Unable to use callback in c code

[Dolaned]

Hi i am trying to call a callback from C but it throws a sigarbt, i currently have another callback that works. I am running: Dart SDK version: 2.10.2 (stable) (Tue Oct 13 15:50:27 2020 +0200) on "macos_x64"

Ill attach the typedef and the function too i am using dart/ffigen to create the library headers

i have a function that only returns a void and that works fine.

thanks again

[Dolaned]

@dcharkes this is what i was referring to in the other thread

[dcharkes]

What thread are you trying to do the callback on? (Thread 27 does not look like a Dart execution thread, with no DartVM things on the stack and a pthread_start.)

You can only do a callback into an isolate with the main Dart execution thread from that isolate. Doing a callback from any other thread results in an abort. This abort is intentional, it is not supported to run multiple threads in one Dart isolate.

If you want to use a thread that you spawn in C, you have to use the message passing system (native ports) to communicate back to Dart:

  Dart_Port send_port;

  Dart_CObject dart_object;
  dart_object.type = Dart_CObject_kInt64;
  dart_object.value.as_int64 = work_addr;

  const bool result = Dart_PostCObject_DL(send_port, &dart_object);

[Dolaned]

ok im not running a seperate isolate from the main one but this still doesnt matter right?

[dcharkes]

No, you're starting a thread in C (_pthread_start) and doing an FFI callback back into Dart, that is not supported.

[Greg-Griffith]

Thank you @dcharkes. Your explanation makes sense. the saveBlocks function pictured above is called by a function that was called from dart, which internally spawned a new C thread which then responds to that callback.

[Dolaned]

so to clarify we would actually put them callbacks in C and then call native ports from them call backs?

[Dolaned]

hi mate is there some documentation on the c side of this? or is it just in the code examples? Thanks.

Edit: i have found this https://www.dynamsoft.com/codepool/dart-native-extension-barcode-sdk.html what would you recommend i do for a callback that is fired from C's side rather than dart making the call to C. Thanks.

[Dolaned]

Hi @dcharkes I think I'm beginning to understand it, one final question though

Is the result from the callback function supposed to come from the callback and the dart port is used to notify dart there is a result or is it all supposed to come from the dart port

Also, how do I compile this for Xcode it tells me it can't find the symbols for arm64 when including the header files

Thanks

[Dolaned]

@dcharkes is the dart sdk stripped back for flutter and that's why it can't find the symbols ?

[dcharkes]

@Dolaned - that's why you use the _DL versions of the symbols. See https://github.com/dart-lang/sdk/commit/7eac9f355ed54cb0aab018814f776c08d20cef44.

[dcharkes]

I'm assuming this is resolved by using the native ports with the _DL symbols. Please reopen if not.

[Dolaned]

yes that did fix the issue, thanks.