Wasm-C FFIΒΆ

main.c:

// Example function
int add(int a, int b)
{
  return a + b;
}

// This function is defined in javascript
extern void print(const char* str);

int main(void)
{
  print("Hello, World!");
  return 0;
}

Compilation:

clang --target=wasm32 -nodefaultlibs -nostdlib -Wl,--no-entry -Wl,--export-all -Wl,--allow-undefined -Wl,--initial-memory=131072 -o main.wasm main.c

index.js:

// Helper function to get a string from a pointer in memory
function getString(ptr, memory) {
    const bytes = new Uint8Array(memory.buffer, ptr);
    let len = 0;
    while (bytes[len] !== 0) len++;
    return new TextDecoder().decode(bytes.slice(0, len));
}

const importObject = {
    env: {
        // Provide functions that can be called from C
        print: (ptr) => {
            const str = getString(ptr, wasmInstance.exports.memory);
            console.log("From C: ", str);
        },
    },
}

let wasmInstance;
WebAssembly.instantiateStreaming(fetch("main.wasm"), importObject).then(
    (obj) => {

        wasmInstance = obj.instance;

        // Call a C function
        const res = wasmInstance.exports.add(1, 2);
        console.log("1 + 2 = ", res);

        // Call main
        wasmInstance.exports.main();
    }
);

index.html:

<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>My Website</title>
    <link rel="stylesheet" href="./style.css">
    <link rel="icon" href="./favicon.ico" type="image/x-icon">
  </head>

  <body>
    <main>
      <h1>Welcome to My Website</h1>
    </main>
    <script src="index.js"></script>
  </body>

</html>

Run a server:

python3 -m http.server