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#### 36.20.1 Describing Data Layout

To control unpacking and packing, you write a data layout specification, a special nested list describing named and typed fields. This specification controls the length of each field to be processed, and how to pack or unpack it. We normally keep bindat specs in variables whose names end in ‘`-bindat-spec`’; that kind of name is automatically recognized as risky.

A field’s type describes the size (in bytes) of the object that the field represents and, in the case of multibyte fields, how the bytes are ordered within the field. The two possible orderings are big endian (also known as “network byte ordering”) and little endian. For instance, the number `#x23cd` (decimal 9165) in big endian would be the two bytes `#x23` `#xcd`; and in little endian, `#xcd` `#x23`. Here are the possible type values:

`u8`
`byte`

  Unsigned byte, with length 1.

`u16`
`word`
`short`

  Unsigned integer in network byte order, with length 2.

`u24`

  Unsigned integer in network byte order, with length 3.

`u32`
`dword`
`long`

  Unsigned integer in network byte order, with length 4. Note: These values may be limited by Emacs’s integer implementation limits.

`u16r`
`u24r`
`u32r`

  Unsigned integer in little endian order, with length 2, 3 and 4, respectively.

`str len`

  String of length len.

`strz len`

  Zero-terminated string, in a fixed-size field with length len.

`vec len [type]`

  Vector of len elements of type type, defaulting to bytes. The type is any of the simple types above, or another vector specified as a list of the form `(vec len [type])`.

`ip`

  Four-byte vector representing an Internet address. For example: `[127 0 0 1]` for localhost.

`bits len`

  List of set bits in len bytes. The bytes are taken in big endian order and the bits are numbered starting with `8 * len âˆ’ 1` and ending with zero. For example: `bits 2` unpacks `#x28` `#x1c` to `(2 3 4 11 13)` and `#x1c` `#x28` to `(3 5 10 11 12)`.

`(eval form)`

  form is a Lisp expression evaluated at the moment the field is unpacked or packed. The result of the evaluation should be one of the above-listed type specifications.

For a fixed-size field, the length len is given as an integer specifying the number of bytes in the field.

When the length of a field is not fixed, it typically depends on the value of a preceding field. In this case, the length len can be given either as a list `(name ...)` identifying a field name in the format specified for `bindat-get-field` below, or by an expression `(eval form)` where form should evaluate to an integer, specifying the field length.

A field specification generally has the form `([name] handler)`, where name is optional. Don’t use names that are symbols meaningful as type specifications (above) or handler specifications (below), since that would be ambiguous. name can be a symbol or an expression `(eval form)`, in which case form should evaluate to a symbol.

handler describes how to unpack or pack the field and can be one of the following:

`type`

  Unpack/pack this field according to the type specification type.

`eval form`

  Evaluate form, a Lisp expression, for side-effect only. If the field name is specified, the value is bound to that field name.

`fill len`

  Skip len bytes. In packing, this leaves them unchanged, which normally means they remain zero. In unpacking, this means they are ignored.

`align len`

  Skip to the next multiple of len bytes.

`struct spec-name`

  Process spec-name as a sub-specification. This describes a structure nested within another structure.

`union form (tag spec)…`

  Evaluate form, a Lisp expression, find the first tag that matches it, and process its associated data layout specification spec. Matching can occur in one of three ways:

  - If a tag has the form `(eval expr)`, evaluate expr with the variable `tag` dynamically bound to the value of form. A non-`nil` result indicates a match.
  - tag matches if it is `equal` to the value of form.
  - tag matches unconditionally if it is `t`.
`repeat count field-specs…`

  Process the field-specs recursively, in order, then repeat starting from the first one, processing all the specifications count times overall. The count is given using the same formats as a field length—if an `eval` form is used, it is evaluated just once. For correct operation, each specification in field-specs must include a name.

For the `(eval form)` forms used in a bindat specification, the form can access and update these dynamically bound variables during evaluation:

`last`

  Value of the last field processed.

`bindat-raw`

  The data as a byte array.

`bindat-idx`

  Current index (within `bindat-raw`) for unpacking or packing.

`struct`

  The alist containing the structured data that have been unpacked so far, or the entire structure being packed. You can use `bindat-get-field` to access specific fields of this structure.

`count`
`index`

  Inside a `repeat` block, these contain the maximum number of repetitions (as specified by the count parameter), and the current repetition number (counting from 0). Setting `count` to zero will terminate the inner-most repeat block after the current repetition has completed.

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