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Re: Adding UNION/MAP -- Feedback and tips pls!
- From: Russell Brennan <RussellJBrennan at gmail dot com>
- Cc: gcc at gcc dot gnu dot org, "fortran at gcc dot gnu dot org" <fortran at gcc dot gnu dot org>
- Date: Wed, 6 Mar 2013 14:07:47 -0500
- Subject: Re: Adding UNION/MAP -- Feedback and tips pls!
- References: <51314549.6080902@earthlink.net> <Prayer.1.3.5.1303020815470.18790@hermes-1.csi.cam.ac.uk> <CAGkQGiL7Qg0RAqJrT5i4LDzjnsr3jQ42Ws2BQFDpPUHyQe3+WQ@mail.gmail.com> <CAKwh3qgyS0nRcGSE5UT+DN+11kjYRE9Vu7SsQ_AGPtuD7VV4dA@mail.gmail.com> <Prayer.1.3.5.1303021519480.2329@hermes-1.csi.cam.ac.uk> <CAAKgPaHZwaoSBVg-hNZxan-uAtnmEBjPxyv9iuh=xdOshFp1uQ@mail.gmail.com> <Prayer.1.3.5.1303041813340.19357@hermes-1.csi.cam.ac.uk> <CAAKgPaGtNQoA=2h+5Z8DmvgsEBKUKX78Ojwi_SY2RG8hUcksOg@mail.gmail.com> <Prayer.1.3.5.1303042034040.18634@hermes-1.csi.cam.ac.uk> <51378C6D.5060609@netcologne.de>
>> "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