Re: [Gm2] translating C bit fields

[Gaius Mulley]

Dragiša Durić <address@hidden> writes:

> Gaius,
>
> IMO - we also need a mechanism to fix a sequence of fields to word boundary.
> Obviousy, a field without "BY" will fit to its default alignment position, but
> what if we have
>
> type = RECORD
>   foo: INTEGER BY 2;
>   bar: CARDINAL BY 4;
>   butalso: CARDINAL BY 2; (* and we need to have this aligned to some 
> boundary…
> *)
> END;
>
> Alignment can be pretty complicated these days when on same architecture some
> people decide to align 64 bit values to  byte positions divisible by 8… And
> other do not. I met such a situation while wrapping Gtk2 to Modula-3… gcc
> aligns such values to divisible-by-4 byte positions, while cm3 aligns to
> divisible-by-8…

Hi Dragiša,

ok - how about:

type = RECORD
   foo: INTEGER BY 2;
   bar: CARDINAL BY 4;
   butalso: CARDINAL MOD 4 BY 2; (* and we need to have this aligned to some 
boundary *)
 END;

so we extend the ebnf to include:

FieldAlignment := [ ByteAlignment | BitAlignment ] =:

BitAlignment := [ 'MOD' ConstExpression ] 'BY' ConstExpression =:

ByteAlignment := "__ATTRIBUTE__" "(" "(" Ident "(" ConstExpression ")" ")" ")"
               =:

where the MOD indicates the byte alignment and BY indicates the bit
length?  MOD 0 indicates packed, MOD 1 the next byte.

regards,
Gaius