stream-element-type

(stream-element-type stream)

The element type a stream carries. A binary file stream answers the integer type it was opened with, widened the way SBCL widens it: (unsigned-byte 8) for any one-octet unsigned type (bit, (unsigned-byte 3), (integer 100 200)), (unsigned-byte 16) for (unsigned-byte 9) through 16, (signed-byte 32) for (signed-byte 20), and so on (see open). Every other stream -- a character file stream, a string stream, a socket, a standard stream -- answers character. Works in all four backends.

(with-open-file (s "data.bin" :element-type '(unsigned-byte 12))
  (stream-element-type s)) ; => (UNSIGNED-BYTE 16)