Calling C code from JavaScript involving Structs

[Planeshifter]

First let me say, this is an amazing project!

Now my question:
I am in the process of transpiling a C library to JS which should then be used as a node.js module. While I managed to export functions accepting or returning standard types such as floats or doubles, the documentation is not clear on how to properly convert functions accepting or returning custom C structs (the Embind and WebIDL facilities seem to be tailored only at C++ code).

Consider the following artificial example:

#include <math.h>

struct Vector3d{
  double x;
  double y;
  double z;
};

double length(struct Vector3d vec){
  double vec_sum = vec.x*vec.x + vec.y*vec.y + vec.z*vec.z;
  return sqrt(vec_sum);
}

struct Vector3d createVec(double x, double y, double z){
  struct Vector3d ret;
  ret.x = x;
  ret.y = y;
  ret.z = z;
  return ret;
}

Ideally, I would like to end up with a constructor function than can create 3d vectors and a length function to calculate their length. This seems to be a quite common task when porting C code, but browsing the Internet I couldn't find a proper explanation of how to proceed? I would greatly appreciate if someone could tell me whether and if so how one can convert such C code to JavaScript. I found a discussion on the Google group (Link) suggesting that it is possible, but was not able to reproduce the necessary steps, so a worked example would be of much help.

Best,
Philipp

[kripken]

In WebIDL, you can use [Value] to allow passing and returning structs by value,

http://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/WebIDL-Binder.html#pointers-references-value-types-ref-and-value

For efficiency, though, it is much better for your APIs to hand off pointers to objects.

[Planeshifter]

Thank you for your quick response. By now, I managed to convert above example using Embind as follows:

#include <math.h>
#include <emscripten/bind.h>

using namespace emscripten;

struct Vector3d{
  double x;
  double y;
  double z;
};

double length(struct Vector3d vec){
  double vec_sum = vec.x*vec.x + vec.y*vec.y + vec.z*vec.z;
  return sqrt(vec_sum);
}

struct Vector3d createVec(double x, double y, double z){
  struct Vector3d ret;
  ret.x = x;
  ret.y = y;
  ret.z = z;
  return ret;
}

EMSCRIPTEN_BINDINGS(my_module) {
    value_object<Vector3d>("Vector3d")
        .field("x", &Vector3d::x)
        .field("y", &Vector3d::y)
        .field("z", &Vector3d::z);
    function("createVec",&createVec);
    function("length", &length);
}

That works just as expected. When trying WebIDL, I was not able to export a function which is not a method belonging to a class. It seems that you always have to define an interface, at least in the examples I came across. Is that correct?

[kripken]

Yes, WebIDL currently exports only classes and structs, I don't think
anyone has added global functions to it.

• Alon

On Tue, Dec 16, 2014 at 11:03 PM, Philipp Burckhardt <
notifications@github.com> wrote:

Thank you for your quick response. By now, I managed to convert above
example using Embind as follows:

#include <math.h>
#include <emscripten/bind.h>

using namespace emscripten;

struct Vector3d{
double x;
double y;
double z;
};

double length(struct Vector3d vec){
double vec_sum = vec.x_vec.x + vec.y_vec.y + vec.z*vec.z;
return sqrt(vec_sum);
}

struct Vector3d createVec(double x, double y, double z){
struct Vector3d ret;
ret.x = x;
ret.y = y;
ret.z = z;
return ret;
}

EMSCRIPTEN_BINDINGS(my_module) {
value_object("Vector3d")
.field("x", &Vector3d::x)
.field("y", &Vector3d::y)
.field("z", &Vector3d::z);
function("createVec",&createVec);
function("length", &length);
}

That works just as expected. When trying WebIDL, I was not able to export
a function which is not a method belonging to a class. It seems that you
always have to define an interface (which will be converted to a JS
constructor function), at least in the examples I came across.

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Reply to this email directly or view it on GitHub
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[Planeshifter]

Thanks for the clarification!
Best,
Philipp