How to pass array of structs from C++ to Dart

[rashedmyt]

I have been trying this for sometime but it always results in seg fault.

The C++ Code:

struct Test {
    int32_t data;
};

extern "C" __attribute__((visibility("default"))) __attribute__((used))
struct Test **getTestData() {
    struct Test **test = (struct Test **) malloc(4*sizeof(struct Test *));
    for (int i = 0; i < 4; i++) test[i] = (struct Test *) malloc(sizeof(struct Test));
    test[0]->data = 21;
    test[1]->data = 14;
    test[2]->data = 50;
    test[3]->data = 34;
    return test;
}

The Dart Code:

import 'dart:ffi';

class Test extends Struct<Test> {
  @Int32()
  int data;

  factory Test.allocate(int data) =>
      Pointer<Test>.allocate().load<Test>()..data = data;
}

final Pointer<Pointer<Test>> Function() nativeTest = cryptoLib
    .lookup<NativeFunction<Pointer<Pointer<Test>> Function()>>("getTestData")
    .asFunction();

String welcome() {
    final res = nativeTest();
    List<String> msgs = List<String>();
    var len = 0;

    while (true) {
        final temp = res.elementAt(len++).load<Pointer<Test>>();
        if (temp == nullptr) break; // I think the problem is here
        msgs.add(temp.load<Test>().data.toString());
    }

    return msgs.join(" ");
}

Whenever I call welcome function in my flutter app.. the app crashes with seg fault. Is there any other way of doing this correctly?

[Daco Harkes]

Hey Chandu,

Shouldn't you null-terminate the data structure on the C side?

  test[0]->data = 21;
    test[1]->data = 14;
    test[2]->data = 50;
    test[3]->data = 34;

    test[4] = nullptr;

(and malloc 5*size etc.)

Kind regards,

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Daco Harkes

[rashedmyt]

That worked.. Thanks. I was also wondering how to read a tuple in c++ on the dart side? I have a vector of tuple's which will be returned from the c++ to the dart. When I put extern "C", the compiler gave error's about conversion of vector to C-style arrays.. So I decided that i need to use pointer to send array's, but I couldn't understand how tuple is read on the dart side..

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Daco Harkes

[Daco Harkes]

Hey Chandu,

std::tuple and std::vector are not part of the ABI (and can be laid out differently in memory by different compilers).

In dart:ffi, we only consider things specified by the ABI.

The work around is to write some C functions to interact with your data structures and pass a pointer to the data structure around.

Kind regards,

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Daco Harkes

[Daco Harkes]

FYI: I created https://github.com/dart-lang/sdk/issues/38788 to point people to. (This is not the first time this question has been asked.)

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Daco Harkes

[rashedmyt]

Oh.. ok. Thanks for pointing out the nullptr in C

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Daco Harkes