[Dart FFI] How to pass a Struct via Dart_PostCObject_DL?

[Xwnine]

Cpp Inside

struct Connected {
    uint32_t fd;
    uint32_t connect_type;
    char* local_address;
    uint16_t local_port;
    char* peer_address;
    uint16_t peer_port;
    uint32_t ip_family;
};

DART_EXPORT bool MockPostCObject() {
    Dart_CObject c_event_code;
    c_event_code.type = Dart_CObject_kInt32;
    c_event_code.value.as_int32 = 1;

    Dart_CObject c_mrt_fd;
    c_mrt_fd.type = Dart_CObject_kInt32;
    c_mrt_fd.value.as_int32 = 0;

    char localBuf[100] = "127.0.0.1";
    char peerBuf[100] = "192.168.1.25";

    Connected *cc = (Connected *)malloc(sizeof(Connected));
    cc->fd = 0;
    cc->connect_type = 1;
    cc->local_address = localBuf;
    cc->local_port = 443;
    cc->peer_address = peerBuf;
    cc->peer_port = 443;

    Dart_CObject c_data;
    c_data.type = Dart_CObject_kInt64;
    c_data.value.as_int64 = reinterpret_cast<intptr_t>(&cc);
 

    Dart_CObject* c_request_arr[] = {&c_event_code, &c_mrt_fd, &c_data};
    Dart_CObject c_request;
    c_request.type = Dart_CObject_kArray;
    c_request.value.as_array.values = c_request_arr;
    c_request.value.as_array.length = sizeof(c_request_arr) / sizeof(c_request_arr[0]);

    bool result = Dart_PostCObject_DL(send_port_, &c_request);
    return result;
}

Dart side

class Connected extends ffi.Struct {
  @ffi.Uint32()
  external int fd;
  @ffi.Uint8()
  external int connectType;
  external ffi.Pointer<ffi.Uint8> local_address;
  @ffi.Uint16()
  external int local_port;
  external ffi.Pointer<ffi.Uint8> peer_address;
  @ffi.Uint16()
  external int peer_port;
}


class CtsWork {
  final int eventCode;
  final int fd;
  final int? data;

  factory CtsWork.fromCppMessage(List message) {
    return CtsWork._(message[0], message[1], message[2]);
  }

  CtsWork._(this.eventCode, this.fd, this.data);

  @override
  String toString() =>
      "CtsWork(eventCode: $eventCode, fd: $fd, dataAddress: ${data.toString()})";
}

void _handleCppWork(dynamic message) {
  if (message == null) {
    return;
  }

  final request = CtsWork.fromCppMessage(message);
  logDebug("CtsMrt handle message: ${request.toString()} ");

  final dataAddress = request.data as int;
  final event = Pointer<Connected>.fromAddress(dataAddress).ref;
  print("connected port: ${event.local_port}, ip: ${event.local_address}, peerPort:${event.peer_port}, peerIp: ${event.peer_address}, connect_type: ${event.connectType}");
}

and conslog output

flutter: [2021-08-31 05:22:52:938][debug][]Port Message: CtsWork(eventCode: 1, fd: 0, dataAddress: 123145350311448)
flutter: connected port: 80, ip: Pointer<Uint8>: address=0x700002dc6e70, peerPort:59054, peerIp: Pointer<Uint8>: address=0x700002dc6e80, connect_type: 0

[mraleph]

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(&cc);

&cc means address of variable cc. What you want to send instead is just the value of cc.

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(cc);

[Xwnine]

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(&cc);

&cc means address of variable cc. What you want to send instead is just the value of cc.

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(cc);

it works !!!
thank you !!!

[Xwnine]

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(&cc);

&cc means address of variable cc. What you want to send instead is just the value of cc.

    c_data.value.as_int64 = reinterpret_cast<intptr_t>(cc);

When should CC be released?
I tried calling Dart_PostCObject_DL(send_port_, &c_REQUEST);
And then delete cc and get an incorrect value：

flutter: [2021-08-31 05:22:52:938][debug][]Port Message: CtsWork(eventCode: 1, fd: 0, dataAddress: 123145350311448)
flutter: connected port: 18960, ip: Pointer: address=0x700002dc6e70, peerPort:443, peerIp: Pointer: address=0x700002dc6e80, connect_type: 0

[mraleph]

@Xwnine you should delete it when you don't need it anymore. If you do delete cc you delete an object pointed by cc - so however tries to use it afterwards gets garbage values. You should delete it on the receiving end when you no longer need it.

[Xwnine]

@Xwnine you should delete it when you don't need it anymore. If you do delete cc you delete an object pointed by cc - so however tries to use it afterwards gets garbage values. You should delete it on the receiving end when you no longer need it.

Thank you very much for your answer. Do you mean to delete it on the Dart side?

Dart Side

void _handleCppWork(dynamic message) {
  if (request.data == null) {
    return;
  }

  /// Get the pointer to cc.  cc was created on the C++ side .Like this  Connected *cc = (Connected *)malloc(sizeof(Connected));

  final pointer = Pointer.fromAddress(request.data as int);
  final event = pointer.cast<Connected>().ref;
  print("connected port: ${event.local_port}, ip: ${event.local_address}, peerPort:${event.peer_port}, peerIp: ${event.peer_address}, connect_type: ${event.connectType}");
  
 /// handle event 
  observer.onConnected(event);
  
  ///I do not need to use CC now, how should I free CC memory?  Like this?
  calloc.free(pointer);
}

[mraleph]

Do you mean to delete it on the Dart side?

Yes. If you allocated the memory with new then you should free it with delete, so you might need to create a helper C function that does that for you and call that through ffi.

If you allocated memory with malloc or calloc then you can just call malloc.free.

[Xwnine]

Do you mean to delete it on the Dart side?

Yes. If you allocated the memory with new then you should free it with delete, so you might need to create a helper C function that does that for you and call that through ffi.

If you allocated memory with malloc or calloc then you can just call malloc.free.

thank u .I'll try creating cc using malloc in C++ and calling calloc.free on the dart side instead