Ensure allocated objects are fully initialized (e.g. ObjC blocks)

[mkustermann]

From looking at the examples (e.g. example/objective_c/avf_audio_bindings.dart):

  ffi.Pointer<_ObjCBlock> _newBlock1(ffi.Pointer<ffi.Void> invoke, ffi.Pointer<ffi.Void> target) {
    final b = pkg_ffi.calloc.allocate<_ObjCBlock>(ffi.sizeOf<_ObjCBlock>());
    b.ref.isa = _objc_concrete_global_block1;
    b.ref.invoke = invoke;
    b.ref.target = target;
    b.ref.descriptor = _objc_block_desc1;
    final copy = _Block_copy(b.cast()).cast<_ObjCBlock>();
    pkg_ffi.calloc.free(b);
    return copy;
  }

as well as the generator in lib/src/code_generator/objc_built_in_functions.dart

  late final newBlock = ObjCInternalFunction('_newBlock', _blockCopyFunc, (Writer w, String name) {
    ...
    return '''
$blockPtr $name($voidPtr invoke, $voidPtr target) {
  final b = ${w.ffiPkgLibraryPrefix}.calloc.allocate<$blockType>(
      ${w.ffiLibraryPrefix}.sizeOf<$blockType>());
  b.ref.isa = ${concreteGlobalBlock.name};
  b.ref.invoke = invoke;
  b.ref.target = target;
  b.ref.descriptor = ${blockDescSingleton.name};
  final copy = ${_blockCopyFunc.name}(b.cast()).cast<$blockType>();
  ${w.ffiPkgLibraryPrefix}.calloc.free(b);
  return copy;
}
''';
  });

It seems that for flags field of an ObjC block isn't initialized

class _ObjCBlock extends ffi.Struct {
  external ffi.Pointer<ffi.Void> isa;
  @ffi.Int()
  external int flags;
  @ffi.Int()
  external int reserved;
  external ffi.Pointer<ffi.Void> invoke;
  external ffi.Pointer<_ObjCBlockDesc> descriptor;
  external ffi.Pointer<ffi.Void> target;
}

The memory allocation will leave the contents of memory as uninitialized, yet those (uninitialized) flags will influence how the block is e.g. copied, etc.

/cc @liamappelbe Was it intentional to not initialize flags?

[dcharkes]

I believe you're looking for #233.

[mkustermann]

I believe you're looking for #233.

It doesn't explain why .flags isn't initialized, does it?

The code seems to allocate an ObjC block (without initializing .flags) and then calls _Block_copy which is reading those flags. Maybe I misunderstand something?

[liamappelbe]

I was assuming that the allocated memory is zero initialized. I'll fix it.