如何找出 WebAssembly 缓冲区的地址并将其返回给 Javascript？

Question

I'm trying to obtain the memory address of a WebAssembly buffer and return it to Javascript, so that I can instaniate its memory as a Javascript ArrayBuffer and directly write in an array in the module's memory at the appropriate address.

The c code:

#include <stdint.h>

uint8_t buff[10][100];

uint64_t addr(int buffer_index){
    return (uint64_t)&buff[buffer_index];
}

I compile it with:

emcc project.c -Os -s WASM=1 -s SIDE_MODULE=1 -o project.wasm

The html:

<script>
var importObject = {
        env: {
            memoryBase: 0,
            tableBase: 0,
            setTempRet0:(x)=>{},
            memory: new WebAssembly.Memory({ initial:256 }),
            table: new WebAssembly.Table({ initial:0, element:'anyfunc' })
        }
    };

    fetch('http://localhost:9000/assets/wasm/project.wasm').then(
        response => response.arrayBuffer()
    ).then(
        bytes => WebAssembly.instantiate(bytes, importObject)
    ).then(
        results => {
            let module=results.instance
            let exports=module.exports
            let addr=exports._addr
            console.log(addr(0))
            console.log(addr(1))
            console.log(addr(2))
        }
    )
</script>

The result of the run:

project.html:21 5242880
project.html:22 5242980
project.html:23 5243080

This looks sane, because the buffer has 100 byte chunks, and the returned addresses are 100 bytes apart.

How can I find out where these addresses point in the module's memory?

Answer 1

Your have provided the module with memory via importObject.env.memory . You simply have to use the offsets / addresses provided to inspect the memory allocated to your buff variable in your C code:

// create a Uint8Array as a 'view' on the module linear memory
// starting at _addr(0), and with a length of 100 elements.
var buffer = new Uint8Array(importObject.env.memory.buffer, exports._addr(0), 100);

for (var i=0; i<buffer.length; i++) {
  var foo = buffer[i];
  // do something with foo here!
}

You can see a more complete example in my project that renders a Mandelbrot fractal .