[ruby-core:116377] Re: [Ruby master Feature#19057] Hide implementation of `rb_io_t`.

[Eric Wong via ruby-core <ruby-core@ml.ruby-lang.org>]

"ioquatix (Samuel Williams) via ruby-core" <ruby-core@ml.ruby-lang.org> wrote:
> Are you saying that in general:
> 
> ```
> struct A {
>   int x;
>   float y;
>   char z;
>   struct S s;
> };
> ```
> 
> and
> 
> ```
> struct B {
>   int x;
>   float y;
> };
> ```

> That reinterpreting a pointer to A as a pointer to B, and accessing
> field `B::y` is never undefined? And that generalises? The only rule I
> knew was the first field, if a struct, is guaranteed to be at offset=0.
> Are you able to link me to the standard?

(oops, battery died last week and I forgot about Ruby entirely :x)

As long as it's the same HW arch, yes.  AFAIK C itself doesn't say anything
about it, it's up to every ABI.  https://refspecs.linuxfoundation.org/elf/
has a bunch of ABIs.  abi386-4.pdf (via pdftotext) states:

  Each member is assigned to the lowest available offset with the appropriate
  alignment. This may require internal padding, depending on the previous
  member.

> While I understand what you are saying may be true, I was not able to
> find confirmation of it in the standard.

It's a lot of arch-specific ABIs to read through and I haven't
read every one; but keep in mind FFI/Rust interop would not work
at all without a predictable C ABI.  I do similar things via
Perl syscall wrapper and pack (same as Ruby Array#pack) on
various architectures the GCC Compiler Farm (cfarm) provides
with no problems.

You can always add STATIC_ASSERT on the offsets to ensure new
platforms are behaving.  I don't expect Ruby will be able to run
at all if there's a hypothetical platform with unpredictable offsets.

All this only applies to C and normal C types (not C++)
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