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

[Eric Weddington]

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.

You should really use the C language bitwise operators (|, &, ~, ^, <<, >>)
and use bit masks to manipulate the bits that you want.

The bit-wise operators have one advantage over using the bit-field
structure. Using bit-wise operators you can operate on multiple,
*non-contiguous* bits simultaneously within a variable. You can't do that
with bit-field structures.

Eric Weddington
(Give me a call if you want, and we can discuss it more)