如何访问线性内存中的数据[重复]

Question

我的.wat 文件有以下两行代码。

(memory $buf 1)
(data (i32.const 0) "hello")

我想将 $buf 作为参数传递给导入的 Javascript 函数。 Javascript 端的代码如下所示：

 function consoleLogString(memory, offset,length){
        var bytes = new Uint8Array(memory.buffer, 0, 5);
        var string = new TextDecoder('utf8').decode(bytes);
        console.log(string);
    }
    var importObj = {console : {log: consoleLogString}};
    fetchAndInstantiate('stringLogger2.wasm',importObj).then(function(instance) {
        instance.exports.writeHi();
    });
}

在 wasm 方面，我希望能够做类似的事情

(import "console" "log" (func $log (param i32 i32 i32)))
(export "writeHi" (func $writeHi))
(memory $buf 1)
(data (i32.const 0) "hello")
(func $writeHi
      ;; how do I push my buffer onto the stack so it can be passed to the function?
      i32.const 0
      i32.const 5
      call $log
)

Answer 1

据我所知，WebAssembly 只支持 i32 / i64 / f32 / f64，所以这意味着你不能传入指针。

Pass a JavaScript array as argument to a WebAssembly function

但是您可以将 memory 导入到 wasm 实例，并且可以将日志功能与导入的 memory 链接>

function consoleLogString(memory, offset, length) {
    var bytes = new Uint8Array(memory.buffer, offset, length);
    var string = new TextDecoder('utf8').decode(bytes);
    console.log(string);
}
const memory = new WebAssembly.Memory({ initial: 2 });
const consoleLogStringWrapper = memory => (offset, length) => consoleLogString(memory, offset, length);
var importObj = {
    imp : {
        mem: memory,
        log: consoleLogStringWrapper(memory)
    }
};
fetchAndInstantiate('stringLogger2.wasm', importObj).then(function(instance) {
    instance.exports.writeHi();
});

您的 wat 将与此类似：

    (module
        (import "imp" "log" (func $log (param i32 i32)))
        (memory (import "imp" "mem") 1)
        (func $writeHi
             i32.const 0
             i32.const 5
             call $log
        )
        (data (i32.const 0) "hello")
        (export "writeHi" (func $writeHi))
    )

使用以下js在wat2wasm上测试：

function consoleLogString(memory, offset, length) {
    var bytes = new Uint8Array(memory.buffer, offset, length);
    var string = new TextDecoder('utf8').decode(bytes);
    console.log(string);
}
const memory = new WebAssembly.Memory({ initial: 2 });
const consoleLogStringWrapper = memory => (offset, length) => consoleLogString(memory, offset, length);
var importObj = {
    imp : {
        mem: memory,
        log: consoleLogStringWrapper(memory)
    }
};

const wasmInstance = new WebAssembly.Instance(wasmModule, importObj);
const { writeHi } = wasmInstance.exports;
writeHi();