dart ffi: Pointer<Void> as function argument

[Maarten Boekhold]

First of all, apologies if the below question shows up twice. I posted this same question earlier today, but it doesn't seem to show up. I'm not sure if that's due to moderation, or if something went wrong on my end.

I'm having big trouble creating FFI bindings for the following C function:

typedef Handle void;

int getDeviceHandle(char *deviceName, Handle *handle);

which is called in C as follows:

Handle handle;

int res = getDeviceHandle("myDevice", &handle);

I'm trying in Dart with variations of:

typedef Handle = Void;

late int Function(

    List<Utf8> deviceName,

    Pointer<Void> handle

) getDeviceHandle;

typedef getDeviceHandle_native_t = Int32 Function(

    List<Utf8> deviceName,

    Pointer<Void> handle

);

// "library" is the opened library

getDeviceHandle = library

    .lookup<NativeFunction<getDeviceHandle_native_t>>('getDeviceHandle')

    .asFunction();

// Now to call getDeviceHandle()

Pointer<Handle> handlePtr = calloc(); // fails at runtime here

String deviceName = "myDevice";

int res = getDeviceHandle(

    deviceName.toNativeUtf8(),

    handlePtr

);

handle = handlePtr.value; // don't know how to extract the actual Handle object.

                          // There is no 'value' property (or 'ref')

Can anybody help me to get this to work? I've searched a lot through packages on pub.dev that use dart:ffi in the hope of finding a similar situation, but haven't been able to find anything that worked for me.

[Daco Harkes]

Dear Maarten,

To clarify, are you using Handle as in the Dart_Handle from dart_api.h? Or is Handle just a name and could it have been called DeviceHandle as well?

Handle from dart_api.h

Currently FFI trampolines and callbacks convert Dart_Handle on the C side to Object on the Dart side. (The reasoning: why have a handle in Dart if you can have a normal pointer instead?)

To use a Dart_Handle:

// no typedef from Handle to void

late Object Function(
    List<Utf8> deviceName,

    Pointer<Int32> returnStatusCode,
) getDeviceHandle;

typedef getDeviceHandle_native_t = Handle Function(
    List<Utf8> deviceName,
    Pointer<Int32> returnStatusCode,
);

By value structs

If it's just a typedef.

One can't dereference a void* in C, nor can one get the value of a Pointer<Void> in Dart.

If you know the structure of the struct then define it as struct

class DeviceHandle extends Struct {

  @Int32()

  external int myField;

  // more fields

}

Then a Pointer<DeviceHandle> can be dereferenced by .ref to get a DeviceHandle which gives access to the myField.

Kind regards,

 •  Daco Harkes
 •  Software Engineer
 •  dacoh...@google.com 

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[Maarten Boekhold]

Hi Daco, your response prompted me to compose a very detailed description of what I'm (trying) to do, which had me jump into the source code of the library I'm trying to create bindings for. This proved very useful as it led me to discover my mistake: I need to deal with a **void, not *void. So in Dart, "Pointer<Pointer<Void>> handle = calloc();". I have much progressed now. Thanks for responding to my query,

Maarten