Re: [avr-gcc-list] Bit-wise structure and unions

[David Gay]

On 11/21/06, Eric Weddington <address@hidden> wrote:
> Hi Keith
>
> [Long time, no hear. ;-)]
>
> Comments below.
>
> > -----Original Message-----
> > From:
> > address@hidden
> > [mailto:address@hidden
> > org] On Behalf Of Keith Gudger
> > Sent: Tuesday, November 21, 2006 11:41 AM
> > To: address@hidden
> > Subject: [avr-gcc-list] Bit-wise structure and unions
> >
> > Below is a program which illustrates the following issue:
> >
> <snip>
>
> > But I would rather not have the union.  Is there any other way around
> > this?  Thanks.
> >
>
> Unfortunately, no. The union would be the standard solution.
>
> Ok, I take that back but only slightly. Why use bit fields at all? They are
> inherently non-portable between compilers (if they work, then you're lucky).
> Inherently, bit fields are tied to an int, which on the avr port is a 16-bit
> type. We're lucky in that you can get GCC to put them into an unsigned char.
> But again, that's compiler dependent.

Actually a platform ABI for C normally specifies such things precisely
to avoid same-platform different-C-compiler portability problems
(there's way more things than just bitfields that are not specified by
the C standard, so such an ABI spec is a necessity). FWIW, avr-gcc
packs bitfields into bytes. The size of int or the type used to
declare the bitfield don't affect this packing (*). Whether this
matches what IAR C is doing is another question...

David Gay
*: If you're curious, avr-gcc doesn't define the magic
PCC_BITFIELD_TYPE_MATTERS flag in its configuration.