Thunder from the hill wrote:
> > > don't cast from "foo *" to "bar *" if sizeof(foo)<sizeof(bar)
> >
> > What is the reason for this? I do it quite routinely ("poor man's
> > inheritance").
>
> This should only be OK if you pad bar before.
Erm, crossed wire :-)
I do it for inheritance in the same way as, say, Xlib with X events.
That's perfectly safe and commonplace.
I didn't understand Albert's reason for prohibiting the mere cast.
(Notions of old machines where the char * representation is different
from int * came to mind, but Linux doesn't run on those... does it?)
Oliver Neukum explained that you shouldn't dereference a pointer to a
larger type because of alignment issues on some machines.
sizeof(foo)<sizeof(bar) captures this rule just fine for the basic data
types (char, int etc.).
But for structures, it's actually possible to have a smaller type with a
larger alignment requirement, and vice versa:
struct small { double x; };
struct large { char y [11]; };
Also, it is certainly permitted to cast "char *" to "int *" if you know
that the underlying object is an "int" or something compatible with one.
So, the general rule `don't cast from "foo *" to "bar *" if
sizeof(foo)<sizeof(bar)' is wrong, and is routinely not followed.
An alternative rule might be `never dereference a "bar *" if it might
not have the correct alignment for "bar" on any platform'.
-- Jamie
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sun Jul 07 2002 - 22:00:18 EST