[pjmlp]

Another good example is Win32, in many cases only the length is initialized and the API does the rest, this allows them to change the ABI across versions without impacting the caller.

[ATsch]

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.

[pjmlp]

I see, thanks.