Re: Casting a pointer to a structure

[Jim Blandy <jimb@red-bean.com>]

> does anyone know if it is possible to cast a pointer to
> a structure into a pointer to the first element of a substructure?
> 
> For example:
> 
> struct env_funcs {
>   int something;
> }
> struct x {
>   struct *env_funcs;
>   int something;
> }
> struct m {
>   struct x x;
>   int something;
> }
> 
> Now is it possible to cast a pointer to m into a pointer to a pointer
> to struct env_funcs?   I know that it is possible to cast a pointer
> to the first element of a structure into a pointer to the whole structure
> but the above construct surprised me.

You may be looking for someone's opinion other than mine, but
anyway...  :)

The C *compiler* will let you cast anything to anything you like.  So
it's a question of what is guaranteed to be safe.

I assume you actually meant to write:

struct env_funcs {
  int something;
}
struct x {
  struct env_funcs *env_funcs;
  int something;
}
struct m {
  struct x x;
  int something;
}

ANSI C guarantees that it is safe to cast a pointer to a structure to
a pointer to the type of that structure's first element.  Thus, it is
safe to cast a `struct m *' to a `struct x *'.  Applying the same rule
again, it is safe to cast a `struct x *' to a `struct env_funcs **'.

I conclude (perhaps erroneously) that you might as well cast a `struct
m *' directly to a `struct env_funcs **', since none of the casts are
actually doing any computation, and the compiler doesn't try to verify
the safety of any of the casts anyway.

I don't have a copy of the standard any more, so I can't say whether
collapsing the two casts into one is kosher.