Christofer,
So in DFDL, unlike regular XSD, only elements can be repeating/optional. You can't have max/minOccurs on a choice or sequence, only on an element. DFDL isn't only for XML. Many data models that the DFDL infoset could be projected into, those data models don't allow repeating or optional entities that aren't named and unitary. If we had allowed repeating anonymous groups, then if you interfaced DFDL directly to a language with tthis, we'd be having to generate names for these anonymous groups. We decided instead to keep the data model for DFDL simpler. If it repeats or is optional it has to be an element. The rationale here is the same reason we left out attributes of XSD. DFDL has only elements. This is because this dual-child tree where a node can have element children AND attribute children that have the same names that do not get mixed up... that's unique to XML, and DFDL is trying to be not so tied to XML/XSD, but able to describe data and project that data into the native data structures of many data models. ...mike beckerle Tresys Technology ________________________________ From: Christofer Dutz <[email protected]> Sent: Tuesday, January 15, 2019 4:15:26 PM To: [email protected] Subject: Re: Daffodil ignoring dfdl:length=0 when dfdl:lengthKind=explicit (Wonder why every response turns out to be a private off-list response ... gotta remember to hit "reply to all") Thanks guys ... that worked like a charm ;-) Now I stumbled into the next little problem I couldn't find any documentation on. Now I have two situations: 1) The "parameterLength" provides the number of bytes all parameters consume, so the parser should continue parsing parameters as long as there are bytes left to read. How can I tell the sequence or the choice allow multiple instances? 2) I have "numItems" specifying the number of item-elements (also different types) so here not the number of bytes controls how many items are parsed, but the plain number of elements. Here too I cant find a way to specify a xs:minOccurs, xs:maxOccurs or dfdl:occursCount ... all seem to be invalid for both sequence and choice. Chris Am 15.01.19, 18:52 schrieb "Steve Lawrence" <[email protected]>: It's not that Daffodil is ignoring dfdl:lengthKind="0", it's just that is allows length of zero to be valid. There are actually some use cases where zero length is valid, and you would want this to cause a backtrack if the children required more than zero bytes. In this case, you just need to make it so that if the length is zero then it does not attempt to parse any child elements. One way to accomplish this is via dfdl:occursCountKind="expression" and dfdl:occursCount, something like so: <xs:element name="payloads" minOccurs="0" maxOccurs="1 dfdl:lengthKind="explicit" dfdl:lengthUnits="bytes" dfdl:length="{../payloadsLength }" dfdl:occursCountKind="expression" dfdl:occursCount="{ if (../payloadsLength eq 0) then 0 else 1 }" <xs:complexType> <xs:sequence> <xs:choice> <xs:element ref="s7:S7RequestPayloadCpuServices"/> <xs:element ref="s7:S7RequestPayloadWriteVar"/> </xs:choice> </xs:sequence> </xs:complexType> </xs:element> This changes the payloads element to be optional (minOccurs=0) and defines the occurrences as either 0 or 1 based on the value of the payloadsLength. If the length is 1, the payloads element will not exist in the infoset and it will not attempt to parse the child data. - Steve On 1/15/19 10:39 AM, Christofer Dutz wrote: > 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 >
