"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. " Yes that's what I was calling the first child rule. In DFDL terms, would that also mean that the first child can not have any leading skip, as that would throw off the alignment? We also need to bear in mind that we have complex element, sequence, first child element, all of which have alignment. 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 18:51 Subject: Re: [DFDL-WG] Possible DFDL spec errata 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@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 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 Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU -- 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. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU