>> "However, if you overlay one variable with another smaller variable,
>> that portion of the initial variable is retained that is not overlaid.
>> Depending on the application, the retained portion of an overlaid variable
>> may or may not contain meaningful data and can be utilized at a later
>> point in the program."
>
>
> Ouch.
>
> This seems to be at odds with C's unions, where it is not allowed to do
> type punning.
As of gcc 4.4.6, the description above seems to match the C behavior:
#include <stdio.h>
#include <stdint.h>
int main() {
union {
uint32_t test32int;
uint16_t test16int;
} testunion;
testunion.test32int = 0xFFFFFFFF;
printf("Before assignment: %0x\n", testunion.test32int);
testunion.test16int = 0x0000;
printf("After assignment: %0x\n", testunion.test32int);
}
Produces the following:
Before assignment: ffffffff
After assignment: ffff0000
