close

(close stream)

Closes a stream previously returned by open and returns t. After closing, the stream handle is no longer valid and must not be used for further read/read-line/write-line calls; closing it again does nothing and returns t. A standard stream, *standard-output* included, survives the close, which returns t. Anything that is not a stream signals a type-error whose expected type is stream. Works on every backend. In most code you should prefer the with-open-file macro, which arranges the matching close for you even when the body exits early.

(let ((s (open "out.txt" :output)))
  (write-line "done" s)
  (close s))

Here the stream is opened for output, written to, and then closed; the close call flushes and releases the file descriptor.