How to use C++ allocated array in Emscripten generated code?

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?

“I don't know how could I access the array” – But the code you are showing does exactly this. What is your question? — 5gon12eder
– 5gon12eder, Commented Jan 24, 2015 at 21:17
My question is how to access the array from javascript after the emscripten conversion. — kovacsv
– kovacsv, Commented Jan 24, 2015 at 21:18
Please add that to your question. It is not clear as currently written, at least to me. Also look again at the “from C it can be used…”, there is no delete[] in C. — 5gon12eder
– 5gon12eder, Commented Jan 24, 2015 at 21:20
Alright, I took back my close as “unclear what you are asking” vote. — 5gon12eder
– 5gon12eder, Commented Jan 24, 2015 at 21:23

Charles Ofria · Accepted Answer · 2015-01-25 04:28:06Z

3

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.

edited Jan 25, 2015 at 4:28

answered Jan 25, 2015 at 0:38

Charles Ofria

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2 Comments

kovacsv Over a year ago

Thank you! This is exactly what is missed from my knowledge. If someone interested, here are my all experiments with pointers: github.com/kovacsv/EmscriptenPointers

Tarun Gehlaut Over a year ago

If changing the function signature is an option, you could use a std::vector instead of the array. Passing around std::vectors is pretty simple using embind

Sohail Si · Accepted Answer · 2016-06-13 15:37:12Z

0

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).

answered Jun 13, 2016 at 15:37

Sohail Si

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