Sdk: dart ffi: Structs pass by value

[am15h]

I want to write dart binding for the following C code:

c api:

TFL_CAPI_EXPORT extern TfLiteQuantizationParams TfLiteTensorQuantizationParams(  
const TfLiteTensor* tensor);  


typedef struct TfLiteQuantizationParams {  
float scale;  
int32_t zero_point;  
} TfLiteQuantizationParams;

I tried doing this:

dart binding:

TfLiteQuantizationParams Function(Pointer<TfLiteTensor> tensor)  
TfLiteTensorQuantizationParams = tflitelib  
.lookup<NativeFunction<_TfLiteTensorQuantizationParams_native_t>>(  
'TfLiteTensorQuantizationParams')  
.asFunction();

typedef _TfLiteTensorQuantizationParams_native_t  
= TfLiteQuantizationParams Function(Pointer<TfLiteTensor> tensor);

But I am facing the following error;

Compiler message:                                                       
../lib/src/bindings/tensor.dart:126:6: Error: Expected type 'NativeFunction<TfLiteQuantizationParams Function(Pointer<TfLiteTensor>)>' to be a valid and instantiated subtype of 'NativeType'.
 - 'NativeFunction' is from 'dart:ffi'.                                 
 - 'TfLiteQuantizationParams' is from 'package:tflite_flutter_plugin/src/bindings/types.dart' ('../lib/src/bindings/types.dart').
 - 'Pointer' is from 'dart:ffi'.                                        
 - 'TfLiteTensor' is from 'package:tflite_flutter_plugin/src/bindings/types.dart' ('../lib/src/bindings/types.dart').
    .asFunction();

dart struct

class TfLiteQuantizationParams extends Struct {  
@Float()  
double scale;

@Int32()  
int zero_point;

@override  
String toString() {  
return 'TfLiteQuantizationParams{scale: $scale, zero_point: $zero_point}';  
}  
}

I need some help to get this working.

Thanks in advance

[am15h]

Related issue https://github.com/dart-lang/sdk/issues/36730. @dcharkes Can you suggest a workaround for my problem?

[dcharkes]

We do not support passing structs by value yet indeed (I'm working on it).

The workaround is to write some wrapper C code that turns it into a pointer.

Dart code:

final tfLiteTensorQuantizationParamsWrapped = tflitelib.lookupFunction<
    Pointer<TfLiteQuantizationParams> Function(Pointer<TfLiteTensor> tensor),
    Pointer<TfLiteQuantizationParams> Function(
        Pointer<TfLiteTensor> tensor)>('TfLiteTensorQuantizationParams');

[dcharkes]

Closing issue as https://github.com/dart-lang/sdk/issues/36730, please reopen if you have any issues with the workaround in the mean time.