add

C WASM - Add Numbers

This example package demonstrates compiling a simple C program, add.c which adds two numbers to WASM, executing it in a host environment, and accessing it's methods.

Project Structure

Dependencies

• NodeJS & NPM is required to build the project dependencies, run the WASM compile, and run tests. NPM comes installed with NodeJS by default
• The EMCC compiler toolchain is required to compile C code to WASM

Before running the project, you must also install the project's dependencies:

npm install

Directories & files

• src/ - Contains the C source code to be compiled into the WASM module, for example add.c
• build/ - Contains the compiled output WASM file resulting from the build process
• test/ - Contains NodeJS test code demonstrating and unit testing the functionality contained in and exposed by the compiled WASM module in build
• package.json - Contains the dependency declarations and NPM build & test scripts. Controls WASM compiler options

Development Workflow

1. Write C Code with exported functions
2. (if needed) Modify package.json's build script to include any new exported C functions (-s EXPORTED_FUNCTIONS=\"[...]\")
3. Compile the C code to WASM
4. Modify tests in test/ to test functionality exported from C
5. Run tests
6. Repeat!

Compiling C to WASM

npm run build

An add.wasm file will be produced in the build directory

Testing

Test the module outside of the context of the blockchain:

npm test

Module Usage Example

How to access and use WASM modules from NodeJS, from test/add.unit.spec.js:

const util = require('../../../../src/util');

const buffer = fs.readFileSync(path.resolve(__dirname, '../build/add.wasm'));
const imports = util.getDefaultImports();

const result = await WebAssembly.instantiate(buffer, imports);

//use result.instance.exports to access functions prefixed by "_"
console.log(result.instance.exports._add(3, 99)); // => 102

API

_add(int x, int y) - int

Add two integers together and returns the result

_addRunning(int x) - int

Add an integer to a running total and returns the resulting running total