Question about fields alignment

[Waldek Hebisch]

Pascal Viandier wrote:
> Hi,
> 
> I have a question about field's alignment in GNU Pascal records on
> different platforms.
> 
> Consider the sample program below:
> 
> Program offsets;
> Type
>   Rec1_t = Record
>              Str16 : String(16);
>              R     : Real;
>            End;
>   Rec2_t = Record
>              Str11 : String(11);
>              Rec1  : Rec1_t;
>            End;
>   Rec3_t = Record
>              Str11 : String(11);
>              Str16 : String(16);
>              R     : Real;
>            End;
<snip>
> This looks good to me. Rec1 and Rec3 map on each other.
> How come the difference in alignment of Rec1 in Rec2 between Sparc and
> Linux?
> I know that Real data is aligned on 8 bytes boundaries on Sparc
> platforms but why is the address of Rec1 aligned on 8 bytes instead of
> 4?

Typical rule is that a record requires bigger or equal alignment as
each of its components.  So Rec1 needs at least Real alignment.

> So the two records (Rec2 and Rec3) containing the same data are of
> different sizes and do not map anymore. This causes some trouble in my
> programs since this kind of data structures are passed back and forth to
> C code, in order to interface with a database engine.

Hmm, GNU Pacal uses the same rules as C, so C structures corresponding
to Rec2 and Rec3 also does not mach -- at most one can be correct.

-- 
                              Waldek Hebisch
hebisch at math.uni.wroc.pl