Search code examples
rustffiraw-pointer

Why do most ffi functions use raw pointers instead of references?

Both in ffi tutorials and in automatically generated interfaces, *const T pointers are used most of the time. As far as I know the difference between &T and *const T is only that *const T doesn't have to fulfill certain conditions like not being null and is unsafe to dereference.

fn main() {
    unsafe {
        do_something(&TestStruct {data: 3})
    }
}

#[repr(C)]
pub struct TestStruct {
    data: u32
}

extern "C" {
    fn do_something(arg: &TestStruct);
}

This code can be compiled and runs. Because external functions are similar in usage to internal ones, i don't understand why raw pointers are used there as the default.

Solution

  • An element of answer can probably be found in the fact that references must be aligned. As using un-aligned references is undefined behaviour, and the alignment of the pointers cannot be guaranteed in FFIs, defaulting to pointers seems to be a sane choice