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Attribute description

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Mikhail Yakshin edited this page Mar 1, 2016 · 10 revisions

Attribute description specifies how to read one particular attribute — typically, a single number, a string, array of bytes, etc. Attribute can also reference other complex structures by specifying user type given in type description. Each attribute is typically compiled into equivalent parsing instruction(s) in target language.

Common attributes

id

  • Contents: a string that matches /^[a-z][a-z0-9_]*$/ — i.e. starts with lowercase letter and then may contain lowercase letters, numbers and underscore
  • Purpose: identify attribute among others
  • Influences: used as variable / field name in target programming language
  • Mandatory: yes

type

repeat

  • Contents: expr or eos
  • Purpose: designate repeated attribute in a structure;
    • if repeat: expr is used, then attribute is repeated the number of times specified in repeat-expr key;
    • if repeat: eos is used, then attribute is repeated until the end of current stream
  • Influences: attribute would be read as array / list / sequence, executing parsing code multiple times
  • Mandatory: no

repeat-expr

  • Contents: expression
  • Purpose: specify number of repetitions for repeated attribute
  • Influences: number of times attribute is parsed
  • Mandatory: yes, if repeat: expr

if

  • Contents: expression
  • Purpose: mark the attribute as optional
  • Influences: attribute would be parsed only if condition specified in if key evaluates (in runtime) to true
  • Mandatory: no

Attributes that depend on type

No type specified

If there's no type specified, attribute will be read just as a sequence of bytes from a stream. Thus, one has to decide on how many bytes to read? There are two ways:

  • Specify amount of bytes to read in size key. One can specify an integer constant or an expression in this field (for example, if the number of bytes to read depends on some other attribute)
  • Set size-eos: true, thus ordering to read all the bytes till the end of current stream.

u*, s*

str

strz

User-specified types

https://doc.kaitai.io/

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