The Unit Specifier

The unit specifier modifies how size is interpreted:

(value (size N) (unit U))

The actual size in bits is N * U. The unit must be from 1 to 256:

lfe> (binary (1 (size 2) (unit 8)))  ; 2 * 8 = 16 bits
#B(0 1)
lfe> (binary (1 (size 16) (unit 1)))  ; 16 * 1 = 16 bits
#B(0 1)

Both produce the same result, because 2 * 8 equals 16 * 1. The unit specifier is primarily useful for:

1. Enforcing alignment: Binary segments default to unit:8, meaning sizes must be multiples of 8 bits
2. Working with specific bit widths: Video codecs often work with pixels or samples of specific bit widths
3. Documentation: (size 2) (unit 8) might be clearer than (size 16) if you’re thinking in terms of “2 bytes”

The default unit values are:

• integer, float, bitstring: unit 1 (arbitrary bit sizes)
• binary, bytes: unit 8 (must be byte-aligned)

If you try to violate alignment constraints (like specifying a binary with a non-multiple-of-8 bit count), the system will object:

lfe> (binary (#B(1 2) (size 15) binary))  ; 15 is not divisible by 8
; Error: the value Binary in size specification of binary segment is expected to be of type integer

The error message is trying to be helpful. It’s not always successful at being helpful, but the attempt is appreciated.