Christofer,
So looking at your schemas I think the concept you are missing is that just
because something is zero length doesn't mean the parser won't try to parse
against it.
A DFDL schema could parse, with zero data, consume zero bits of course, yet
still populate an infoset with default values, empty strings, zero-length
hexBinary byte arrays, nilled elements, or computed elements. The parser
doesn't turn on/off based on zero length being available.
What you need do is use minOccurs="0", and dfdl:occursCountKind="expression"
and dfdl:occursCount="{ if (../parametersLength gt 0) then 1 else 0 }"
What is maybe a little confusing is that you actually must specify both - the
length AND whether there are occurrences or not. If there are no occurrences,
the length won't be used, but if there are, the parser must be able to
determine how long they are.
...mike beckerle
Tresys
________________________________
From: Christofer Dutz <[email protected]>
Sent: Tuesday, January 15, 2019 10:39:03 AM
To: [email protected]
Subject: Daffodil ignoring dfdl:length=0 when dfdl:lengthKind=explicit
Hi all,
after working though the 6 tutorials on DFDL in general I think I have a much
greater understanding on how I have to do things. I even managed to get my S7
protocol messages schema in a somewhat working condition.
Right now I’m having one problem:
A S7 Messages consists of a header, a number of variable length parameters and
a number of variable length payloads.
As parameters and payloads are of variable length, the header contains a
“parametersLength” and “payloadsLength” field which contains the number of
bytes the parameters and payloads require in total.
So I defined something like this:
<xs:element name="parametersLength" type="s7:short"/>
<xs:element name="payloadsLength" type="s7:short"/>
<xs:element name="parameters" dfdl:lengthKind="explicit"
dfdl:lengthUnits="bytes" dfdl:length="{../parametersLength}">
<xs:complexType>
<xs:sequence>
<xs:choice>
<xs:element ref="s7:S7GeneralParameterSetupCommunication"/>
<xs:element ref="s7:SS7RequestParameterCPUService"/>
<xs:element ref="s7:S7RequestParameterReadVar"/>
<xs:element ref="s7:S7RequestParameterWriteVar"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="payloads" dfdl:lengthKind="explicit" dfdl:lengthUnits="bytes"
dfdl:length="{../payloadsLength}">
<xs:complexType>
<xs:sequence>
<xs:choice>
<xs:element ref="s7:S7RequestPayloadCpuServices"/>
<xs:element ref="s7:S7RequestPayloadWriteVar"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
</xs:element>
However in a request, that doesn’t contain any payloads, daffodil still tries
to parse the payloads, even if the “length” of the sequence is set to explicit
and to a length of 0 … why is it doing that?
Each parameter and payloads first byte contains the code that tells the parser
what type it is and each of the elements in my schema use a discriminator to
tell the parser which input it is requiring.
<xs:element name="S7RequestPayloadCpuServices">
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:discriminator test="{./type eq 0}"/>
</xs:appinfo>
</xs:annotation>
<xs:complexType>
<xs:sequence>
<xs:element name="type" type="s7:byte"/>
<xs:element name="transportSize" type="s7:byte"/><!-- fixed="9"-->
<xs:element name="length" type="s7:byte"/>
<xs:element name="sslId" type="s7:short"/>
<xs:element name="sslIndex" type="s7:short"/>
</xs:sequence>
</xs:complexType>
</xs:element>
Hope it’s correct to do things like that …
The effect is that Daffodil has correctly parsed all the parameters and as
there is no payload (payloadLength = 0) it shouldn’t try to parse a payload,
but it does and when reading the first byte it instantly fails as there is no
data to parse anymore.
Chris
