missing_exception_value

The method {0} must have an exceptional return value (the second argument) when the return type of the function is neither 'void', 'Handle', nor 'Pointer'.

The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction or NativeCallable.isolateLocal doesn't have a second argument (the exceptional return value) when the type to be returned from the invocation is neither void, Handle, nor Pointer.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type returned by f is expected to be an 8-bit integer but the call to fromFunction doesn't include an exceptional return argument:

import 'dart:ffi';
​
int f(int i) => i * 2;
​
void g() {
  Pointer.fromFunction<Int8 Function(Int8)>(f);
}

Add an exceptional return type:

import 'dart:ffi';
​
int f(int i) => i * 2;
​
void g() {
  Pointer.fromFunction<Int8 Function(Int8)>(f, 0);
}