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
9165) in big endian would be the two bytes
and in little endian,
#x23. Here are the possible
Unsigned byte, with length 1.
Unsigned integer in network byte order, with length 2.
Unsigned integer in network byte order, with length 3.
Unsigned integer in network byte order, with length 4. Note: These values may be limited by Emacs’s integer implementation limits.
Unsigned integer in little endian order, with length 2, 3 and 4, respectively.
String of length 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]).
Four-byte vector representing an Internet address. For example:
[127 0 0 1] for localhost.
List of set bits in len bytes. The bytes are taken in big
endian order and the bits are numbered starting with
len - 1 and ending with zero. For example:
(2 3 4 11 13) and
(3 5 10 11 12).
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
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
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:
Unpack/pack this field according to the type specification type.
Evaluate form, a Lisp expression, for side-effect only. If the field name is specified, the value is bound to that field name.
Skip len bytes. In packing, this leaves them unchanged, which normally means they remain zero. In unpacking, this means they are ignored.
Skip to the next multiple of len bytes.
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:
(eval expr), evaluate expr with the variable
tagdynamically bound to the value of form. A non-
nilresult indicates a match.
equalto the value of form.
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.
(eval form) forms used in a bindat specification,
the form can access and update these dynamically bound variables
Value of the last field processed.
The data as a byte array.
Current index (within
bindat-raw) for unpacking or packing.
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.
repeat block, these contain the maximum number of
repetitions (as specified by the count parameter), and the
current repetition number (counting from 0). Setting
zero will terminate the inner-most repeat block after the current
repetition has completed.