Re: Seeking an example that illustrates the need for dfdl:contentLength

[Beckerle, Mike]

Simply modify the length of the logical data. Now your pre existing length is 
incorrect and must be computed.

Or start from just the logical data. How did you get the length in the first 
place?

It required knowledge that the string is delimited by a single nul byte, as 
opposed to having a prefixed length, or having its length implied by the 
overall length minus 20 which is the length of the identifier.

You have to know the format to compute the length.








________________________________
From: Roger L Costello <coste...@mitre.org>
Sent: Monday, March 29, 2021 7:32:01 AM
To: users@daffodil.apache.org <users@daffodil.apache.org>
Subject: Seeking an example that illustrates the need for dfdl:contentLength


Hi Folks,



My (binary) input file consists of:

  *   Two bytes (little endian) indicating the length of the following person 
data
  *   Person info, consisting of:
     *   Person name, null terminated
     *   Person digital identifier, 20 hex digits



At the end of this message is my DFDL schema. It parses the input fine. Parsing 
generates this XML:



<SimpleDataFormat>

  <length>29</length>

  <personInfo>

    <name>John Doe</name>

    <digital-id>AE6E4D1209F17B460503904FAD297B31E9CF6362</digital-id>

  </personInfo>

</SimpleDataFormat>



It also unparses fine.



There was no need to use contentLength. How can I modify/add-to the example so 
that contentLength must be used to properly unparse? I want the example to 
require this:



<xs:element name="length" type="xs:nonNegativeInteger"

    dfdl:lengthKind="explicit"

    dfdl:length="2"

    dfdl:lengthUnits="bytes"

    dfdl:outputValueCalc='{ dfdl:contentLength(../personInfo) }'

/>

<xs:element name="personInfo"

    dfdl:lengthKind="explicit"

    dfdl:length="{../length}"

    >

    ...

</xs:element>



Here is my current DFDL schema:



<xs:element name="SimpleDataFormat"

    dfdl:lengthKind="implicit"

    >

    <xs:complexType>

        <xs:sequence>

            <xs:element name="length" type="xs:nonNegativeInteger"

                dfdl:lengthKind="explicit"

                dfdl:length="2"

                dfdl:lengthUnits="bytes"

            />

            <xs:element name="personInfo"

                dfdl:lengthKind="explicit"

                dfdl:length="{../length}"

                >

                <xs:complexType>

                    <xs:sequence>

                        <xs:element name="name" type="xs:string"

                            dfdl:encoding="ASCII"

                            dfdl:terminator="%NUL;"

                            dfdl:emptyValueDelimiterPolicy="terminator"

                            dfdl:lengthKind="delimited"

                        />

                        <xs:element name="digital-id" type="xs:hexBinary"

                            dfdl:lengthKind="explicit"

                            dfdl:length="20"

                            dfdl:lengthUnits="bytes"

                        />

                    </xs:sequence>

                </xs:complexType>

            </xs:element>

        </xs:sequence>

    </xs:complexType>

</xs:element>