Re: union versus bit manipulation

[Glynn Clements <glynn@gclements.plus.com>]

Robert P. J. Day wrote:

>   looking for advice on the following issue.  some code i've inherited
> defines an unsigned 16-bit value that's meant to be interpreted in the
> following way in terms of the internal bit structure:
> 
>   1-bit       class
>   2-bit       type
>   13-bit      value
> 
> however, in addition to needing to access the individual components of
> this object, the code (sadly) also needs to treat the whole thing as
> an unsigned 16-bit value to use as a key into a larger data structure.
> 
>   at the moment, defining the whole thing as an unsigned 16-bit object
> and using bit operations works fine, but i was considering redefining
> the type to use a union thusly:
> 
> union thing {
>         uint16_t thingval ;
>         struct S {
>                 unsigned val    : 13 ;
>                 unsigned type   : 2 ;
>                 unsigned class  : 1 ;
>         } s ;
> } ;
> 
> 
>   the major problems i see are that 1) i'd obviously need to guarantee
> that the fields in the struct are packed to make sure they still
> correspond to the appropriate 16-bit value, and 2) i need to make it
> portable across different endian architectures (i'm compiling the code
> on am x86 for a Power PC board).
> 
>   given the cautions associated with structure packing and alignment,
> as well as endianness, is it even worth the trouble to think of
> something like this?

No.

> or should i just leave the object as a uint16_t and stick with the
> bit operations?

Yes.

Apart from layout issues, you risk introducing aliasing bugs if your
code is compiled with optimisation.

-- 
Glynn Clements <glynn@gclements.plus.com>