Rfcs: It's not possible to get the size of the offset from an arbitrary structure to it's field.

[steveklabnik]

Issue by sstewartgallus
_Wednesday Jul 31, 2013 at 04:38 GMT_

_For earlier discussion, see https://github.com/rust-lang/rust/issues/8144_

_This issue was labelled with: in the Rust repository_

---

I have the following code at http://gitorious.org/p-x/p-x/blobs/master/src/gl.rs#line465 but it is very ugly. I have to create a data structure so I can calculate the offset from the structure to it's field, and pass it to the low level OpenGL code. To understand the issue better the documentation at http://www.khronos.org/opengles/sdk/docs/man/xhtml/glVertexAttribPointer.xml for the glVertexAttribPointer function may be useful.

impl <'self, S: Zero, T: AttributeValue> DisabledAttribute<'self, T> {
    pub fn vertex_pointer<C: Component<S, T>, State>(
        &self,
        _: &BufferTarget<'self, State, S>,    // Not used just needs to be bound
        normalized: bool,
        component: &C
    ) {
        let handle = self.raw_attribute;

        // Compute the stride, and offset
        let s = &Zero::zero();
        let t = component.access(s);
        let offset = borrow::to_uint(t) - borrow::to_uint(s);

        do self.linked_program.raw_program.context.send_and_recv {
            AttributeValue::vertex_attrib_pointer::<T>(
                handle,
                normalized,
                sys::size_of::<S>(),
                offset
            )
        }
    }
}

The definition of Component:

pub trait Component<S, T> {
    fn access<'r>(&self, &'r S) -> &'r T;
}

A sample implementation:

struct Positions;
impl Component<assets::Vertex, [f32, ..3]> for Positions {
    fn access<'r>(&self, s: &'r assets::Vertex) -> &'r [f32, ..3] {
        &s.position
    }
}

[aidanhs]

Somewhat related - https://github.com/Diggsey/rust-field-offset

[gtors]

The workaround:

macro_rules! offset_of {
    ($ty:ty, $field:ident) => {
        unsafe { &(*(0 as *const $ty)).$field as *const _ as usize }
    }
}

source: https://stackoverflow.com/questions/40310483/how-to-get-pointer-offset-in-bytes/40310851

[eddyb]

@gtors That might be UB. I don't know where the previous discussion was.

[tomaka]

For the record @gtors's solution caused segfaults in glium.

[eddyb]

I believe using an union / mem::uninitialized + mem::drop could work, but you have to be careful about panicking while you have an uninitialized variable on the stack.

[sfackler]

We don't have to deal with Drop impls in this case, but the uninitialized approach appears to work in practice: https://github.com/rust-lang-nursery/unix-socket/blob/master/src/lib.rs#L21. It compiles down to a constant.