[DFDL-WG] DFDL pointer & offset proposal
Bradd Kadlecik
braddk at us.ibm.com
Wed Sep 25 16:08:16 EDT 2019
Properties for indirection:
The following properties allow for data to be accessed in a non-sequential
or non-contiguous format by using indirection values to locate the data
contents. When parsing, the indirection value is used to access the
contents in the data stream to populate the infoset. When unparsing, an
indirection value is created to store the contents in the data stream when
creating the data stream from the infoset. The indirection value does not
exist in the infoset, but only in the data stream.
The LeadingAlignment and TrailingAlignment regions apply to the indirection
value in the data stream. The initiator and terminator as well as all
regions between the LeftFraming and RightFraming regions are applied to the
data content accessed through the indirection value.
Property Name | Description
----------------------+--------------------------------------------------------
indirectKind | Enum
|
| Specifies the type of indirection value used to
| access the the contents in the data stream.
|
| Valid values ‘pointer’, ‘offset’, ‘none’.
|
| ‘pointer’ means a numeric value gives an
| implementation-dependent starting position of the
| content.
|
| ‘offset’ means a numeric value that when added to
| indirectBase gives the starting position of the
| content.
|
| ‘none’ means no indirection is used and is the
| default value.
|
| Annotation: dfdl:choice, dfdl:element, dfdl:group,
| dfdl:simpleType, dfdl:sequence
----------------------+--------------------------------------------------------
indirectType | QName
|
| Name of a simple type derived from xs:integer or any
| subtype of it.
|
| This type, with its DFDL annotations specifies the
| representation of the indirection value.
|
| It is a schema definition error if the xs:simpleType
| specifies any of:
| · dfdl:lengthKind 'delimited', 'endOfParent', or
| 'pattern'
| · dfdl:lengthKind 'explicit' where length is an
| expression
| · dfdl:outputValueCalc
| · dfdl:initiator or dfdl:terminator other than
| empty string
| · dfdl:alignment other than '1'
| · dfdl:leadingSkip or dfdl:trailingSkip other
| than '0'.
|
| Annotation: dfdl:choice, dfdl:element, dfdl:group,
| dfdl:simpleType, dfdl:sequence
----------------------+--------------------------------------------------------
indirectBase | String
|
| Provides the relative path to a prior element upon
| which the offset is based.
|
| Required only when dfdl:indirectKind is ‘offset’.
|
| Annotation: dfdl:choice, dfdl:element, dfdl:group,
| dfdl:simpleType, dfdl:sequence
----------------------+--------------------------------------------------------
indirectEmptyValue | Integer
|
| Specifies the indirection value that indicates when
| the data content is empty. If this property is
| specified, all underlying elements must contain a
| default value specification or it is a schema
| definition error.
|
| Annotation: dfdl:choice, dfdl:element, dfdl:group,
| dfdl:simpleType, dfdl:sequence
The following example illustrates how to describe a pointer to a
null-terminated string (common in languages like C).
<xs:element name="myString" type="xs:string"
dfdl:lengthKind="delimited" dfdl:encoding="UTF-8"
dfdl:terminator="%NUL;" dfdl:indirectKind="pointer"
dfdl:indirectType="ptr32_t" dfdl:indirectEmptyValue="0"
default=""/>
<xs:simpleType name="ptr32_t" dfdl:representation="binary"
dfdl:lengthKind="explicit" dfdl:length="4"
dfdl:byteOrder="bigEndian">
<xs:restriction base="integer"/>
</xs:simpleType>
The data stream may look like the following for a string value of “test”.
Location Hex values
00000000 0012A000
0012A000 7465737400
t e s t
The following example defines an array of 3 pointers to complex elements
defined by ns0:myStruct.
<xs:element name="myArray" type="ns0:myStruct"
dfdl:lengthKind="implicit" dfdl:indirectKind="pointer"
dfdl:indirectType="ptr32_t" dfdl:indirectEmptyValue="0"
minOccurs="3" maxOccurs="3" dfdl:occursCountKind="fixed"/>
<xs:simpleType name="ptr32_t" dfdl:representation="binary"
dfdl:lengthKind="explicit" dfdl:length="4"
dfdl:byteOrder="bigEndian">
<xs:restriction base="integer"/>
</xs:simpleType>
The data stream may look like the following with the contents of each
occurrence in a different location and the contents of the 2nd occurrence
being empty.
Location Hex values
00000000 00147000 00000000 00146000
00146000 ...
00147000 ...
The following example defines an offset to a 6 byte hexBinary value with
the offset being calculated from the start of the current element.
<xs:element name="myData" type="xs:hexBinary"
dfdl:lengthKind="explicit" dfdl:length="6"
dfdl:indirectKind="offset" dfdl:indirectType="uint"
dfdl:indirectBase="."/>
<xs:simpleType name="uint" dfdl:representation="binary"
dfdl:lengthKind="explicit" dfdl:length="4"
dfdl:byteOrder="bigEndian">
<xs:restriction base="unsignedInt"/>
</xs:simpleType>
The data stream may look like the following for a hexBinary value of
“123456789ABC”.
Location Hex values
00000100 00000108
00000208 123456789ABC
Regards,
Bradd Kadlecik
z/TPF Development
Phone: 1-845-433-1573 2455 South Rd
E-mail: braddk at us.ibm.com Poughkeepsie, NY
12601-5400
United States
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