[WASM] Write to WebAssembly Memory from JavaScript

Posted 2020-09-26 Answer1215

We write a function that converts a string to lowercase in WebAssembly, demonstrating how to set the input string from javascript.

WASM Fiddle: https://wasdk.github.io/WasmFiddle/?h1s69

Demo Repo: https://github.com/guybedford/wasm-intro

 

We want to create a funcrtion ‘toLowerCase‘, which enable JS to write in Memory.

To write data into WASM, we need to variables in C code, one is ‘inStr‘ which get original input (for example ‘Hello World‘), another is ‘outStr‘ which will transform to lower case string (for example: ‘hello world‘).

C code:

void consoleLog (char* offset, int len);

char inStr[20];
char outStr[20];

char* getInStrOffset () {
  return &inStr[0];
}

void toLowerCase() {
  for (int i = 0; i < 20; i++ ) {
    char c = inStr[i];
    if (c > 64 &&  c < 91) {
      c = c + 32;
    }
    outStr[i] = c;
  }
  consoleLog(&outStr[0], 20);
}

 

JS: Some code to get wasm instance.

var wasmModule = new WebAssembly.Module(wasmCode);
var wasmInstance = new WebAssembly.Instance(wasmModule, {
  env: {
    consoleLog (offset, len) {
      const strBuf = new Uint8Array(mem.buffer, offset, len);
      log(new TextDecoder().decode(strBuf));
    }
  }
});
const mem = wasmInstance.exports.memory;

 

Now we need to write data from JS to WASM memory, the way to do it is just to put data into ‘inStr‘:

const mem = wasmInstance.exports.memory;

function writeString (str, offset) {
  const strBuf = new TextEncoder().encode(str);
  const outBuf = new Uint8Array(mem.buffer, offset, strBuf.length);
  



      for (let i = 0; i < strBuf.length; i++) {
        outBuf[i] = strBuf[i];
      }
  }
}

writeString(‘Hello Web Assembly‘, wasmInstance.exports.getInStrOffset());

Because what ‘wasmInstance.exports.getInStrOffset()‘ return us is the first char address of ‘inStr‘, then we use for loop to write data into ‘inStr‘.

After writting the data, then we can call ‘toLowerCase‘ to see the result:

var wasmModule = new WebAssembly.Module(wasmCode);
var wasmInstance = new WebAssembly.Instance(wasmModule, {
  env: {
    consoleLog (offset, len) {
      const strBuf = new Uint8Array(mem.buffer, offset, len);
      log(new TextDecoder().decode(strBuf));
    }
  }
});
const mem = wasmInstance.exports.memory;

function writeString (str, offset) {
  const strBuf = new TextEncoder().encode(str);
  const outBuf = new Uint8Array(mem.buffer, offset, strBuf.length);
  



      for (let i = 0; i < strBuf.length; i++) {
        outBuf[i] = strBuf[i];
      }
  }
}

writeString(‘Hello Web Assembly‘, wasmInstance.exports.getInStrOffset());
wasmInstance.exports.toLowerCase();