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
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848



From:        Mike Beckerle <mbeckerle.dfdl@gmail.com>
To:        Steve Hanson/UK/IBM@IBMGB
Cc:        dfdl-wg@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@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


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