wasm-arrays

A couple of helper functions to make WebAssembly array parameters easy to use.

Read more

Medium article going through the reasoning and implementation:

https://becominghuman.ai/passing-and-returning-webassembly-array-parameters-a0f572c65d97

Importing

Browser

Add <script src="/dist/wasm-arrays.min.js"></script> and use ccallArrays or cwrapArrays See the demo.html file included to see a few examples.

NOTE: You will need to serve the html file through a server. There is one already included, so you can run node server, then navigate to localhost:1337 to view.

Nodejs

const {ccallArrays, cwrapArrays} = require("./wasm-arrays.js")

You can see the test.js file to see more nodejs examples.

Usage

There are a few examples included in the demo.html, test.js and emscripten.cpp files, for their respective use cases

The included functions aim to mimick the WebAssembly ccall and cwrap functions. They therefore work the same way.

If you need to pass an array parameter, you need to just add the parameter to the list of parameters.

In the C++ code, this will be turned into two parameters:

1. Array pointer
2. Array size

Example:

JavaScript

const result = ccallArrays("addNums", "number", ["array"], [[1,2,3,4,5,6,7]])

C++

EMSCRIPTEN_KEEPALIVE
int addNums (float *buf, int bufSize) {
 
    int total = 0;
 
    for (int i=0; i<bufSize; i++) {
        total+= buf[i];
    }
 
    return total;
}

To return an array from WebAssembly, you need to specify the return parameter as "array". You also need to specify the size of the returned array.

Example:

JavaScript

const res = ccallArrays("doubleValues", "array", ["array"], [[1,2,3,4,5]], {heapIn: "HEAP8", heapOut: "HEAP8", returnArraySize: 5})
console.log(res) // [2,4,6,8,10]

C++

EMSCRIPTEN_KEEPALIVE
int8_t* doubleValues (int8_t *buf, int bufSize) {
 
    int8_t values[bufSize];
 
    for (int i=0; i<bufSize; i++) {
        values[i] = buf[i] * 2;
    }
 
    auto arrayPtr = &values[0];
    return arrayPtr;
}

Heaps

You can also specify the heaps used for the arrays going in and out. The following table matches the heaps to the C++ types, for quick reference:

Heap	C++
HEAP8	int8_t
HEAPU8	uint8_t
HEAP16	int16_t
HEAPU16	uint16_t
HEAP32	int32_t
HEAPU32	uint32_t
HEAPF32	float
HEAPF64	double