That should work perfectly with multiple structs. Define #[repr("C")] struct MyThing { length: usize, data: MaybeUninit<MyInner> }

That should work perfectly with multiple structs. Define #[repr("C")] struct MyThing { length: usize, data: MaybeUninit<MyInner> } Then initialize with sizeof(data) and MaybeUninit::uninitialized(). When the call is complete, assume_init() and access the fields of the result struct as normal.