problem when using a FFI 'C' callback function

[AlexBelchUK]

I'm on dart 2.7.1 Windows 10 - I'm getting a problem when using a FFI 'C' callback function.
I tried to pass a function pointer to a 'C' method that uses a function call back
When to callback is performed, I'm getting an error message from dart c:\b\s\w\ir\cache\builder\sdk\runtime\vm\runtime_entry.cc: 3217: error: Cannot invoke native callback outside an isolate.
I've tried using an isolate in the registration call but the error still happens

I've seen in issue 37301 that someone has done a callback...

I was wondering if the patch was in 2.7.1 or still on a dev branch ?
Can FFI 'C' callbacks to dart be done yet ?

[dcharkes]

Can FFI 'C' callbacks to dart be done yet ?

Yes.

From which thread are you trying to do a callback? Is this the Dart thread, or are you trying to do a callback from another thread?

[AlexBelchUK]

I'm trying the callback from another thread in 'C' outside the Dart thread.
How does the error 'Cannot invoke native callback outside an isolate.'
happen ?

[dcharkes]

Yes, doing a callback from another thread is not supported. Dart has one mutator thread per isolate. If a Dart thread is already running in the isolate, and you would call back from C into Dart with another thread, there would be 2 threads executing Dart code in a single isolate. That would break the concurrency assumptions.

Instead, you have to call back into Dart on the main Dart thread. One can notify the main Dart thread asynchronously through a SendPort. Please check this sample: https://dart-review.googlesource.com/c/sdk/+/134704.

[AlexBelchUK]

Thanks for the help, I'll follow the example and add adaptor functions to register the port and then to receive callbacks on the 'C' side. I'll use the Dart send message 'C' functions to pass parameter messages into the Dart isolate. Thanks again.

[shuisheng]

hi @dcharkes,
I have dart client and call c++ dll
I use async callback via port, pass data to dart from c++ thread with Dart_PostCObject.
after I compiled c++ code, then open c++ dll in dart, prompt cannot open the c++ dll

then when I remove the call to Dart_PostCObject, that issue not occur

[shuisheng]

It's my mistake, I should use Dart_PostCObject_DL instead of Dart_PostCObject. now it can work

[houzhj58]

since dart 3.1, can use NativeCallable