Emscripten pass array to C++ and change its values

Question

It's my first time working with webassembly and I got to the following problem. A solution, which I dont know how to apply, exists in the official docs of emscripten. I also scanned through the web to find a working solution but none has worked for me so far.

Basically I need to get some array out of my C++ code after it has executed. For this purpose, I made a new function within my C++ code which copies the relevant data to some array which has been allocated within emscripten.

The C++ code looks like this:

void copy_to_array(std::vector<float> &vec, float* res){

    std::memcpy(res, vec.begin().base(), sizeof(float) * vec.size());
    for(int i = 0; i < vec.size(); i++){
        std::cout << "copying array entry: " << i << " : " << vec[i] << " -> " << res[i] << std::endl;
    }
}
EMSCRIPTEN_KEEPALIVE void wasm_get_displacement_x(float* res){
    copy_to_array(wasm_result_displacement_x, res);
}

I made a function to copy the result from the internal vector wasm_result_displacement_x to the given float array res.

I know that the values of the array are not 0 since the std::cout call shows the following:

My code within javascript looks like this:

function getFloatArrayFromFunctionCall(func_name, size) {
    var res_ptr = Module._malloc(size * 4);
    Module.ccall(func_name, null, ["number"], res_ptr);
               
    let mem_view = Module.HEAPF32.subarray(res_ptr, res_ptr + size);

    console.log(mem_view);
    Module._free(res_ptr);
}
getFloatArrayFromFunctionCall("wasm_get_displacement_x", n_nodes)

It seems like I made a mistake when trying to read the content of the module memory. Using the getValue approach based on this answer doesnt work either.

I am very happy for any help or advice!

I made an example how to create c++ objects in java script which wrap arrays of objects using emscripten and WebIDL. https://github.com/werto87/emscripten_webidl — Koronis Neilos, Apr 07 '22 at 15:54
I am not sure where to look in your code. Does it have a concrete solution for my problem? — Finn Eggers, Apr 07 '22 at 16:07
"Basically I need to get some array out of my C++ code after it has executed" You can look in "https://github.com/werto87/emscripten_webidl/blob/main/html/some.html". It shows how to work with java script and c++ and arrays. — Koronis Neilos, Apr 07 '22 at 16:19
It is a totally different way of communicating between the two of them. In particular I am working with ccall. — Finn Eggers, Apr 07 '22 at 16:20

score 1 · Answer 1 · answered Apr 07 '22 at 17:08

Okay so I figured out the solution by trial and error. Module.ccall is supposed to take an array of arguments it reads.

 function getFloatArrayFromFunctionCall(func_name, size) {
     var res_ptr = Module._malloc(size * 4);
     Module.ccall(func_name, null, ["number"], [res_ptr]);
     var view = Module.HEAPF32.subarray(res_ptr >> 2, (res_ptr >> 2) + size);
     Module._free(res_ptr);
     return view;
 }

solves the problem. Hope this is helpfull for anyone stumbling across the same problem.

Now I get `Module._malloc is not a function` console.log from Browser when I call my C function — canbax, Feb 25 '23 at 14:44

Emscripten pass array to C++ and change its values

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