[DFDL-WG] Possible DFDL spec errata

[Mike Beckerle]

I don't get it. Only the first child (aka "first anything" in the
structure) gets aligned and can have the conflict of its local alignment
specification, and that of some enclosing structure.

If some later field gets aligned, then that's not relevant at all, as you
treat that normally as another instance of alignment recursively (it could,
afterall, also be a structure with sub fields having their own alignments,
etc.)

Is that the "first child rule"

...mike

On Tue, May 29, 2012 at 1:45 PM, Steve Hanson <smh at uk.ibm.com> wrote:

> Hi Mike
>
> The one rule that C does apply is the first child rule. C does not allow
> padding before the first child in a struct, so the alignment of the first
> child must not be greater than the alignment of the parent.  But it won't
> complain if the third element's alignment is greater than the parent's, for
> example.  So the rule needs to be relaxed, I think the only question is
> whether we drop the rule altogether or go with the first child rule.
>
> Regards
>
> Steve Hanson
> Architect, Data Format Description Language (DFDL)
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK*
> **smh at uk.ibm.com* <smh at uk.ibm.com>
> tel:+44-1962-815848
>
>
>
> From:        Mike Beckerle <mbeckerle.dfdl at gmail.com>
> To:        Steve Hanson/UK/IBM at IBMGB
> Cc:        dfdl-wg at ogf.org
> Date:        29/05/2012 18:08
> Subject:        Re: [DFDL-WG] Possible DFDL spec errata
> ------------------------------
>
>
>
>
>
> On Tue, May 29, 2012 at 12:03 PM, Steve Hanson <*smh at uk.ibm.com*<smh at uk.ibm.com>>
> wrote:
> These were on the agenda for today's cancelled call.
> *
> 2. Is parent/child alignment rule too strict?*
>  Spec property description for alignment says "The alignment of a child
> component must be less than
>  or equal the alignment of its parent element, sequence or choice".
> Experiments with creating DFDL schema
>  from C structures have shown that this rule is violated. It seems
> unnecessary.  Specifically, C ensures that the alignment
>  of objects in a repeating structure is preserved by rounding up the
> length of the structure, rather than aligning it.
>
> I think the point of this restriction was to insure that we didin't have
> conflicting alignments. Where the begining of the structure was aligned,
> say 64, but the first element was aligned at some incompatible boundary.
> Like 128. It would almost certainly be an error if the first element in a
> structure has a larger alignment constraint than the overall structure, and
> the user should pick where to place the constraint, and not constrain both,
> except when the constraints on one are implied by the constraints of
> another.
>
> Example: I have a 4k page aligned structure. The first element is some
> type of double float coming from a global simple type def. The global
> simple type definition that says my specific kind of double floats are
> always 8-byte aligned. The 4k page is obviously 4096 byte aligned. There
> should be no conflict here. The page alignment "Wins" and the begining of
> the structure is 4k aligned, and that satisifies the 8-byte alignment of my
> specialized double float.
>
> The point of having any restriction at all, is to catch conflicts. Eg.,
> what if I had something that was 64-byte aligned, and I put it inside a
> structure that has only 2-byte alignment....., this is probably an error.
>
> If this isn't going to catch any errors, then this restriction is
> unnecessary.
>
> *
>  3. Does DFDL need to support signed integer types with lengthUnits 'bits'?
> *
>  Spec only allows unsigned integers to have lengthUnits 'bits', but it is
> possible in C structures to delare
>  bit fields for signed integers. They behave like two's complement. It
> looks like the spec is being
>  too restrictive in its types for 'bits'.
>
> I missed this over restriction. Signed are definitely needed.
>
>
> --
> Mike Beckerle | OGF DFDL WG Co-Chair
> Tel:  781-330-0412
>
>
> Unless stated otherwise above:
> IBM United Kingdom Limited - Registered in England and Wales with number
> 741598.
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>



-- 
Mike Beckerle | OGF DFDL WG Co-Chair
Tel:  781-330-0412
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