[DFDL-WG] Need dfdl:inputValueCalc and dfdl:outputValueCalc on same element

[Steve Hanson]

If I understand the example correctly, you have added 'payloadLength' to 
the infoset as a convenience to the user, but at the same time  you are 
referencing it from the 'payload' element's dfdl:length property. Your 
example could be changed to use the 'length' element instead:

<element name="length" type="xs:int"
  dfdl:outputValueCalc="{ dfdl:contentLength(../payload) + 20 }">
<element name="payloadLength" type="xs:int"
  dfdl:inputValueCalc="{ ../length - 20 }" />
<element name="payload" dfdl:length="{ ../length - 20 }">
   <complexType>.....</complexType>
</element>

If we allow both input & output value calc on a element, it is saying that 
on unparsing the element's value in the infoset is ignored, and its value 
never appear in the data. So it's a place to hold a transient value, don't 
we have variables for that? 

Regards
 
Steve Hanson
IBM Integration Bus, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   19/07/2016 18:49
Subject:        [DFDL-WG] Need dfdl:inputValueCalc and 
dfdl:outputValueCalc on same    element
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>




So in PCAP format we find something like this:


<element name="length" type="xs:int"
  dfdl:outputValueCalc="{ dfdl:contentLength(../payload) + 20 }">
<element name="payloadLength" type="xs:int"
  dfdl:inputValueCalc="{ ../length - 20 }" />
<element name="payload" dfdl:length="{ ../payloadLength }">
   <complexType>.....</complexType>
</element>


To make this unparse we really need the payloadLength element to carry 
both an IVC and an OVC. 

Otherwise when unparsing the dfdl:length expression on the payload element 
can't be meaningful. We have an erratum that states that length 
expressions are always re-evaluated when unparsing, so as to produce a 
target length that can be used to insert padding/fillByte. 

However, the spec currently says you cannot have both IVC and OVC on the 
same element.

I do not recall why we have this restriction, other than trying to 
prohibit things we didn't think were necessary, but it seems in this 
situation to be needed.

Comments?


Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
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