38.20.2 Functions to Unpack and Pack Bytes

In the following documentation, spec refers to a data layout specification, bindat-raw to a byte array, and struct to an alist representing unpacked field data.

function bindat-unpack spec bindat-raw \&optional bindat-idx

This function unpacks data from the unibyte string or byte array bindat-raw according to spec. Normally, this starts unpacking at the beginning of the byte array, but if bindat-idx is non-nil, it specifies a zero-based starting position to use instead.

The value is an alist or nested alist in which each element describes one unpacked field.

function bindat-get-field struct \&rest name

This function selects a field’s data from the nested alist struct. Usually struct was returned by bindat-unpack. If name corresponds to just one argument, that means to extract a top-level field value. Multiple name arguments specify repeated lookup of sub-structures. An integer name acts as an array index.

For example, if name is (a b 2 c), that means to find field c in the third element of subfield b of field a. (This corresponds to struct.a.b[2].c in C.)

Although packing and unpacking operations change the organization of data (in memory), they preserve the data’s total length, which is the sum of all the fields’ lengths, in bytes. This value is not generally inherent in either the specification or alist alone; instead, both pieces of information contribute to its calculation. Likewise, the length of a string or array being unpacked may be longer than the data’s total length as described by the specification.

function bindat-length spec struct

This function returns the total length of the data in struct, according to spec.

function bindat-pack spec struct \&optional bindat-raw bindat-idx

This function returns a byte array packed according to spec from the data in the alist struct. It normally creates and fills a new byte array starting at the beginning. However, if bindat-raw is non-nil, it specifies a pre-allocated unibyte string or vector to pack into. If bindat-idx is non-nil, it specifies the starting offset for packing into bindat-raw.

When pre-allocating, you should make sure (length bindat-raw) meets or exceeds the total length to avoid an out-of-range error.

function bindat-ip-to-string ip

Convert the Internet address vector ip to a string in the usual dotted notation.

(bindat-ip-to-string [127 0 0 1])
     ⇒ "127.0.0.1"