>>27
sizeof(void) is not defined.
6.5.3.4 The sizeof operator
Constraints
The sizeof operator shall not be applied to an expression that has function type or an
incomplete type (...)
And
void is an incomplete type which cannot be completed:
6.2.5 Types
The void type comprises an empty set of values; it is an incomplete type that cannot be completed.
(Sorry, I don't know how to properly quote things here.)