)
set_target_properties(hello PROPERTIES SUFFIX ".wasm")
target_compile_options(hello PRIVATE "-fno-exceptions" "-pthread")
target_link_options(hello PRIVATE
"-pthread"
"-mexec-model=reactor"
"-Wl,--import-memory"
"-Wl,--max-memory=2147483648"
"-Wl,--export-dynamic"
"-Wl,--export=napi_register_wasm_v1"
"-Wl,--export-if-defined=node_api_module_get_api_version_v1"
"-Wl,--export=malloc"
"-Wl,--export=free"
"-Wl,--export=emnapi_thread_crashed"
"-Wl,--import-undefined"
"-Wl,--export-table"
)
elseif((CMAKE_C_COMPILER_TARGET STREQUAL "wasm32") OR (CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-unknown-unknown"))
target_link_libraries(hello emnapi-basic-mt)
set_target_properties(hello PROPERTIES SUFFIX ".wasm")
target_compile_options(hello PRIVATE "-fno-exceptions" "-matomics" "-mbulk-memory")
target_link_options(hello PRIVATE
"-nostdlib"
"-Wl,--no-entry"
"-Wl,--export=napi_register_wasm_v1"
"-Wl,--export-if-defined=node_api_module_get_api_version_v1"
"-Wl,--export=emnapi_async_worker_create"
"-Wl,--export=emnapi_async_worker_init"
"-Wl,--import-memory,--shared-memory,--max-memory=2147483648,--import-undefined"
"-Wl,--export-dynamic,--export=malloc,--export=free,--export-table"
)
endif()
`

`bash


emscripten

emcmake cmake -DCMAKE_BUILD_TYPE=Release \
-DEMNAPI_FIND_NODE_ADDON_API=ON \
-DEMNAPI_WORKER_POOL_SIZE=4 \
-G Ninja -H. -Bbuild

wasi-sdk with thread support

cmake -DCMAKE_TOOLCHAIN_FILE=$WASI_SDK_PATH/share/cmake/wasi-sdk-pthread.cmake \
-DWASI_SDK_PREFIX=$WASI_SDK_PATH \
-DEMNAPI_FIND_NODE_ADDON_API=ON \
-DCMAKE_BUILD_TYPE=Release \
-G Ninja -H. -Bbuild

cmake -DCMAKE_TOOLCHAIN_FILE=node_modules/emnapi/cmake/wasm32.cmake \
-DLLVM_PREFIX=$WASI_SDK_PATH \
-DCMAKE_BUILD_TYPE=Release \
-G Ninja -H. -Bbuild

cmake --build build
`

And additional work is required during instantiating wasm compiled with non-emscripten.

`js
// emnapi main thread (could be in a Worker)
instantiateNapiModule(input, {
context: getDefaultContext(),
/**
* emscripten
* 0: no effect
* > 0: the same effect to UV_THREADPOOL_SIZE
* non-emscripten
* 0: single thread mock
* > 0 schedule async work in web worker
*/
asyncWorkPoolSize: 4, // 0: single thread mock, > 0: schedule async work in web worker
wasi: new WASI(/ ... /),

/**
* Setting this to true or a delay (ms) makes
* pthread_create() do not return until worker actually start.
* It will throw error if emnapi runs in browser main thread
* since browser disallow blocking the main thread (Atomics.wait).
* @defaultValue false
*/
waitThreadStart: isNode || (isBrowser && !isBrowserMainThread),

/**
* Reuse the thread worker after thread exit to avoid re-creatation
* @defaultValue false
*/
reuseWorker: {
/**
* @see {@link https://emscripten.org/docs/tools_reference/settings_reference.html#pthread-pool-size | PTHREAD_POOL_SIZE}
*/
size: 0,

/**
* @see {@link https://emscripten.org/docs/tools_reference/settings_reference.html#pthread-pool-size-strict | PTHREAD_POOL_SIZE_STRICT}
*/
strict: false
},

onCreateWorker () {
return new Worker('./worker.js')
// Node.js
// const { Worker } = require('worker_threads')
// return new Worker(join(__dirname, './worker.js'), {
// env: process.env,
// execArgv: ['--experimental-wasi-unstable-preview1']
// })
},
overwriteImports (importObject) {
importObject.env.memory = new WebAssembly.Memory({
initial: 16777216 / 65536,
maximum: 2147483648 / 65536,
shared: true
})
}
})
`

`js
// worker.js
(function () {
let fs, WASI, emnapiCore

const ENVIRONMENT_IS_NODE =
typeof process === 'object' && process !== null &&
typeof process.versions === 'object' && process.versions !== null &&
typeof process.versions.node === 'string'

if (ENVIRONMENT_IS_NODE) {
const nodeWorkerThreads = require('worker_threads')

const parentPort = nodeWorkerThreads.parentPort

parentPort.on('message', (data) => {
globalThis.onmessage({ data })
})

fs = require('fs')

Object.assign(globalThis, {
self: globalThis,
require,
Worker: nodeWorkerThreads.Worker,
importScripts: function (f) {
(0, eval)(fs.readFileSync(f, 'utf8') + '//# sourceURL=' + f)
},
postMessage: function (msg) {
parentPort.postMessage(msg)
}
})

WASI = require('wasi').WASI
emnapiCore = require('@emnapi/core')
} else {
importScripts('./node_modules/memfs-browser/dist/memfs.js')
importScripts('./node_modules/@tybys/wasm-util/dist/wasm-util.min.js')
importScripts('./node_modules/@emnapi/core/dist/emnapi-core.js')
emnapiCore = globalThis.emnapiCore

const { Volume, createFsFromVolume } = memfs
fs = createFsFromVolume(Volume.fromJSON({
'/': null
}))

WASI = globalThis.wasmUtil.WASI
}

const { instantiateNapiModuleSync, MessageHandler } = emnapiCore

const handler = new MessageHandler({
onLoad ({ wasmModule, wasmMemory }) {
const wasi = new WASI({ fs })

return instantiateNapiModuleSync(wasmModule, {
childThread: true,
wasi,
overwriteImports (importObject) {
importObject.env.memory = wasmMemory
}
})
}
})

globalThis.onmessage = function (e) {
handler.handle(e)
// handle other messages
}
})()
`

Preprocess Macro Options

$3

This is UV_THREADPOOL_SIZE equivalent at compile time, if not predefined, emnapi will read asyncWorkPoolSize option or UV_THREADPOOL_SIZE from Emscripten environment variable at runtime:

`js
Module.init({
// ...
asyncWorkPoolSize: 2
})

// if asyncWorkPoolSize is not specified
Module.preRun = Module.preRun || [];
Module.preRun.push(function () {
if (typeof ENV !== 'undefined') {
ENV.UV_THREADPOOL_SIZE = '2';
}
});
`

`js
// wasi
instantiateNapiModule({
// ...
asyncWorkPoolSize: 2
})
// if asyncWorkPoolSize is not specified
new WASI({
env: {
UV_THREADPOOL_SIZE: '2'
}
})
`

It represent max of EMNAPI_WORKER_POOL_SIZE async work (napi_queue_async_work) can be executed in parallel. Default is not defined.

You can set both PTHREAD_POOL_SIZE and EMNAPI_WORKER_POOL_SIZE to number of CPU cores in general.
If you use another library function which may create N child threads in async work,
then you need to set PTHREAD_POOL_SIZE to EMNAPI_WORKER_POOL_SIZE * (N + 1).

This option only has effect if you use -pthread.
Emnapi will create EMNAPI_WORKER_POOL_SIZE threads when initializing,
it will throw error if PTHREAD_POOL_SIZE < EMNAPI_WORKER_POOL_SIZE && PTHREAD_POOL_SIZE_STRICT == 2.

See Issue #8 for more detail.

$3

This option only has effect if you use -pthread, Default is 0.
Tell emnapi how to delay async work in uv_async_send / uv__async_close.

- 0: Use setImmediate() (Node.js native setImmediate or browser MessageChannel and port.postMessage)
- 1: Use Promise.resolve().then()

$3

This option only has effect if you use emscripten -pthread. Default is 1 if emscripten version >= 3.1.9, else 0.

- 0

Use JavaScript implementation to send async work from worker threads, runtime code will access the Emscripten internal PThread object to add custom worker message listener.

- 1:

Use Emscripten proxying API to send async work from worker threads in C. If you experience something wrong, you can switch set this to 0` and feel free to create an issue.