如何在Emscripten生成的代码中使用C ++分配的数组？

Question

I have a C++ code like this:

extern "C" {

void MyCoolFunction (int** values)
{
    int howManyValuesNeeded = 5;
    *values = new int[howManyValuesNeeded];
    for (int i = 0; i < howManyValuesNeeded; i++) {
        (*values)[i] = i;
    }
}

}

From C++ it can be used like this:

int *values = NULL;
MyCoolFunction (&values);
// do something with the values
delete[] values;

Of course the real code is much more complicated, but the point is that the function allocates an int array inside, and it decides what the array size will be.

I translated this code with Emscripten, but I don't know how could I access the array allocated inside the function from javascript. (I already know how to use exported functions and pointer parameters with Emscripten generated code, but I don't know how to solve this problem.)

Any ideas?

Answer 1

In Emscripten, memory is stored as a giant array of integers, and pointers are just indexes into that array. Thus, you can pass pointers back and forth between C++ and Javascript just like you do integers. (It sounds like you know how to pass values around, but if not, go here .)

Okay. Now if you create a pointer on the C++ side (as in your code above) and pass it over to Javascript, Emscripten comes with a handful of helper functions to allow you to access that memory. Specifically setValue and getValue .

Thus, if you passed your values variable into JS and you wanted to access index 5, you would be able to do so with something like:

var value5 = getValue(values+(5*4), 'i32');

Where you have to add the index times the number of bytes (5*4) to the pointer, and indicate the type (in this case 32 bit ints) of the array.

Answer 2

You can call the delete from JavaSCript by wrapping it inside another exported function.

extern "C" { ...
    void MyCoolFunction (int** values);
    void finsih_with_result(int*);
}

void finsih_with_result(int *values) {
    delete[] values;
}

Alternatively you may also directly do this on JavaScript side: Module._free(Module.HEAPU32[values_offset/4]) (or something like that; code not tested).