write-sequence

(write-sequence sequence stream &key start end)

Writes the elements of sequence to stream and returns the sequence. Writing starts at index :start (default 0) and stops before index :end (default the sequence length). The :start/:end keywords must be literal; their values may be arbitrary expressions. Works in all three backends.

When sequence is a string, the bounded slice is written as characters (like write-string), so it works with a text output stream such as the one from with-output-to-string:

When sequence is a one-dimensional array of integers between 0 and 255, it expands into a write-byte loop, so it requires a stream opened with :direction :output :element-type '(unsigned-byte 8).

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

(let ((buf (make-array 4)))
  (setf (aref buf 0) 222) (setf (aref buf 1) 173)
  (setf (aref buf 2) 190) (setf (aref buf 3) 239)
  (with-open-file (out "data.bin" :direction :output :element-type '(unsigned-byte 8))
    (write-sequence buf out)))  ; => the array

This writes the four bytes DE AD BE EF to data.bin. Use :start/:end to write only a slice of the array.