Amit Kapadia

Citizen Science developer for the Zooniverse. Usually found in a planetarium or climbing a crag.

Emscripten: Pointers and Pointers

13 Sep 2013

Emscripten is a Mozilla Research project that compiles LLVM bytecode to Javascript. Any language (e.g. C and C++) that compiles to LLVM as an intermediary may be ported to Javascript for use in the browser. This is a brief write up on porting functions to JS and exposing their functionality.

C to Javascript

Porting functions from C/C++ to Javascript is straight-forward. As an example, the C function below multiplies two numbers.

extern "C" {
  int float_multiply(float x, float y) {
    return x * y;

Running the following on the command line will port this function to Javascript outputting the code to multiply.js.

emcc multiply.cpp -o multiply.js -s EXPORTED_FUNCTIONS="['_float_multiply']"

Note that the EXPORTED_FUNCTIONS flag specifies an array of functions to export, each with a leading underscore. If the function is not explicitly listed then Emscripten will consider it dead code, and strip it from the output.

After including multiply.js in a project, the function may be accessed by calling cwrap from Module, an object that Emscripten defines in its output. This function exposes float_multiply by specifying the function name, the return type, and an array of argument types.

var float_multiply = Module.cwrap('float_multiply', 'number', ['number', 'number']);
var result = float_multiply(5.2, 4.5);

Using cwrap (or ccall) is simple when functions use only primitive types as arguments, however many interesting functions use pointers as a means to operate over an array.


Using pointers with a ported function needs a bit more machinery. For example, the C function below is ported using emcc in the same way as float_multiply, but it’s slightly more complex to use from Javascript.

int float_multiply_array(float factor, float *arr, int length) {
  for (int i = 0; i <  length; i++) {
    arr[i] = factor * arr[i];
  return 0;

Emscripten’s cwrap and ccall functions use only primitives to define the arguments of the ported function. Since an array (or typed array) is not a primitive type, it must be passed to the function by another mechanism – it must be passed as a number pointing to a block of memory internally managed by Emscripten.

// Import function from Emscripten generated file
float_multiply_array = Module.cwrap(
  'float_multiply_array', 'number', ['number', 'number', 'number']

// Create example data to test float_multiply_array
var data = new Float32Array([1, 2, 3, 4, 5]);

// Get data byte size, allocate memory on Emscripten heap, and get pointer
var nDataBytes = data.length * data.BYTES_PER_ELEMENT;
var dataPtr = Module._malloc(nDataBytes);

// Copy data to Emscripten heap (directly accessed from Module.HEAPU8)
var dataHeap = new Uint8Array(Module.HEAPU8.buffer, ptr, nDataBytes);
dataHeap.set(new Uint8Array(data.buffer));

// Call function and get result
float_multiply_array(2, dataHeap.byteOffset, data.length);
var result = new Float32Array(dataHeap.buffer, dataHeap.byteOffset, data.length);

// Free memory

First the data must be copied to Emscripten’s memory heap. A number representing the data’s byte offset on the heap is passed as an argument to float_multiply_array.

With these techniques, many C/C++ algorithms may be ported to Javascript, but thus far only pointers have been addressed. Many algorithms use pointers of pointers for multi-dimensional arrays (e.g. to represent an image or volume).

Pointers of Pointers

Much of the above is described in the Emscripten Wiki, but there isn’t much discussion on pointers of pointers. Extrapolating from the above examples, consider this function, and note the use of the double pointer, float **arr.

int float_multiply_matrix(float **arr, int ilength, int jlength) {
  float *row;
  for (int i = 0; i < ilength; i++) {
    row = arr[i];
    for (int j = 0; j < jlength; j++) {
      row[j] = 2.0 * row[j];
  return 0;

Porting the function is done with emcc in the same way as the previous two functions, but using this function in Javascript requires slightly more code.

// Import function from Emscripten generated file
var float_multiply_matrix = Module.cwrap(
  'float_multiply_matrix', 'number', ['number', 'number', 'number']

// Create example data to test float_multiply_matrix
var width = 10;
var height = 5;
var data = new Float32Array(width * height);
for (var i = 0; i < width * height; i++) {
  data[i] = i;

// Get data byte size, allocate memory on Emscripten heap, and get pointer
var nDataBytes = data.length * data.BYTES_PER_ELEMENT;
var dataPtr = Module._malloc(nDataBytes);

// Copy data to Emscripten heap
var dataHeap = new Uint8Array(Module.HEAPU8.buffer, dataPtr, nDataBytes);
dataHeap.set( new Uint8Array(data.buffer) );

// Create array of pointers that reference each row in the data
// Note the use of Uint32Array. The pointer is limited to 2147483648 bytes
// or only 2GB of memory :(
var pointers = new Uint32Array(height);
for (var i = 0; i < pointers.length; i++) {
  pointers[i] = dataPtr + i * data.BYTES_PER_ELEMENT * width;

// Allocate bytes needed for the array of pointers
var nPointerBytes = pointers.length * pointers.BYTES_PER_ELEMENT
var pointerPtr = Module._malloc(nPointerBytes);

// Copy array of pointers to Emscripten heap
var pointerHeap = new Uint8Array(Module.HEAPU8.buffer, pointerPtr, nPointerBytes);
pointerHeap.set( new Uint8Array(pointers.buffer) );

// Call the function by passing a number pointing to the byte location of 
// the array of pointers on the Emscripten heap.  Emscripten knows what to do!
float_multiply_matrix(pointerHeap.byteOffset, height, width);

var result = new Float32Array(dataHeap.buffer, dataHeap.byteOffset, data.length);

// Free memory

The technique in the above example requires a careful preparation of data, and an understanding of byte offsets on Emscripten’s heap. Emscripten offers a lot in ways of memory management making it fun to all malloc and free from a Javascript.

The intent of using this tool is to port scientific algorithms to JS for use in web applications. Next week is .astronomy – a conference bringing together astronomers and developers to share and implement unique ideas. One idea is to port an astronomical source extraction algorithm to Javascript. More on that soon.