Passing/returning array pointers to emscripten compiled C code

[fjc...@gmail.com]

Hello --

I'm new to Emscripten/javascript as well as this discussion community.  I apologize, in advance, if my situation has already been addressed.    

From a windows 7 environment, I've used emcc to compile a simple c program which accepts an array and modifies it (see below).   

double* displayArray(double *doubleVector) {

for (int cnt = 0; cnt < 3; cnt++) 

printf("doubleVector[%d] = %f\n", cnt, doubleVector[cnt]);

doubleVector[0] = 98;

doubleVector[1] = 99;

doubleVector[2] = 100;

for (int cnt1 = 0; cnt1 < 3; cnt1++) 

  printf(" modified doubleVector[%d] = %f\n", cnt1, doubleVector[cnt1]);

return doubleVector;

}

int main() {

double d1, d2, d3;

double array1[3];

double *array2;

array1[0] = 1.00000;

array1[1] = 2.000000;

array1[2] = 3.000000;

array2 = displayArray(array1);

for (int cntr =0; cntr < 3; cntr++)

printf("array1[%d] = %f\n", cntr, array1[cntr]);

for (int cnt = 0; cnt < 3; cnt++)

printf("array2[%d] = %f\n", cnt, array2[cnt]);

    return 1;

}

Using the -o options for emcc , I generated a .html file which I loaded to a browser (Chrome).  

python emcc displayArray7.c -o displayArray7.html -s EXPORTED_FUNCTIONS="['_main', '_displayArray']"

Upon loading, I see that the output being generated within the browser window is as expected (see below).  

doubleVector[0] = 1.000000

doubleVector[1] = 2.000000

doubleVector[2] = 3.000000

 modified doubleVector[0] = 98.000000

 modified doubleVector[1] = 99.000000

 modified doubleVector[2] = 100.000000

array1[0] = 98.000000

array1[1] = 99.000000

array1[2] = 100.000000

array2[0] = 98.000000

array2[1] = 99.000000

array2[2] = 100.000000

However, when using the module.cwrap() command via javascript console and attempting to invoke the function directly (outside of main()) , 

> displayArray=Module.cwrap('displayArray','[number]',['[number]'])

function () { return ccallFunc(func, returnType, argTypes, Array.prototype.slice.call(arguments)); }

> result = displayArray([1.0,2.0,3.0])

[1, 2, 3]

> result

[1, 2, 3]

I am seeing the following being generated/displayed in the browser which is NOT what I expect to see.   

doubleVector[0] = 0.000000

doubleVector[1] = 0.000000

doubleVector[2] = 0.000000

modified doubleVector[0] = 100.000000

modified doubleVector[1] = 100.000000

modified doubleVector[2] = 100.000000   

I have the following questions:

• Do I have the syntax correct for the return type and parameter listing correct in my call to Module.cwrap()
• I've successfully run the simple, straight-forward example of int_sqrt() in the "Interacting with code" section of the tutorial which deals with passing non-pointer variables to the int_sqrt() routine.  Is there something different that is happening when arrays and/or pointers are passed to  (or returned from) the emscripten-generated javascript code?  
• How is that the generated output in the browser of the function, displayArray(), works (as expected) when called from main(); but not via the javascript console?  

I am new to Emscripten/javascript so any info/assistance will be greatly appreciated.  

Thanks in advance,

FJC

[Alon Zakai]

ccall and cwrap are quite limited. Best to look in the source (src/preamble.js) where the comments explain the types that are supported. In particular the only array type supported so far is 8-bit.

Embind is a more general tool to connect the two worlds, see https://github.com/kripken/emscripten/wiki/Interacting-with-code

But for a double array like in that example, I would just write low-level code manually. Pointers are just integers. You can malloc some space, write into that, then ccall and pass the pointer (which is just a number), etc.

- Alon

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[fjc...@gmail.com]

Hi Alon -

Thanks for the response/info.   I've successfully run the simple example illustrated in the Embind.   Am currently looking into this...

I'm not sure I quite understand what you mean by "low-level code" that you mention below.   Perhaps an example of this low-level code would help to illustrate and help me to understand?  

Thanks again,

FJC

[Alon Zakai]

Here, look at what zee.js does

https://github.com/kripken/zee.js/blob/master/post.js

It mallocs a buffer, does some work on it using normal JS typed array stuff, then passes a "pointer" to it to a compiled C function.

- Alon