[DFDL-WG] Spec section 12.3.7.2 - binary reps and zero length

[Mike Beckerle]

This is one more among a number of problematic issues in the way the DFDL
Spec describes length.

I did a bunch of work on this a while back, but we decided that the
required changes to improve on what we have would be too extensive for this
draft of the spec.

I do believe we need to specify where length of zero is allowed or not. In
your analysis below you suggest that currently we are allowing packed
numbers of length zero. Since we allow prefixed and delimited for packed
numbers I think allowing length of zero is ok, and probably needed to avoid
all sorts of corner case checking.

It doesn't bother me that types/property combinations that disallow length
0 implicitly disallow use of default values. It does suggest a need for a
schema-definition error, however, if a default value is specified, but the
types and properties are such that it cannot be defaulted when parsing. Can
the default value end up being used when unparsing? I would think in the
case of array with minOccurs > 0 one might use it when unparsing.

The intention of Table 22 is that it gives the minimum length in bits, but
if length units is bytes, then one must divide by 8 and round up. So if
length units is 'bytes' then the minimum length for type xs:short is 1, and
the maximum length is 2.

To fix this, probably the simplest thing is to add two more columns to the
table giving minimum length when length units is bytes, and maximum length
when length units is bytes. That will be clearer than whatever prose we try
to come up with to explain this.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
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On Tue, Jul 8, 2014 at 10:55 AM, Steve Hanson <smh at uk.ibm.com> wrote:

> Section 9 of the specification talks about empty etc representations,
> where the meanings of a length of zero are defined.  However section
> 12.3.7.2 has sub-sections for certain binary reps where it is illegal to
> have a length of zero.  Is this deliberate?  The cases where 0 is allowed
> are also those which can have lengthKind 'delimited' so that might be the
> reason. Note that disallowing length of zero prevents use of a default
> value.
>
> 12.3.7.2.1: Binary numbers. Table 22 has minimum length in bits => 0 not
> allowed.
>
> 12.3.7.2.2: Floating point numbers: Says length must be exactly 4 bytes or
> 8 bytes => 0 not allowed.
>
> 12.3.7.2.3: Packed numbers. No minimum length stated => 0 allowed.
>
> 12.3.7.2.4: Binary booleans. Refers to 12.3.7.2.1 => 0 not allowed.
>
> 12.3.7.2.5: Binary calendars. Says length must be exactly 4 bytes or 8
> bytes => 0 not allowed.
>
> 12.3.7.2.6: Packed calendars. No minimum length stated => 0 allowed.
>
> 12.3.7.2.7: Opaque binary. No minimum length stated => 0 allowed.
>
> Noticed this when modelling an example of a header with user-defined
> information at the end with the length given by a binary count.
>
>     <xs:complexType name="Type_Header">
>       *<xs:sequence>*
>         <xs:element dfdl:length="75" name="Filler" type="xs:hexBinary"/>
>         <xs:element dfdl:length="2" name="HeaderLength"
> type="xs:nonNegativeInteger"/>
>         <xs:element dfdl:length="30" name="Filler2" type="xs:hexBinary"/>
>          <xs:element dfdl:length="{../HeaderLength - 108}" name="UserInfo"
> type="xs:hexBinary"
>                     minOccurs="0" dfdl:occursCountKind="implicit"/>
>       *</xs:sequence>*
>     </xs:complexType>
>
> This could equally be modelled using occursCountKind 'expression' but that
> seems unnecessary if the length is 0:
>
>     <xs:complexType name="Type_Header">
>       *<xs:sequence>*
>         <xs:element dfdl:length="75" name="Filler" type="xs:hexBinary"/>
>         <xs:element dfdl:length="2" name="HeaderLength"
> type="xs:nonNegativeInteger"/>
>         <xs:element dfdl:length="30" name="Filler2" type="xs:hexBinary"/>
>          <xs:element dfdl:length="{../HeaderLength - 108}" name="UserInfo"
> type="xs:hexBinary"
>                     minOccurs="0" dfdl:occursCountKind="expression"
> dfdl:occursCount="if (../HeaderLength - 108 eq 0) then 0 else 1"/>
>       *</xs:sequence>*
>     </xs:complexType>
>
> Also what do the minimum lengths in Table 22 mean when lengthUnits is
> bytes ?
>
> Regards
>
> Steve Hanson
> Architect, *IBM DFDL*
> <http://www.ibm.com/developerworks/library/se-dfdl/index.html>
> 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
> 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
>
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