How do I interface with a C function that returns a struct?

[Jeff D]

In version 1.12, you could set a emcc compiler flag RUNTIME_TYPE_INFO = 1 and it would emit a javascript object containing the size and layouts of the types.

Example of the struct info...

C:

typedef struct SFLocation {
    double x;
    double y;
    SFLinearUnit unit;
    bool empty;
} SFLocation;

JS (NOTE - this is the feature that is no longer available):

   "SFLocation" : {
      "alignSize" : 8,
      "fields" : [ "double", "double", "i32", "i8" ],
      "flatIndexes" : [ 0, 8, 16, 20 ],
      "flatSize" : 24,
      "packed" : false
   }

So when I need to call a C function that has this interface...

C:

SFLocation ScreenGetCursorLocation(Screen * this);

JS:

function _ScreenGetCursorLocation($agg$result, $this)

I have to allocate memory for $agg$result somewhere.  The two problems I have at runtime without the type info is...

  1. I don't know the size of the struct, so I don't know how much memory to allocate.
  2. I can't retrieve values from the struct because I don't know the offsets or field types anymore  (emscripten's "getValue" function requires an 'address' and 'type' as arguments).

Is there a better way of doing this now with the newer compiler that I am not aware of?

Here is how I used to be able to do it:

var GetCursorLocation = Module.cwrap("ScreenGetCursorLocation", "number", ["number", "number"]);

var metadata = Runtime.typeInfo["SFLocation"];
var locationPtr = _malloc(metadata.flatSize);

GetCursorLocation(locationPtr, screen);

var x = getValue(locationPtr + metadata.flatIndexes[0], metadata.fields[0]);
var y = getValue(locationPtr + metadata.flatIndexes[1], metadata.fields[1]);
// ETC, ETC

_free(locationPtr);

[Alon Zakai]

The WebIDL binder and embind might be a good fit for what you want,

http://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#interacting-with-code-binding-cpp

They let you get and set fields on structs/classes. Or, you can create C methods to do that (or return the size/offset etc.), compile and call those, which is essentially what the WebIDL binder and embind do under the hood.

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[Jeff D]

WebIDL looks promising... I was definitely not looking forward to writing a ton sizeof() and offsetof() functions!

After playing around with it a bit, I think I can make this work if I mark all my struct WebIDL interfaces as [NoDelete] and write C methods for allocating and freeing the structs.  

Thanks for the advice.

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[Jeff D]

Throwing this out there in case anybody comes across the same issue.

C:

typedef struct SFColor {

    float r;

    float g;

    float b;

} SFColor;

typedef struct SFAreaInfo

{

    char * name;

    double multiplier;

    SFColor color;

    bool livingArea;

bool perimeter;

} SFAreaInfo;

WebIDL:

[NoDelete]

interface SFColor{

  attribute float r;

  attribute float g;

  attribute float b;

};

[NoDelete]

interface SFAreaInfo{

  attribute DOMString name;

  attribute double multiplier;

  [Value]attribute SFColor color;

  attribute boolean livingArea;

  attribute boolean perimeter;

};

I initially got tripped up on the nested struct because without the [Value] flag it tries to assign a 'SFColor *'(pointer) to the color field.

After that, for the generated .cpp file, I wrote a script to remove the 'extern "C" {' stuff, and stripped out the "VoidPtr" destructor that it automatically creates (it uses the delete keyword).