Clarification anyone? -- was Re: Linux and aliasing?

[Martin v. Loewis]

> I've been following this thread from the beginning, but I don't have the
> ANSI/ISO standard for C, and I'd like to know more about what kind of code is
> conformant and what isn't.

Tim already answered that, but then also explained what optimizations
the compiler can do. Here's just the rule, in different wording:

<rule>
Any access to an object of some type through a pointer of a different,
incompatible type is invalid (unless the different type is char*).
</rule>
(Look Ma, I didn't say 'alias':)

<example>
So if you have 

  struct foo *my_foo = get_some_foo();
  struct bar *my_bar = (struct bar*)my_foo;

then writing

  my_bar->some_date = 42;

is incorrect (actually, undefined). However, writing

  struct foo *my_foo2 = (struct foo*)my_bar;
  my_foo2->some_data_inside_foo = -42;

is fine: the object behind my_foo2 is really of type 'struct foo'.
</example>

Hope this clarifies it,
Martin