On Sat, 02 Jun 2012 09:10:06 -0400
Ernie Wright <ern...@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 - ยง6.3:
"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
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