Question

I'm trying to pass a value from JavaScript to WASM, but it's not working. Here's my C code:

extern int x;
int foo() {
  return x;
}

In JavaScript, I instantiate the module and set x equal to 5:

var importObj = {
  env: { 
    memory: new WebAssembly.Memory({initial: 256, maximum: 256}),     
    _x: 5
  }
};    

WebAssembly.instantiateStreaming(fetch('test.wasm'), importObj)
.then(result =>
    console.log('Output: ' + result.instance.exports._foo())
);

This doesn't produce any errors, but the logged message is Output: 0 instead of Output: 5. Any ideas?

Answer 1

你用什么来编译你的C代码到wasm？我试图让这个工作在https://webassembly.studio上，并发现函数foo从内存位置0加载值。所以我能够通过从JS：

初始化实例内存来生成Output: 5

WebAssembly.instantiateStreaming(fetch('test.wasm'), importObj)
  .then(result => {
    const mem = new Uint32Array(result.instance.exports.memory.buffer)
    mem[0] = 5
    console.log('Output: ' + result.instance.exports._foo())
  });

说实话，我希望extern int x成为可以通过importObj传递的wasm的导入全局。我希望这仍然有助于走上正确的道路。

Answer 2

当前的lld实现在链接时不支持未定义的数据符号。如果您通过--allow-undefined，您将得到未定义的符号，其地址为0。

我们一直在讨论添加对导入数据符号的支持：https://github.com/WebAssembly/tool-conventions/issues/48

Passing data to WebAssembly

2 个答案: