How to define extern type?

Question

I have some c functions with a struct pointer argument.

extern "C" {
    fn InitSomeStruct() -> *SomeStruct;
    fn SomeFunction(v: *SomeStruct);
    fn DestroySomeStruct(v: *SomeStruct);
}

fn main() {
    unsafe {
        let s = InitSomeStruct();
        SomeFunction(s);
        DestroySomeStruct(s);
    }
}

The implementation of SomeStruct is unknown. How to declare and use external struct like SomeStruct from the rust code?

Answer 1

The convention is to use an empty enum for opaque FFI types, that is:

enum SomeStruct {}

An empty struct like struct SomeStruct; is also used sometimes.