Re: Identifying a structure by reading it's first field.

[Chris Vine <chris cvine freeserve co uk>]

On Sat, 02 Jun 2012 09:10:06 -0400
Ernie Wright <erniew comcast net> wrote:
> On 6/2/2012 6:16 AM, Chris Vine wrote:
>
> > You are probably also interested in the strict aliasing rule, [...]
> > This is an alarmingly often overlooked rule.
>
> In part because it wasn't in the standard prior to C99, which isn't
> supported to the degree C90 is.

It was in C89 - §6.3:

  "An object shall have its stored value accessed only by an lvalue
  that has one of the following types:

  *     the declared type of the object,

  *     a qualified version of the declared type of the object,

  *     a type that is the signed or unsigned type corresponding to the
        declared type of the object,

  *     a type that is the signed or unsigned type corresponding to a
        qualified version of the declared type of the object,

  *     an aggregate or union type that includes one of the
        aforementioned types among its members (including, recursively,
        a member of a subaggregate or contained union), or

  *     a character type."

It is the fifth bullet point which permits your usage.  Not many
1990-vintage compilers used strict aliasing for optimization, but most
(all?) now do with a sufficiently high optimizing level.

Chris