write-byte

(write-byte byte stream)

Writes one byte -- an integer between 0 and 255 -- to a binary output stream (a stream opened with :direction :output :element-type '(unsigned-byte 8)) and returns the byte. Works in all four backends. The byte is written raw, with no newline or other framing added.

stream takes the same designators every other stream operation takes: t is the process standard output, nil means the current *standard-output* (which holds t unless you bind it), and *error-output* is standard error. (write-byte b *standard-output*) therefore puts raw octets on standard output, in order with anything princ and format write there.

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

(with-open-file (out "data.bin" :direction :output :element-type '(unsigned-byte 8))
  (write-byte 137 out)  ; => 137
  (write-byte 80 out)
  (write-byte 78 out)
  (write-byte 71 out))

(write-byte 137 *standard-output*)  ; one raw octet on stdout

This writes the four bytes 89 50 4E 47 (the start of a PNG signature) to data.bin. The interpreter and JVM signal an error for a value outside 0-255.