Dart FFI and arrays

[Chris Norman]

Hi all,

I'm currently writing some Dart bindings for a library written in C++. I am trying to bind a function defined as follows:

// From synthizer.h:

SYZ_CAPI syz_ErrorCode syz_echoSetTaps(syz_Handle handle, unsigned int n_taps, struct syz_EchoTapConfig *taps);

Far as I can tell, `taps` is supposed to be an array of ` syz_EchoTapConfig` structs.

Here is my function (it's mercifully short):

  /// Sets the taps of the echo.

  void setTaps(List<EchoTapConfig>? taps) {

    if (taps == null || taps.isEmpty) {

      synthizer

          .check(synthizer.synthizer.syz_echoSetTaps(handle.value, 0, nullptr));

    } else {

      final a = Array<Pointer<syz_EchoTapConfig>>(taps.length);

      for (var i = 0; i < taps.length; i++) {

        final t = taps[i];

        final tc = calloc<syz_EchoTapConfig>();

        tc.ref

          ..delay = t.delay

          ..gain_l = t.gainL

          ..gain_r = t.gainR;

        a[i] = tc; // This line causes a crash.

        print(a);

      }

      synthizer.check(

          synthizer.synthizer.syz_echoSetTaps(handle.value, taps.length, a[0]));

    }

  }

I'm reasonably sure my problem lies with my use of the Array class, and (more specifically), then StructArray extension, which gives it the [] operator.

Can anyone suggest what I can do next? I have tried googling, but I can't find aexamples of using the Array class, neither in the examples for dart:ffi, or the ffi package.

If you need more info, please tell me. I'm by no means a C++ programmer, and I'm not really sure what info to provide to be useful rather than extraneously verbose.

Thanks in advance.

Take care,

Chris Norman

[Simon]

Indeed, the problem is that you're manually creating instances of Array. As the documentation points out, the constructors are only meant to be used as annotations on a struct class and we're not supposed to invoke then in normal code.

You can allocate multiple structs in Dart, which is what the C library probably expects:

  } else {

     final a = calloc<syz_EchoTapConfig>(taps.length);

     for (var i = 0; i < taps.length; i++) {

       final t = taps[i];

       final tc = a[i];

       tc

         ..delay = t.delay

         ..gain_l = t.gainL

         ..gain_r = t.gainR;

     }

    synthizer.check(

        synthizer.synthizer.syz_echoSetTaps(handle.value, taps.length, a));

     calloc.free(a);

   }