DataScript - A Specification and Scripting Language for Binary Data

Question

1. What have the authors contributed in "Datascript–a specification and scripting language for binary data" ?

2. What are some of the constraints that are included in binary formats?

3. What is the purpose of the constraints in DataScript?

4. What does the code generator do when parsing input?

Accepted Answer

DataScript consists of two components: a constraint-based specification language that uses DataScript types to describe the physical layout of data and a language binding that provides a simple programming interface to script binary data.. The authors show examples that demonstrate that DataScript is simple, yet powerful enough to describe many commonly used formats.

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Many binary formats include constraints that guarantee the consistency of the data, such as parameter ranges, length constraints, checksums and others.

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DataScript borrows its set of operators, associativity, and precedence rules from Java and C in the hope that doing so will facilitate adoption by tapping into programmers’ existing skill sets.

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When parsing input, it simply sets a mark in the input stream and reads the necessary numbers of bytes for the next field, performs byte-order swapping if necessary, and initializes the field.

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Java is often used for mobile code, which a virtual machine may load from untrusted sources, hence the class loader must verify the well-formedness of the class file.

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DataScript provides a forall operator that can be attached to arrays, and whose use is necessary when constraints cannot be attached to an array element because they refer to the array itself, whose definition occurs lexically after the element’s definition.

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For instance, the SymbolTable type uses the “sh size” field to compute the number of entries in the table if the section contains a symbol table.

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Each field in a composite type can have a constraint associated with it, which is specified as an arbitrary boolean predicate separated from the field name by a colon.

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The type definition then uses fields such as “sh offset” and “sh type” to compute the offset and to discriminate the section type.

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The authors have introduced DataScript, a specification language for describing physical data layouts, and the authors presented a Java language binding for DataScript specifications.

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Many data formats are not sequential, but contain headers or directories whose entries store offsets into the file at which other structures start.

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A constraint defined in this way can only be applied to instances that are contained inside an instance of the type defining the constraint.

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If the enclosing composite type has no byte order attribute, big endian or network byte order is assumed (as is the case for Java class files).

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This process involves identifying constants that are stored as multibyte integers in the file, reading parameters such as lengths and offsets, reading arrays of simple and variable-record types, reading flags and indices and checking that they are valid and within range, and the verification of constraints that ensure the internal consistency of a class file.

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A set type forms a subset of an underlying primitive type, which specifies the byte order and signedness used to store and interpret its values.

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For this reason, it generates stubs in C. Unlike DataScript, PacketTypes does not address generating data, it also does not provide an visitor interface for writing scripts the way DataScript’s Java binding does.

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The size and signedness is encoded in the keyword, i.e., uint32 is a 32-bit unsigned integer, while int16 is a signed 16-bit integer.

DataScript - A Specification and Scripting Language for Binary Data

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Most frequently asked questions

1. What have the authors contributed in "Datascript–a specification and scripting language for binary data" ?

2. What are some of the constraints that are included in binary formats?

3. What is the purpose of the constraints in DataScript?

4. What does the code generator do when parsing input?

5. What is the role of the class loader in a Java virtual machine?

6. What is the definition of a forall operator?

7. What type uses the “sh size” field to compute the number of entries in the table?

8. What type of constraint can be used to define a field?

9. What is the type definition used to compute the offset?

10. What is the name of the specification language for describing physical data layouts?

11. What are some data formats that are not sequential?

12. What type of constraint can be used to define a record type?

13. What is the order of the byte in a composite type?

14. What is the process of identifying constants in a class file?

15. What is the byte order of a set type?

16. Why does PacketTypes not provide a visitor interface for writing scripts?

17. What is the sign of a uint32?

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Citations

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References

Design Patterns: Elements of Reusable Object-Oriented Software

The Java Virtual Machine Specification

Packet types: abstract specification of network protocol messages

Java Virtual Machine

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