read-sequence

(read-sequence sequence stream &key start end)

Fills sequence -- a one-dimensional array created with make-array -- with bytes read from a binary input stream, and returns the index of the first element that was not filled (the fill position). Reading starts at index :start (default 0) and stops before index :end (default the array length) or at end of file, whichever comes first. The :start/:end keywords must be literal; their values may be arbitrary expressions. Works in all three backends; it expands into a read-byte loop, so it requires a stream opened with :element-type '(unsigned-byte 8).

Because it touches the filesystem, read-sequence is shown here statically rather than as a runnable example:

(let ((buf (make-array 8)))
  (with-open-file (in "data.bin" :element-type '(unsigned-byte 8))
    (read-sequence buf in))  ; => 4 when data.bin has 4 bytes
  (aref buf 0))              ; => the first byte

A return value smaller than the array length means the file ended early; elements at and beyond the fill position keep their previous values.