CODEWITHSUNDEEP
emscriptenwebassembly
Joe
Joe

Reputation: 385

Emscripten - How to get uint8_t array from C to JavaScript?

I have an array type of uint8_t in C. A function called getResultArray will return this array. How can I get this array in JavaScript?

uint8_t * getResultBuffer() { return resultBuffer }

Upvotes: 3

Views: 2001

Answers (1)

Adi Levin
Adi Levin

Reputation: 5233

The pointer returned from the C function is an offset into the ArrayBuffer that Emscripten uses to represent memory. For viewing as uint8_t, access the memory using Module.HEAPU8.

Here is an example, using em++:

fill_array.cpp:

#include "stdint.h"

extern "C" {
    uint8_t* fill_array(int n);
}

uint8_t* fill_array(int n) {
    uint8_t* arr = new uint8_t[n];
    for(uint8_t i=0;i<n;++i)
        arr[i] = i;
    return arr;
}

index.html:

<!doctype html>
<html>
<body>
    <script>
        var Module = {
          onRuntimeInitialized: function() {
            var fill_array = Module.cwrap('fill_array', 'number', [])
            var n = 16;
            var ptr_from_wasm = fill_array(n);
            var js_array = Module.HEAPU8.subarray(ptr_from_wasm, ptr_from_wasm + n);
            alert(js_array);
          },
        };
    </script>
    <script async type="text/javascript" src="index.js"></script>
</body>
</html>

Results in the following:

0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15

For this to work, you'll need to add the following arguments to em++:

-s EXPORTED_FUNCTIONS='["_fill_array"]' -s EXTRA_EXPORTED_RUNTIME_METHODS='["ccall", "cwrap"]'

See full source code in This repo

Upvotes: 4

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