'Pointer<NativeFunction<T Function()>> ptr; T Function() func = ptr.asFunction(); ' When T must be 'Pointer' to pass compilation checking.

[SilverFruity]

• Dart analyzer and linter

Test on dartpad

Test on The latest main branch

Executing this code with dart compiled from the latest commit on the main branch of the github repository has the same problem, as follows:

➜  sdk git:(main) ✗ PAGER=cat git log -1
commit f860d84ad77fa0910ae5be18e2211ac06119cf8a (HEAD -> main, origin/master, origin/main, origin/lkgr, origin/base, origin/HEAD)
Author: Nicholas Shahan <nshahan@google.com>
Date:   Thu Dec 28 03:19:30 2023 +0000

    [test] Use Uris in place of Strings in test
    
    Provides consistent behavior for identical and equals across all
    backends. The test was failing on JavaScript backends because
    Strings that are equal are also identical.
    
    Change-Id: I85929f37746d8f7e192410e3d2f6530ce05176c5
    Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343860
    Reviewed-by: Nate Bosch <nbosch@google.com>
    Auto-Submit: Nicholas Shahan <nshahan@google.com>
    Commit-Queue: Nate Bosch <nbosch@google.com>
➜  sdk git:(main) ✗ ./tools/build.py --no-goma --mode release --arch x64 create_sdk
Generating Xcode projects took 47ms
Done. Made 428 targets from 121 files in 1574ms
buildtools/ninja/ninja -C xcodebuild/ReleaseX64 create_sdk
ninja: Entering directory `xcodebuild/ReleaseX64'
ninja: no work to do.
The build took 2.026 seconds
➜  sdk git:(main) ✗ cat test.dart
import "dart:ffi";

void main() {
   Pointer<NativeFunction<Pointer<Void> Function()>> isAlive = Pointer<NativeFunction<Pointer<Void> Function()>>.fromAddress(0x100000);
   Pointer<Void> Function() isAliveFunc = isAlive.asFunction();

   Pointer<NativeFunction<Void Function()>> isAlive1 = Pointer<NativeFunction<Void Function()>>.fromAddress(0x100000);
   Void Function() isAliveFunc1 = isAlive1.asFunction();
}
➜  sdk git:(main) ✗ ./xcodebuild/ReleaseX64/dart-sdk/bin/dart test.dart            
test.dart:8:44: Error: Expected type 'Void Function()' to be 'void Function()', which is the Dart type corresponding to 'NativeFunction<Void Function()>'.
 - 'Void' is from 'dart:ffi'.
 - 'NativeFunction' is from 'dart:ffi'.
   Void Function() isAliveFunc1 = isAlive1.asFunction();
                                           ^
➜  sdk git:(main) ✗ 

[halildurmus]

Using Void in Dart type signatures is not allowed, you should use void instead. You also need to specify the Dart type signatures for functions as type arguments in the asFunction method.

Try this:

void main() {
  final isAlive =
      Pointer<NativeFunction<Pointer<Void> Function()>>.fromAddress(0x100000);
  final isAliveFunc = isAlive.asFunction<Pointer Function()>();

  final isAlive1 =
      Pointer<NativeFunction<Void Function()>>.fromAddress(0x100000);
  final isAliveFunc1 = isAlive1.asFunction<void Function()>();

  final testObject = Pointer<TestResult>.fromAddress(0x100000);
  final func = testObject.ref.dispose
      .asFunction<Pointer Function(Pointer<TestResult>)>();
  final func1 =
      testObject.ref.dispose1.asFunction<void Function(Pointer<TestResult>)>();
}

I skipped specifying types in variable declarations to reduce verbosity. Simply use final or var, whichever you prefer.

[mraleph]

cc @dcharkes We should consider upgrading error messages to make them more novice friendly.

I really wish we have a simpler way to managing Dart and C signatures, maybe making primitive types to be extension types could work:

extension type const Int32._(int _) {
}

extension type Pointer<T>._(Address _) {
}

Then if you have a function type Int32 Function(Int32, Pointer<Int32>) which represents both native and Dart signature of the function.

Maybe we should file a bug to explore FFI 2.0 APIs which don't require writing two signatures.

[dcharkes]

Maybe we should file a bug to explore FFI 2.0 APIs which don't require writing two signatures.

I've filed an issue with some links. Also, there was some old (internal) design doc for dart:ffi2 which I linked there.

@SilverFruity were you able to get it working?

My suggestion would be to use package:ffigen, it can generate everything correctly for you.

[SilverFruity]

My current ffi code generated by dart:ffigen works fine, but is avoiding asFunction secondary signatures by Pointer<Void> as follows:

struct TestList {
  void *(*add)(struct TestList *, void *element);
  void *(*dispose)(struct TestList *);
};

Pointer<Void> Function(Pointer<TestList>) add = list.ref.add.asFunction();
add(list, null);
Pointer<Void> Function(Pointer<TestList>) dispose = list.ref.dispose.asFunction();
dispose(list);

[dcharkes]

I'm not entirely sure what your code is trying to do.

Are you using dynamic dispatch on purpose? Storing the function pointer in the TestList struct?
If you don't want to use dynamic dispatch, your code should probably be:

struct TestList {
};

void add(struct TestList *, void *element);
void dispose(struct TestList *);

In which case you wouldn't have to use asFunction.

I assume you want to return void.

If you want dynamic dispatch and return void, your C and Dart code should probably look like this:

struct TestList {
  void (*add)(struct TestList *, void *element);
  void (*dispose)(struct TestList *);
};

void Function(Pointer<TestList>) add = list.ref.add.asFunction();
add(list, null);
void Function(Pointer<TestList>) dispose = list.ref.dispose.asFunction();
dispose(list);

[SilverFruity]

Yes, I'm storing the function pointer in a TestList struct for dynamic dispatch, and then want to be able to call it directly using asFunction.