65.1.7 The c-data form

C-data in general is hard to describe, it is everything which is a contiguous piece of memory, called a block of memory. Data of this type is represented byte-wise, the internal representation is an emacs string encoded as binary.

The syntax is a cons-cell with the symbol c-data in the car and a non-negative integer in the cdr. For a (yet) unknown length of a data block one can simply use the non-cons’d version 'c-data.

;; block of 40 bytes of arbitrary data
'(c-data . 40)

;; indefinite block of data
'c-data